diff --git a/cmd/opensave-app/frontend/src/components/ConflictModal.svelte b/cmd/opensave-app/frontend/src/components/ConflictModal.svelte
index 4a9311ff..f24a7baa 100644
--- a/cmd/opensave-app/frontend/src/components/ConflictModal.svelte
+++ b/cmd/opensave-app/frontend/src/components/ConflictModal.svelte
@@ -1,6 +1,6 @@
 <script>
   import { dialogOut } from '../lib/motion.js';
-  import { conflicts, conflictResolution, games, peers, settings, toast, view } from '../lib/stores.js';
+  import { conflicts, conflictResolution, games, peers, settings, toast, view, askConfirm } from '../lib/stores.js';
   import { isHandheld } from '../lib/devices.js';
   import { visited } from '../lib/later.js';
   import { api } from '../lib/api.js';
@@ -103,9 +103,16 @@
   $: diffFiles = conflict?.diffFiles ?? [];
   $: diffTotal = conflict?.diffTotal ?? 0;
   $: diffCapped = diffTotal > diffFiles.length;
-  $: changedCount = diffFiles.filter((d) => d.status === 'changed').length;
-  $: onlyLocalCount = diffFiles.filter((d) => d.status === 'only-local').length;
-  $: onlyRemoteCount = diffFiles.filter((d) => d.status === 'only-remote').length;
+  // The uncapped counts when the device sends them; counted from the capped
+  // list otherwise (an older build), which is exact below the cap.
+  const countOr = (n, status) => (typeof n === 'number' ? n : diffFiles.filter((d) => d.status === status).length);
+  $: changedCount = countOr(conflict?.changedTotal, 'changed');
+  $: onlyLocalCount = countOr(conflict?.onlyLocalTotal, 'only-local');
+  $: onlyRemoteCount = countOr(conflict?.onlyRemoteTotal, 'only-remote');
+  // "Keep theirs" deletes every file only this device has. Said before the
+  // click, in numbers, and loudly once it is a lot.
+  $: theirsDeletes = onlyLocalCount;
+  $: theirsDeletesMany = theirsDeletes >= 100;
 
   // Bytes that differ, per side: a changed file counts its own size on each
   // side, a file present on only one side counts only there.
@@ -159,6 +166,31 @@
     }
   }
 
+  // Neither side is the right save — a copy that arrived half-way, a mixture
+  // — and this device holds the right one: it goes to every device, which
+  // each keep a snapshot of their own first. A device on which the save was
+  // changed after the update is still asked.
+  async function useEverywhere() {
+    if (!current || busy) return;
+    const [gameId] = current;
+    const name = gameName;
+    const ok = await askConfirm(
+      `Make this device's save of ${name} the one every device uses, ${peerName} included? Each replaces its save with this one — every file, including removing files this device does not have — and keeps a snapshot of its own first.`,
+      { title: 'Use this save everywhere?', confirmText: 'Use this save everywhere', danger: true }
+    );
+    if (!ok) return;
+    busy = true;
+    try {
+      await api.post(`/api/games/${gameId}/use-everywhere`);
+      toast(`${name}: this save goes to your other devices`, 'success');
+      showDiff = false;
+    } catch (e) {
+      toast(e.message, 'error');
+    } finally {
+      busy = false;
+    }
+  }
+
   const fmtTime = (t) => (t ? new Date(t).toLocaleString() : '—');
   const fmtMs = (ms) => (ms ? new Date(ms).toLocaleString() : 'unknown');
   const fmtSize = (n) =>
@@ -217,12 +249,28 @@
           <div class="v-time">last change {fmtMs(remoteMs)}</div>
         </div>
       </div>
-      {#if diffCapped}
+      {#if diffCapped && typeof conflict.onlyLocalTotal !== 'number'}
         <p class="diff-capped">
           Counts above cover the first {diffFiles.length} of {diffTotal} differing files.
         </p>
       {/if}
 
+      <ul class="consequences">
+        <li>
+          <strong>Keep mine</strong> / <strong>Keep both</strong>: nothing is deleted.
+          {#if onlyRemoteCount > 0}The {onlyRemoteCount} file{onlyRemoteCount === 1 ? '' : 's'} only on {peerName} {onlyRemoteCount === 1 ? 'is' : 'are'} brought here;{/if}
+          {#if changedCount > 0}where both have a file, yours is kept.{/if}
+        </li>
+        <li class:danger={theirsDeletesMany}>
+          <strong>Keep theirs</strong>:
+          {#if theirsDeletes > 0}
+            <strong>deletes {theirsDeletes} file{theirsDeletes === 1 ? '' : 's'}</strong> that only this device has (snapshotted first).
+          {:else}
+            nothing only this device has is lost.
+          {/if}
+        </li>
+      </ul>
+
       {#if conflict.diffTotal > 0}
         <button class="diff-toggle" on:click={() => (showDiff = !showDiff)}>
           <Chevron open={showDiff} /> What's different ({conflict.diffTotal} file{conflict.diffTotal === 1 ? '' : 's'})
@@ -250,11 +298,16 @@
       <div class="actions">
         <button class="btn ghost later" disabled={busy} on:click={putOff} title="Nothing of this game syncs until you decide. Open the game to decide.">Decide later</button>
         <button class="btn" disabled={busy} on:click={() => resolve('keep-local')}>Keep mine</button>
-        <button class="btn" disabled={busy} on:click={() => resolve('keep-remote')}>Keep theirs</button>
+        <button class="btn" class:danger={theirsDeletesMany} disabled={busy} on:click={() => resolve('keep-remote')}
+          title={theirsDeletes > 0 ? `Deletes ${theirsDeletes} file(s) only this device has` : ''}>Keep theirs</button>
         <button class="btn primary" disabled={busy} on:click={() => resolve('merge-branch')}>
           Keep both (recommended)
         </button>
       </div>
+      <p class="hint-line">
+        Neither is the right save, and this device has it?
+        <button class="linkish" disabled={busy} on:click={useEverywhere}>Use this device's save everywhere</button>
+      </p>
       <p class="hint-line">
         <History size={15} class="inline-icon" /> Nothing is lost whichever you pick. <strong>“Keep both”</strong> (recommended) keeps
         playing yours — {peerName} receives it too — and keeps {peerName}'s version here on a branch you can switch
@@ -362,6 +415,22 @@
     color: var(--text-faint);
     margin: -8px 0 12px;
   }
+  .consequences {
+    margin: 0 0 14px;
+    padding-left: 18px;
+    font-size: 0.8rem;
+    color: var(--text-dim);
+  }
+  .consequences li {
+    margin: 2px 0;
+  }
+  .consequences li.danger,
+  .btn.danger {
+    color: var(--danger, #e5484d);
+  }
+  .btn.danger {
+    border-color: currentColor;
+  }
   .v-time {
     font-size: 0.74rem;
     color: var(--text-faint);
@@ -451,4 +520,13 @@
     color: var(--text-faint);
     line-height: 1.5;
   }
+  .linkish {
+    background: none;
+    border: none;
+    padding: 0;
+    font: inherit;
+    color: var(--accent-text);
+    text-decoration: underline;
+    cursor: pointer;
+  }
 </style>
diff --git a/cmd/opensave-app/frontend/src/lib/gamestatus.js b/cmd/opensave-app/frontend/src/lib/gamestatus.js
index 4d920c01..2be2f839 100644
--- a/cmd/opensave-app/frontend/src/lib/gamestatus.js
+++ b/cmd/opensave-app/frontend/src/lib/gamestatus.js
@@ -21,6 +21,9 @@ import { playLength } from './format.js';
 export function gameStatus(game, { peers = {}, activity, conflicted = false, now = Date.now() } = {}) {
   // Before anything else: with its folder gone nothing else about it can
   // move — no snapshot, no sync, no decision taken.
+  // …unless the whole drive is absent here: the game's saves live on another
+  // device's D:\ or E:\, and there is nothing on this one to worry about.
+  if (game.savePathMissing && game.saveDriveMissing) return { state: 'elsewhere', label: 'Not on this device', tone: 'muted' };
   if (game.savePathMissing) return { state: 'missing', label: 'Save folder missing', tone: 'warn' };
   // Every save file deleted here at once, held back from the other devices
   // until someone says whether that was meant (lib/emptied.js).
@@ -177,7 +180,7 @@ const stamp = (s) => (s ? Date.parse(s) : null);
 const snapshotsOf = (game) => Object.values(game.branches ?? {}).flatMap((b) => b.snapshots ?? []);
 
 // What needs looking at, in the order the summary above the library reads it.
-const URGENCY = { missing: 0, emptied: 1, conflict: 2, error: 3, syncing: 4, restoring: 4, unsynced: 5, empty: 6, paused: 7 };
+const URGENCY = { missing: 0, emptied: 1, conflict: 2, error: 3, syncing: 4, restoring: 4, unsynced: 5, empty: 6, paused: 7, elsewhere: 10 }; // unlisted states (synced, local…) rank 9
 
 /**
  * The orders the library can be put in. Each compares two rows, {game,
diff --git a/cmd/opensave-app/frontend/src/lib/gamestatus.test.js b/cmd/opensave-app/frontend/src/lib/gamestatus.test.js
index d144943c..810b197b 100644
--- a/cmd/opensave-app/frontend/src/lib/gamestatus.test.js
+++ b/cmd/opensave-app/frontend/src/lib/gamestatus.test.js
@@ -119,6 +119,18 @@ describe('a missing save folder', () => {
     expect(librarySummary(rows)).toEqual({ tone: 'warn', headline: "Hades's save folder is missing" });
     expect(sortRows(rows, 'attention').map((r) => r.game.name)).toEqual(['Hades', 'Celeste']);
   });
+
+  it('on a drive this device does not have is not a warning, and stays out of the headline', () => {
+    const g = game({ id: 'a', name: 'ARK', savePathMissing: true, saveDriveMissing: true });
+    const s = gameStatus(g);
+    expect(s).toMatchObject({ state: 'elsewhere', tone: 'muted', label: 'Not on this device' });
+    const rows = [
+      { game: g, status: s },
+      { game: game({ id: 'c', name: 'Celeste', branches: snaps(minsAgo(5)) }), status: { state: 'synced' } }
+    ];
+    expect(librarySummary(rows).tone).not.toBe('warn');
+    expect(sortRows(rows, 'attention').map((r) => r.game.name)).toEqual(['Celeste', 'ARK']);
+  });
 });
 
 describe('SORTS', () => {
diff --git a/cmd/opensave-app/frontend/src/lib/notifications.js b/cmd/opensave-app/frontend/src/lib/notifications.js
index e8633fd5..ece29614 100644
--- a/cmd/opensave-app/frontend/src/lib/notifications.js
+++ b/cmd/opensave-app/frontend/src/lib/notifications.js
@@ -46,7 +46,8 @@ export function waitingOnYou({ games = {}, conflicts = {}, locationConflicts = [
     out.push({ id: `offer:${o.gameId}:${o.snapshotId}`, tone: 'info', gameId: o.gameId, title: `A newer save for ${o.gameName ?? name(o.gameId)}`, detail: `From ${o.deviceName ?? 'another device'}, through the cloud`, go: { view: 'home' } });
   }
   for (const g of Object.values(games)) {
-    if (g.savePathMissing) {
+    // A drive this device doesn't have is not waiting on anyone.
+    if (g.savePathMissing && !g.saveDriveMissing) {
       out.push({ id: `missing:${g.id}`, tone: 'warn', gameId: g.id, title: `${g.name}'s save folder is missing`, detail: 'Nothing is synced for it until it is back', go: { view: 'game', params: { gameId: g.id } } });
     }
   }
diff --git a/cmd/opensave-app/frontend/src/lib/notifications.test.js b/cmd/opensave-app/frontend/src/lib/notifications.test.js
index a5828324..1e32a9f7 100644
--- a/cmd/opensave-app/frontend/src/lib/notifications.test.js
+++ b/cmd/opensave-app/frontend/src/lib/notifications.test.js
@@ -21,6 +21,11 @@ describe('notifications', () => {
     expect(w.find((x) => x.id.startsWith('damaged')).title).toBe("1 snapshot of Hades can't be restored");
   });
 
+  it('does not ask about a game whose drive this device does not have', () => {
+    const w = waitingOnYou({ games: { ark: { id: 'ark', name: 'ARK', savePathMissing: true, saveDriveMissing: true, branches: {} } } });
+    expect(w).toEqual([]);
+  });
+
   it('turns activity into events, unread since the bell was last opened', () => {
     const now = Date.UTC(2026, 8, 25, 12);
     const items = [
diff --git a/cmd/opensave-app/frontend/src/lib/snapshots.js b/cmd/opensave-app/frontend/src/lib/snapshots.js
index 9ad3a4ab..9dc80b5b 100644
--- a/cmd/opensave-app/frontend/src/lib/snapshots.js
+++ b/cmd/opensave-app/frontend/src/lib/snapshots.js
@@ -89,6 +89,9 @@ export function snapshotKind(snap, now = new Date()) {
   if (/^initial snapshot$/i.test(comment)) return { kind: 'start', label: 'Automatic', title: 'When tracking started' };
   // The save as a play session left it (daemon/sessions.go).
   if (/^after playing/i.test(comment)) return { kind: 'session', label: 'After playing', title: comment };
+  // Taken while a game was played, in case the session never got to end;
+  // removed once it did (daemon/sessions.go).
+  if (/^checkpoint while playing/i.test(comment)) return { kind: 'session', label: 'While playing', title: 'Checkpoint while playing' };
   if (safetyPattern.test(comment)) {
     return { kind: 'safety', label: 'Safety copy', title: plainTitle(comment) };
   }
diff --git a/cmd/opensave-app/frontend/src/lib/snapshots.test.js b/cmd/opensave-app/frontend/src/lib/snapshots.test.js
index d5e26ca2..75c9bdf7 100644
--- a/cmd/opensave-app/frontend/src/lib/snapshots.test.js
+++ b/cmd/opensave-app/frontend/src/lib/snapshots.test.js
@@ -64,6 +64,10 @@ describe('snapshotKind', () => {
     });
   });
 
+  it('names the checkpoints taken while a game is played', () => {
+    expect(snapshotKind(auto('Checkpoint while playing'))).toMatchObject({ kind: 'session', title: 'Checkpoint while playing' });
+  });
+
   it('says a save changed for the watcher’s own snapshots', () => {
     expect(snapshotKind(auto(''))).toMatchObject({ kind: 'auto', title: 'Save changed' });
     expect(snapshotKind(auto('Auto backup'))).toMatchObject({ kind: 'auto', title: 'Save changed' });
diff --git a/cmd/opensave-app/frontend/src/views/Devices.svelte b/cmd/opensave-app/frontend/src/views/Devices.svelte
index 641c5f3b..b03a143f 100644
--- a/cmd/opensave-app/frontend/src/views/Devices.svelte
+++ b/cmd/opensave-app/frontend/src/views/Devices.svelte
@@ -76,6 +76,15 @@
   };
 
   const fmtTime = (t) => (t ? new Date(t).toLocaleString() : 'never');
+
+  // How this device reaches the peer, and how fast it has found that to be.
+  const linkKinds = { lan: 'home network', vpn: 'VPN (e.g. Tailscale)', relay: 'internet relay', internet: 'internet' };
+  const fmtRate = (b) => (b >= 1048576 ? (b / 1048576).toFixed(1) + ' MB/s' : Math.max(1, Math.round(b / 1024)) + ' KB/s');
+  function linkLabel(link) {
+    const kind = linkKinds[link.kind] ?? link.kind;
+    if (!link.bytesPerSec) return `${kind} · speed not measured yet`;
+    return `${kind} · ${fmtRate(link.bytesPerSec)} (measured ${fmtTime(link.measuredMs)})`;
+  }
 </script>
 
 <div class="head">
@@ -99,12 +108,22 @@
             <span class="badge" class:online={peer.status === 'online'} class:offline={peer.status !== 'online'}>
               {peer.status}
             </span>
+            {#if peer.needsUpdate}
+              <span class="badge warn" title="Its OpenSave keeps no save versions: nothing is taken from it, and its deletions are refused, until it is updated">
+                needs update
+              </span>
+            {/if}
           </div>
           <div class="peer-meta">
             {peer.address === 'relay' ? 'internet relay' : `${peer.address}:${peer.port}`}
             · last synced {fmtTime(peer.lastSynced)}
             {#if peer.appVersion}· OpenSave {peer.appVersion}{/if}
           </div>
+          {#if peer.link}
+            <div class="peer-meta" title="Saves go to the fastest connections first, and a device that is behind takes the newer save from the fastest device that has it.">
+              {linkLabel(peer.link)}
+            </div>
+          {/if}
           <!-- Per device, next to how it is reached, because that is the
                context the answer depends on. -->
           <div class="peer-protection">
diff --git a/cmd/opensave-app/frontend/src/views/game/ConfigTab.svelte b/cmd/opensave-app/frontend/src/views/game/ConfigTab.svelte
index 459703f1..8d6f8bfb 100644
--- a/cmd/opensave-app/frontend/src/views/game/ConfigTab.svelte
+++ b/cmd/opensave-app/frontend/src/views/game/ConfigTab.svelte
@@ -25,8 +25,12 @@
     autoSync: game.autoSync ?? true,
     maxSnapshots: game.maxSnapshots ?? 5,
     maxManualSnapshots: game.maxManualSnapshots ?? 0,
-    syncIgnore: game.syncIgnore ?? ''
+    syncIgnore: game.syncIgnore ?? '',
+    syncDeviceSettings: game.syncDeviceSettings ?? false
   };
+  // The files the game database names as this game's settings rather than its
+  // save (graphics, resolution): left out of syncing unless asked for.
+  $: deviceSettings = game.deviceSettings ?? [];
   let locations = [];
 
   // Cover preview: a custom URL wins, else the proxied Steam art for the App
@@ -76,7 +80,8 @@
         autoSync: cfg.autoSync,
         maxSnapshots: Number(cfg.maxSnapshots),
         maxManualSnapshots: Number(cfg.maxManualSnapshots),
-        syncIgnore: cfg.syncIgnore ?? ''
+        syncIgnore: cfg.syncIgnore ?? '',
+        syncDeviceSettings: !!cfg.syncDeviceSettings
       })
     );
   }
@@ -160,6 +165,32 @@
         can't push them out. <strong>0 = keep forever</strong> (the default).
       </span>
     </div>
+    {#if deviceSettings.length > 0}
+      <div class="field device-block">
+        <label for="c-devsettings">Device settings</label>
+        <ul class="device-settings">
+          {#each deviceSettings as s}
+            <li class:given-back={s.source === 'save'}>
+              <code>{s.pattern.replace(/^!?\//, '')}</code>
+              <span class="source">
+                {#if s.source === 'database'}game database{:else if s.source === 'detected'}detected
+                  — {s.reason}{:else}syncs again — {s.reason}{/if}
+              </span>
+            </li>
+          {/each}
+        </ul>
+      </div>
+      <label class="check">
+        <input id="c-devsettings" type="checkbox" bind:checked={cfg.syncDeviceSettings} />
+        Sync these settings between devices too
+      </label>
+      <span class="hint device-hint">
+        These are this game's settings, not its save — named by the game database or found by
+        what they hold — so each device keeps its own (graphics, resolution). They are still in
+        every snapshot. A file that turns out to change along with the save syncs again by
+        itself; to sync one yourself, untick it under “Pick from your save folder” below.
+      </span>
+    {/if}
     <SyncIgnoreEditor {game} bind:value={cfg.syncIgnore} severalFolders={locations.length > 0} />
     <SaveLocations {game} {runner} bind:locations />
     <div class="save">
@@ -206,6 +237,29 @@
   .fields .field {
     margin-bottom: 14px;
   }
+  .device-settings {
+    margin: 4px 0 6px;
+    padding-left: 18px;
+    color: var(--text-dim, inherit);
+  }
+  .device-settings code {
+    font-size: 0.9em;
+  }
+  .device-settings .source {
+    font-size: 0.82em;
+    color: var(--text-faint);
+    margin-left: 6px;
+  }
+  .device-settings .given-back code {
+    text-decoration: line-through;
+  }
+  .device-block {
+    margin-bottom: 4px !important;
+  }
+  .device-hint {
+    display: block;
+    margin-bottom: 14px;
+  }
   .hint-warn {
     color: var(--warn);
   }
diff --git a/cmd/opensave-app/frontend/src/views/game/GameHeader.svelte b/cmd/opensave-app/frontend/src/views/game/GameHeader.svelte
index 19750020..327f2a34 100644
--- a/cmd/opensave-app/frontend/src/views/game/GameHeader.svelte
+++ b/cmd/opensave-app/frontend/src/views/game/GameHeader.svelte
@@ -13,10 +13,12 @@
   import ArrowLeft from 'lucide-svelte/icons/arrow-left';
   import Play from 'lucide-svelte/icons/play';
   import RefreshCw from 'lucide-svelte/icons/refresh-cw';
+  import Upload from 'lucide-svelte/icons/upload';
   import FolderOpen from 'lucide-svelte/icons/folder-open';
   import Pencil from 'lucide-svelte/icons/pencil';
   import Star from 'lucide-svelte/icons/star';
   import TriangleAlert from 'lucide-svelte/icons/triangle-alert';
+  import Info from 'lucide-svelte/icons/info';
   import { collections, isFavourite, toggleFavourite } from '../../lib/collections.js';
 
   export let game;
@@ -24,6 +26,14 @@
   export let runner;
   const { busy, run } = runner;
 
+  // A game OpenSave cannot find here: its program, chosen by hand, says where
+  // it is installed and what runs when it is played (sessions, launch).
+  async function pickProgram() {
+    const exe = await native.selectFile(`Program of “${game.name}”`);
+    if (!exe) return;
+    await run(`Program set for ${game.name}`, () => api.patch(`/api/games/${game.id}`, { exePath: exe }));
+  }
+
   $: activity = $syncActivity[game.id];
 
   // Is my Deck up to date with THIS save? One entry per paired device, in
@@ -42,6 +52,20 @@
   const syncNow = () => run('Sync triggered', () => api.post(`/api/games/${game.id}/sync`));
   const launchGame = () => run('Launching…', () => api.post(`/api/games/${game.id}/launch`));
 
+  // For when the devices hold states nobody can rank — a copy that arrived
+  // only half-way, a mixture of two saves — and the person knows which device
+  // has the right one. Every other device takes this save whole, keeping a
+  // snapshot of its own first; one that changed the save on its own since is
+  // still asked.
+  async function useEverywhere() {
+    const ok = await askConfirm(
+      `Make this device's save of ${game.name} the one every other device uses? They each replace theirs with it — every file, including removing files this device does not have — and keep a snapshot of their own first. A device where the save was changed after this update is asked instead.`,
+      { title: 'Use this save everywhere?', confirmText: 'Use this save everywhere', danger: true }
+    );
+    if (!ok) return;
+    await run(`${game.name}: this save goes to your other devices`, () => api.post(`/api/games/${game.id}/use-everywhere`));
+  }
+
   let editPath = false;
   let pathDraft = '';
 
@@ -118,7 +142,12 @@
         · <span title={new Date(game.lastPlayedAt).toLocaleString()}>played {timeAgo(game.lastPlayedAt, now)}, {playLength(game.playtimeMs)} in all</span>
       {/if}
       {#if game.installed === 'not-found'}
-        · <span title="Steam on this PC does not have it, and it is in no folder games are kept in. Set its program under Configuration to launch it from somewhere else.">not found on this PC</span>
+        · <button
+          class="linkish"
+          disabled={$busy}
+          title="OpenSave cannot tell where this game is installed here, so it cannot see when it is being played. Choose its program to fix both."
+          on:click={pickProgram}>not found on this PC — choose its program</button
+        >
       {/if}
     </div>
     {#if syncedWith.length > 0}
@@ -137,6 +166,7 @@
     {#if canLaunch(game)}
       <button class="btn" disabled={$busy} on:click={launchGame}><Play size={15} />Launch</button>
     {/if}
+    <button class="btn" disabled={$busy} on:click={useEverywhere} title="Every other device replaces its save of this game with this one"><Upload size={15} />Use this save everywhere</button>
     <button class="btn primary" disabled={$busy} on:click={syncNow}><RefreshCw size={15} />Sync now</button>
   </div>
 </div>
@@ -188,7 +218,18 @@
   </div>
 {/if}
 
-{#if game.savePathMissing && !editPath}
+{#if game.savePathMissing && game.saveDriveMissing && !editPath}
+  <!-- The drive itself isn't on this device: the game lives on another PC's
+       D:\ or E:\. Nothing was lost here, so this informs rather than warns. -->
+  <div class="missing elsewhere" role="status">
+    <Info size={16} />
+    <div>
+      <strong>This game isn't on this device.</strong>
+      Its saves are on {game.savePath.slice(0, 2)}, which this PC doesn't have, so it's skipped here and keeps syncing
+      between your other devices. If it is installed here, point the game at its save folder with Edit.
+    </div>
+  </div>
+{:else if game.savePathMissing && !editPath}
   <!-- The folder is not there. It is not created again: an empty folder in
        its place would read as every file deleted, and syncing that would
        delete them on your other devices too. -->
@@ -204,6 +245,15 @@
 {/if}
 
 <style>
+  .linkish {
+    background: none;
+    border: none;
+    padding: 0;
+    font: inherit;
+    color: var(--accent-text);
+    text-decoration: underline;
+    cursor: pointer;
+  }
   .title-row {
     display: flex;
     align-items: center;
@@ -288,6 +338,13 @@
   .missing strong {
     color: var(--text);
   }
+  .missing.elsewhere {
+    border-color: var(--border);
+    background: transparent;
+  }
+  .missing.elsewhere :global(svg) {
+    color: var(--text-faint);
+  }
   .emptied-actions {
     display: flex;
     flex-wrap: wrap;
diff --git a/cmd/opensave-app/frontend/src/views/game/SyncIgnoreEditor.svelte b/cmd/opensave-app/frontend/src/views/game/SyncIgnoreEditor.svelte
index 13d14bd8..742d059f 100644
--- a/cmd/opensave-app/frontend/src/views/game/SyncIgnoreEditor.svelte
+++ b/cmd/opensave-app/frontend/src/views/game/SyncIgnoreEditor.svelte
@@ -133,7 +133,10 @@
             <span class="file-name">
               {#if f.location}<span class="file-loc">{f.location} ›</span>{/if}{f.path}
             </span>
-            <span class="verdict">{f.excluded ? "won't sync" : 'syncs'}</span>
+            <span class="verdict" title={f.reason ?? ''}>
+              {#if !f.excluded}syncs{:else if f.source === 'database'}won't sync · settings (game database){:else if f.source === 'detected'}won't
+                sync · settings (detected){:else}won't sync{/if}
+            </span>
           </label>
         {/each}
       </div>
@@ -141,6 +144,8 @@
         Ticking a file writes the pattern for you, anchored so it can only ever mean that
         one file. Unticking a file caught by a wildcard adds a <code>!</code> exception
         rather than deleting the wildcard.
+        Files marked <em>settings</em> are left out as this game's device settings; unticking
+        one syncs it after all.
         {#if severalFolders}
           A pattern applies to <strong>every one of this game's folders</strong>, so a
           name that appears in two of them is excluded in both.
diff --git a/cmd/opensave-app/main.go b/cmd/opensave-app/main.go
index 09e46f07..4f8ca678 100644
--- a/cmd/opensave-app/main.go
+++ b/cmd/opensave-app/main.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"runtime"
+	"runtime/debug"
 
 	"github.com/wailsapp/wails/v2"
 	"github.com/wailsapp/wails/v2/pkg/options"
@@ -45,6 +46,8 @@ func main() {
 	// removes its leftover binary) before claiming the single-instance lock.
 	cleanupReplacedBinary()
 
+	tuneGC()
+
 	app := NewApp()
 
 	err := wails.Run(&options.App{
@@ -97,3 +100,17 @@ func main() {
 		os.Exit(1)
 	}
 }
+
+// tuneGC trades a little CPU for a smaller footprint. OpenSave is a
+// background app, and Go's default (let the heap double before collecting)
+// meant a large save's manifests showed up as twice their size in Task
+// Manager. The soft limit makes the runtime collect and hand memory back to
+// the OS harder as it approaches 1GiB. Explicit GOGC / GOMEMLIMIT win.
+func tuneGC() {
+	if os.Getenv("GOGC") == "" {
+		debug.SetGCPercent(50)
+	}
+	if os.Getenv("GOMEMLIMIT") == "" {
+		debug.SetMemoryLimit(1 << 30)
+	}
+}
diff --git a/e2e/batch_bench_test.go b/e2e/batch_bench_test.go
new file mode 100644
index 00000000..0299c4e8
--- /dev/null
+++ b/e2e/batch_bench_test.go
@@ -0,0 +1,59 @@
+package e2e
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/internal/p2p"
+	"github.com/opensave/opensave/internal/p2p/syncengine"
+	"github.com/opensave/opensave/testutil"
+)
+
+// TestBench_ManySmallFiles times pulling a save of many small files between
+// two real daemons, batched and file by file. Opt-in, because it is a
+// measurement rather than a check: OPENSAVE_BENCH_FILES=5000 go test ./e2e -run TestBench -v
+func TestBench_ManySmallFiles(t *testing.T) {
+	n, _ := strconv.Atoi(os.Getenv("OPENSAVE_BENCH_FILES"))
+	if n <= 0 {
+		t.Skip("set OPENSAVE_BENCH_FILES to run")
+	}
+	for _, mode := range []struct {
+		name  string
+		proto int
+	}{{"batched", syncengine.ProtoBatchFiles}, {"file-by-file", syncengine.ProtoMultiRoot}} {
+		t.Run(mode.name, func(t *testing.T) {
+			restore := p2p.SetServedProto(mode.proto)
+			defer p2p.SetServedProto(restore)
+
+			a := testutil.NewTestDaemon(t, "Bench-A")
+			b := testutil.NewTestDaemon(t, "Bench-B")
+			a.PairWith(b)
+			chunk := strings.Repeat("x", 3500) // a Project Zomboid map chunk is ~3.5KB
+			for i := 0; i < n; i++ {
+				a.WriteSave(fmt.Sprintf("map/map_%d_%d.bin", i/100, i%100), chunk+strconv.Itoa(i))
+			}
+			restoreSyncOnTrack := suppressSyncOnTrack(a, b)
+			gameID := a.TrackGame("Bench")
+			b.API(http.MethodPost, "/api/games", map[string]string{"name": "Bench", "savePath": b.SaveDir}, nil)
+			restoreSyncOnTrack()
+
+			start := time.Now()
+			b.API(http.MethodPost, "/api/games/"+gameID+"/sync", nil, nil)
+			count := func() int {
+				m, _ := filepath.Glob(filepath.Join(b.SaveDir, "map", "*.bin"))
+				return len(m)
+			}
+			if !testutil.WaitFor(60*time.Minute, func() bool { return count() >= n }) {
+				t.Fatalf("only %d of %d files arrived", count(), n)
+			}
+			el := time.Since(start)
+			t.Logf("%s: %d files in %s = %.2f ms/file", mode.name, n, el.Round(time.Millisecond), float64(el.Microseconds())/1000/float64(n))
+		})
+	}
+}
diff --git a/e2e/cli_features_test.go b/e2e/cli_features_test.go
index 6c656fa1..15877e00 100644
--- a/e2e/cli_features_test.go
+++ b/e2e/cli_features_test.go
@@ -176,10 +176,10 @@ func TestCLI_SnapshotAndRollbackRestoresEverything(t *testing.T) {
 	c.mustRun("add", "History Game", dir)
 	c.mustRun("snapshot", "history-game", "-m", "known good")
 
-	// Two: the one tracking took automatically, and the one just asked for.
-	// The manual one is newest, and is the one to roll back to.
+	// One: the one asked for holds the same files as the one tracking took,
+	// and takes its place (snapshot/content.go). It is the one to roll back to.
 	ids := c.snapshotIDs("history-game")
-	if len(ids) != 2 {
+	if len(ids) != 1 {
 		t.Fatalf("expected the initial snapshot plus the manual one, got %v", ids)
 	}
 	snap := ids[0]
@@ -410,6 +410,9 @@ func TestCLI_SnapshotAll(t *testing.T) {
 	c.mustRun("add", "All One", one)
 	c.mustRun("add", "All Two", two)
 	before := len(c.snapshotIDs("all-one")) + len(c.snapshotIDs("all-two"))
+	// Played on: a snapshot of the same files would be the one already there.
+	c.saveDir("all-one", map[string]string{"a.sav": "1, played on"})
+	c.saveDir("all-two", map[string]string{"b.sav": "2, played on"})
 
 	out := c.mustRun("snapshot", "--all", "before", "the", "reinstall")
 	if !strings.Contains(out, "2 game") {
@@ -630,7 +633,9 @@ func TestCLI_SnapshotDeleteAndPrune(t *testing.T) {
 	dir := c.saveDir("pruning", map[string]string{"slot1.sav": "v1"})
 	c.mustRun("add", "Pruning", dir)
 
-	for _, v := range []string{"v1", "v2", "v3"} {
+	// Each a change: a snapshot of the same files as one already there would
+	// be that one (snapshot/content.go).
+	for _, v := range []string{"v2", "v3", "v4"} {
 		c.saveDir("pruning", map[string]string{"slot1.sav": v})
 		c.mustRun("snapshot", "pruning", "-m", v)
 	}
@@ -671,6 +676,7 @@ func TestCLI_UntrackAllRequiresConfirmation(t *testing.T) {
 
 	dir := c.saveDir("guard", map[string]string{"slot1.sav": "keep me"})
 	c.mustRun("add", "Guarded", dir)
+	c.saveDir("guard", map[string]string{"slot1.sav": "keep me, played on"})
 	c.mustRun("snapshot", "guarded")
 
 	c.mustFail("untrack-all")
@@ -694,7 +700,7 @@ func TestCLI_UntrackAllRequiresConfirmation(t *testing.T) {
 	if out := c.mustRun("status"); strings.Contains(out, "Guarded") {
 		t.Errorf("untrack-all --yes did not clear the library:\n%s", out)
 	}
-	if got := c.readSave(dir, "slot1.sav"); got != "keep me" {
+	if got := c.readSave(dir, "slot1.sav"); got != "keep me, played on" {
 		t.Errorf("untrack-all touched the save on disk: %q", got)
 	}
 }
diff --git a/e2e/cloud_and_peers_test.go b/e2e/cloud_and_peers_test.go
index eaf6ee1f..5fdb30a5 100644
--- a/e2e/cloud_and_peers_test.go
+++ b/e2e/cloud_and_peers_test.go
@@ -197,6 +197,9 @@ func TestCloud_SyncLocalRepairsATruncatedRemoteCopy(t *testing.T) {
 
 	a.WriteSave("slot1.sav", strings.Repeat("save data ", 500))
 	gameID := a.TrackGame("Repair Game")
+	// A change first: a snapshot of the same files as the one tracking took
+	// would be that one (snapshot/content.go).
+	a.WriteSave("slot1.sav", strings.Repeat("more save data ", 500))
 	a.API(http.MethodPost, "/api/games/"+gameID+"/snapshot", map[string]any{"comment": "full"}, nil)
 
 	name := waitForUpload(t, cloudDir)[0]
diff --git a/e2e/history_test.go b/e2e/history_test.go
index b2b7bcff..e5e98b70 100644
--- a/e2e/history_test.go
+++ b/e2e/history_test.go
@@ -1,11 +1,15 @@
 package e2e
 
 import (
+	"archive/zip"
 	"fmt"
+	"io"
 	"net/http"
 	"testing"
 	"time"
 
+	"github.com/opensave/opensave/internal/snapshot"
+
 	"github.com/opensave/opensave/testutil"
 )
 
@@ -14,6 +18,7 @@ type snapshotWire struct {
 	Comment      string `json:"comment"`
 	Branch       string `json:"branch"`
 	IsSystemAuto bool   `json:"isSystemAuto"`
+	ZipPath      string `json:"zipPath"`
 }
 
 type gameWire struct {
@@ -82,11 +87,49 @@ func TestHistory_SnapshotAndRollback(t *testing.T) {
 	}
 
 	// Rolling back is itself non-destructive: the pre-rollback state must
-	// still be reachable, or "undo" would be a one-way door.
-	snaps := snapshotsOn(a, gameID, "main")
-	if len(snaps) < 3 {
-		t.Errorf("expected a safety snapshot of the pre-rollback state, have %d snapshots", len(snaps))
+	// still be reachable, or "undo" would be a one-way door. Asked of the
+	// snapshots themselves, not counted: a state already snapshotted is not
+	// archived a second time.
+	if !snapshotHolds(t, a, gameID, "slot1.sav", "level-9") || !snapshotHolds(t, a, gameID, "inventory.dat", "cursed sword") {
+		t.Error("the pre-rollback state is in no snapshot")
+	}
+}
+
+// snapshotHolds reports whether any snapshot on a game's main branch holds the
+// archive entry name (a path in the save; ".opensave-locations/<name>/<path>"
+// in another location) with exactly content.
+func snapshotHolds(t *testing.T, td *testutil.TestDaemon, gameID, name, content string) bool {
+	t.Helper()
+	for _, s := range snapshotsOn(td, gameID, "main") {
+		path, done, err := snapshot.OpenArchive(s.ZipPath)
+		if err != nil {
+			continue
+		}
+		zr, err := zip.OpenReader(path)
+		if err != nil {
+			done()
+			continue
+		}
+		for _, f := range zr.File {
+			if f.Name != name {
+				continue
+			}
+			rc, err := f.Open()
+			if err != nil {
+				continue
+			}
+			b, _ := io.ReadAll(rc)
+			rc.Close()
+			if string(b) == content {
+				zr.Close()
+				done()
+				return true
+			}
+		}
+		zr.Close()
+		done()
 	}
+	return false
 }
 
 // Branches let two playthroughs coexist. Switching between them has to carry
diff --git a/e2e/keep_both_test.go b/e2e/keep_both_test.go
index 897bedd8..c3b5f35e 100644
--- a/e2e/keep_both_test.go
+++ b/e2e/keep_both_test.go
@@ -54,9 +54,12 @@ func branchHolds(t *testing.T, st branchState, branch, rel string) string {
 	if len(snaps) == 0 {
 		return ""
 	}
-	path, done, err := snapshot.OpenArchive(snaps[0].ZipPath)
+	// The API lists a branch's snapshots oldest first (gamePayload), so the
+	// newest — what a switch puts back — is the last.
+	newest := snaps[len(snaps)-1]
+	path, done, err := snapshot.OpenArchive(newest.ZipPath)
 	if err != nil {
-		t.Fatalf("open %s: %v", snaps[0].ZipPath, err)
+		t.Fatalf("open %s: %v", newest.ZipPath, err)
 	}
 	defer done()
 	zr, err := zip.OpenReader(path)
diff --git a/e2e/multiroot_scenarios_test.go b/e2e/multiroot_scenarios_test.go
index e6659650..3ab77668 100644
--- a/e2e/multiroot_scenarios_test.go
+++ b/e2e/multiroot_scenarios_test.go
@@ -10,6 +10,7 @@ import (
 
 	"github.com/opensave/opensave/internal/delta"
 	"github.com/opensave/opensave/internal/p2p/syncengine"
+	"github.com/opensave/opensave/internal/snapshot"
 	"github.com/opensave/opensave/testutil"
 )
 
@@ -426,9 +427,11 @@ func TestMultiRootScenario_ResolveALocationConflictKeepingTheirs(t *testing.T) {
 	if len(a.Daemon.P2P.Sync.ActiveRootConflicts()) != 0 {
 		t.Error("the conflict is still open after being resolved")
 	}
-	// Undoable: a snapshot was taken before anything was overwritten.
-	if after := len(snapshotsOn(a, gameID, "main")); after <= before {
-		t.Errorf("snapshots %d -> %d; no safety snapshot was taken before overwriting", before, after)
+	// Undoable: what was overwritten is in a snapshot — taken before the
+	// overwrite, or already there from when it was written.
+	_ = before
+	if !snapshotHolds(t, a, gameID, snapshot.RootPrefix+"config/settings.ini", "A-version") {
+		t.Error("the overwritten version is in no snapshot")
 	}
 	if got := a.ReadSave("save.sav"); got != "primary v1" {
 		t.Errorf("resolving a location changed the save folder: %q", got)
diff --git a/e2e/presence_timing_test.go b/e2e/presence_timing_test.go
new file mode 100644
index 00000000..4d9402d6
--- /dev/null
+++ b/e2e/presence_timing_test.go
@@ -0,0 +1,53 @@
+package e2e
+
+import (
+	"os"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/testutil"
+)
+
+// TestPresence_GoneDeviceShowsOffline measures how long a device that went
+// away keeps being shown online. Opt-in measurement:
+// OPENSAVE_PRESENCE_TIMING=1 go test ./e2e -run TestPresence_ -v
+func TestPresence_GoneDeviceShowsOffline(t *testing.T) {
+	if os.Getenv("OPENSAVE_PRESENCE_TIMING") == "" {
+		t.Skip("set OPENSAVE_PRESENCE_TIMING to run")
+	}
+	a := testutil.NewTestDaemon(t, "Presence-A")
+	b := testutil.NewTestDaemon(t, "Presence-B")
+	a.PairWith(b)
+	bID := b.NodeID()
+
+	status := func() string {
+		p, err := a.Daemon.Store.GetPeer(bID)
+		if err != nil {
+			return "?"
+		}
+		return p.Status
+	}
+	if !testutil.WaitFor(60*time.Second, func() bool { return status() == "online" }) {
+		t.Fatalf("B never showed online on A: %q", status())
+	}
+
+	gone := time.Now()
+	b.Server.Stop()
+	b.Daemon.Stop()
+
+	last := status()
+	for time.Since(gone) < 10*time.Minute {
+		if s := status(); s != last {
+			t.Logf("%6.1fs after B stopped: %s -> %s", time.Since(gone).Seconds(), last, s)
+			last = s
+		}
+		if last == "offline" {
+			break
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
+	t.Logf("B shown online for %.1fs after it stopped", time.Since(gone).Seconds())
+	if last != "offline" {
+		t.Fatalf("B still %q 10 minutes after it stopped", last)
+	}
+}
diff --git a/internal/api/routes.go b/internal/api/routes.go
index 65a28d0c..f7bd3c59 100644
--- a/internal/api/routes.go
+++ b/internal/api/routes.go
@@ -1,6 +1,7 @@
 package api
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -49,6 +50,7 @@ func (s *Server) routes(r chi.Router) {
 
 	r.Post("/api/games/{gameId}/snapshot", s.handleCreateSnapshot)
 	r.Post("/api/games/{gameId}/rollback", s.handleRollback)
+	r.Post("/api/games/{gameId}/use-everywhere", s.handleUseSaveEverywhere)
 	r.Get("/api/games/{gameId}/save-files", s.handleGameSaveFiles)
 	r.Get("/api/games/{gameId}/snapshot/{snapshotId}/files", s.handleSnapshotFiles)
 	r.Post("/api/games/{gameId}/snapshot/{snapshotId}/restore-file", s.handleRestoreFile)
@@ -56,6 +58,7 @@ func (s *Server) routes(r chi.Router) {
 	r.Patch("/api/games/{gameId}/snapshot/{snapshotId}", s.handleEditSnapshot)
 	r.Get("/api/games/{gameId}/sessions", s.handleGameSessions)
 	r.Get("/api/snapshots/check", s.handleSnapshotChecks)
+	r.Post("/api/snapshots/merge-duplicates", s.handleMergeDuplicates)
 	r.Get("/api/games/{gameId}/snapshot/{snapshotId}/compare/{otherId}", s.handleCompareSnapshots)
 	r.Post("/api/snapshots/check", s.handleVerifySnapshots)
 	r.Post("/api/games/{gameId}/session", s.handleMarkSession)
@@ -377,7 +380,7 @@ func (s *Server) handleUpdateGame(w http.ResponseWriter, r *http.Request) {
 
 	oldSavePath := game.SavePath
 	oldAutoSync := game.AutoSync
-	oldIgnore := game.SyncIgnore
+	oldIgnore := s.Daemon.IgnoreText(game)
 	oldAppID := game.AppID
 	if err := readJSON(r, &game); err != nil {
 		writeError(w, http.StatusBadRequest, err.Error())
@@ -418,7 +421,7 @@ func (s *Server) handleUpdateGame(w http.ResponseWriter, r *http.Request) {
 		forgetCoverMiss(game.AppID)
 	}
 
-	if game.SyncIgnore != oldIgnore {
+	if s.Daemon.IgnoreText(game) != oldIgnore {
 		// The merge bases were computed over a save that included files the
 		// new rules exclude, so neither device could ever match them again —
 		// which reads as permanent divergence and prompts a conflict on every
@@ -454,6 +457,9 @@ func (s *Server) handleUpdateGame(w http.ResponseWriter, r *http.Request) {
 		writeError(w, notFoundToStatus(err), err.Error())
 		return
 	}
+	if s.Daemon.IgnoreText(game) != oldIgnore {
+		s.Daemon.NoteExclusions(game)
+	}
 
 	// Re-watch if the save location or autoSync flag changed.
 	if game.SavePath != oldSavePath || game.AutoSync != oldAutoSync {
@@ -637,6 +643,50 @@ func (s *Server) handleRollback(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, snap)
 }
 
+// snapshotID is the request's snapshot id, followed when it is kept as an
+// alias of the snapshot holding the same files (snapshot/content.go). For
+// reading a snapshot only: deleting or editing takes the id as given, so an
+// old id can never reach the snapshot that took its place.
+func (s *Server) snapshotID(r *http.Request) string {
+	return s.resolveID(chi.URLParam(r, "snapshotId"))
+}
+
+func (s *Server) resolveID(id string) string {
+	if resolved, ok := s.Daemon.Store.ResolveSnapshotID(id); ok {
+		return resolved
+	}
+	return id
+}
+
+// handleMergeDuplicates keeps every game's identical snapshots once, now
+// rather than at the next start (daemon.MergeSnapshotDuplicates).
+func (s *Server) handleMergeDuplicates(w http.ResponseWriter, r *http.Request) {
+	merged, freed := s.Daemon.MergeSnapshotDuplicates(true)
+	s.BroadcastGamesUpdate()
+	writeJSON(w, http.StatusOK, map[string]any{"merged": merged, "freedBytes": freed})
+}
+
+// handleUseSaveEverywhere makes this device's save the one every other device
+// takes (syncengine.UseSaveEverywhere), and starts a sync to hand it out. The
+// sync runs in the background: a large save takes a while, and the answer the
+// person needs is that it was accepted. A paused device hands it out when it
+// resumes.
+func (s *Server) handleUseSaveEverywhere(w http.ResponseWriter, r *http.Request) {
+	gameID := chi.URLParam(r, "gameId")
+	vec, err := s.Daemon.P2P.Sync.UseSaveEverywhere(gameID)
+	if err != nil {
+		writeError(w, http.StatusConflict, err.Error())
+		return
+	}
+	s.Daemon.P2P.GoSync(func(ctx context.Context) {
+		ctx, cancel := context.WithTimeout(ctx, 30*time.Minute)
+		defer cancel()
+		_, _ = s.Daemon.P2P.SyncGame(ctx, gameID)
+	})
+	s.BroadcastGamesUpdate()
+	writeJSON(w, http.StatusOK, map[string]any{"version": vec.String()})
+}
+
 func (s *Server) handleCreateBranch(w http.ResponseWriter, r *http.Request) {
 	gameID := chi.URLParam(r, "gameId")
 	// copyCurrentSave defaults to true when the field is absent: a caller that
@@ -973,7 +1023,7 @@ func (s *Server) handleSyncResume(w http.ResponseWriter, r *http.Request) {
 // handlePreviewRestore says what restoring a snapshot would change, file by
 // file, without changing anything.
 func (s *Server) handlePreviewRestore(w http.ResponseWriter, r *http.Request) {
-	preview, err := s.Daemon.Snapshots.PreviewRestore(chi.URLParam(r, "gameId"), chi.URLParam(r, "snapshotId"))
+	preview, err := s.Daemon.Snapshots.PreviewRestore(chi.URLParam(r, "gameId"), s.snapshotID(r))
 	if err != nil {
 		writeError(w, notFoundToStatus(err), err.Error())
 		return
diff --git a/internal/api/routes_backup.go b/internal/api/routes_backup.go
index 2be3bcaa..0b7df6cd 100644
--- a/internal/api/routes_backup.go
+++ b/internal/api/routes_backup.go
@@ -25,7 +25,7 @@ import (
 // granular-restore browser in the UI).
 func (s *Server) handleSnapshotFiles(w http.ResponseWriter, r *http.Request) {
 	gameID := chi.URLParam(r, "gameId")
-	snapshotID := chi.URLParam(r, "snapshotId")
+	snapshotID := s.snapshotID(r)
 
 	snap, err := s.Daemon.Store.GetSnapshot(snapshotID)
 	if err != nil || snap.GameID != gameID {
@@ -59,7 +59,7 @@ func (s *Server) handleSnapshotFiles(w http.ResponseWriter, r *http.Request) {
 // save location, taking a safety snapshot first.
 func (s *Server) handleRestoreFile(w http.ResponseWriter, r *http.Request) {
 	gameID := chi.URLParam(r, "gameId")
-	snapshotID := chi.URLParam(r, "snapshotId")
+	snapshotID := s.snapshotID(r)
 
 	var body struct {
 		RelPath string `json:"relPath"`
@@ -723,7 +723,7 @@ func (s *Server) importBackupV2(zr *zip.Reader, manifest *backupManifest, mode s
 			if err == nil && mode == "overwrite" {
 				// A backup file may come from another device: this one's
 				// excluded files stay its own (Manager.RestoreKeeping).
-				_, err = s.Daemon.Snapshots.RestoreKeeping(gameID, snapID, ignore.Parse(local.SyncIgnore))
+				_, err = s.Daemon.Snapshots.RestoreKeeping(gameID, snapID, ignore.Parse(s.Daemon.IgnoreText(local)))
 				res.Path = local.SavePath
 				res.Action = "restored"
 			} else if err == nil {
diff --git a/internal/api/routes_cloud.go b/internal/api/routes_cloud.go
index d8253cee..05e868bd 100644
--- a/internal/api/routes_cloud.go
+++ b/internal/api/routes_cloud.go
@@ -346,7 +346,7 @@ func (s *Server) handleCloudRestore(w http.ResponseWriter, r *http.Request) {
 
 	// A cloud copy may be another device's: this device's excluded files
 	// stay its own (Manager.RestoreKeeping).
-	if _, err := s.Daemon.Snapshots.RestoreKeeping(gameID, snapID, ignore.Parse(game.SyncIgnore)); err != nil {
+	if _, err := s.Daemon.Snapshots.RestoreKeeping(gameID, snapID, ignore.Parse(s.Daemon.IgnoreText(game))); err != nil {
 		writeError(w, http.StatusInternalServerError, fmt.Sprintf("downloaded but restore failed: %v", err))
 		return
 	}
diff --git a/internal/api/routes_peers.go b/internal/api/routes_peers.go
index 4eb9b594..256cd05b 100644
--- a/internal/api/routes_peers.go
+++ b/internal/api/routes_peers.go
@@ -243,13 +243,15 @@ func (s *Server) handleProbePeer(w http.ResponseWriter, r *http.Request) {
 }
 
 // syncReason names why a sync did not happen, for a client to act on rather
-// than parse the message: "paused" (this device paused syncing), "held" (the
+// than parse the message: "paused" (this device paused syncing), "playing" (the game is being played here; it syncs when the session ends), "held" (the
 // game's save was emptied here and waits for an answer), "offline" (no other
 // device answered), or "error" for anything else.
 func syncReason(err error) string {
 	switch {
 	case errors.Is(err, syncengine.ErrPaused):
 		return "paused"
+	case errors.Is(err, syncengine.ErrPlayingHere):
+		return "playing"
 	case errors.Is(err, syncengine.ErrHeld):
 		return "held"
 	case errors.Is(err, p2p.ErrNoPeersOnline):
diff --git a/internal/api/routes_savefiles.go b/internal/api/routes_savefiles.go
index 663b63fc..7f1ae236 100644
--- a/internal/api/routes_savefiles.go
+++ b/internal/api/routes_savefiles.go
@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/opensave/opensave/internal/daemon"
 	"github.com/opensave/opensave/internal/ignore"
 )
 
@@ -37,6 +38,11 @@ type saveFile struct {
 	MtimeMs   int64  `json:"mtimeMs"`
 	// Excluded reports whether the current rules stop this file syncing.
 	Excluded bool `json:"excluded"`
+	// Why: "rule" (the game's own exclusion list), "database" (the game
+	// database names it as the game's settings) or "detected" (found to be
+	// its settings by what it holds). Empty when it syncs.
+	Source string `json:"source,omitempty"`
+	Reason string `json:"reason,omitempty"`
 }
 
 // saveFileListCap bounds the response. A save folder with more files than
@@ -60,11 +66,31 @@ func (s *Server) handleGameSaveFiles(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	text := game.SyncIgnore
 	if raw, ok := r.URL.Query()["rules"]; ok {
-		text = raw[0] // present-but-empty means "no rules", not "use the saved ones"
+		game.SyncIgnore = raw[0] // present-but-empty means "no rules", not "use the saved ones"
 	}
+	// With the game's device settings, which are left out the same way.
+	text := s.Daemon.IgnoreText(game)
 	rules := ignore.Parse(text)
+	ownRules := ignore.Parse(game.SyncIgnore)
+	var settings []daemon.DeviceSetting
+	if !game.SyncDeviceSettings {
+		settings = s.Daemon.DeviceSettings(game)
+	}
+	// which says what excludes a file the rules exclude: the game's own
+	// rules first, as they apply last; then the last device-settings entry
+	// that names it.
+	which := func(path string) (string, string) {
+		if !ownRules.Empty() && ownRules.Match(path) {
+			return "rule", ""
+		}
+		for i := len(settings) - 1; i >= 0; i-- {
+			if settings[i].Source != "save" && ignore.Parse(settings[i].Pattern).Match(path) {
+				return settings[i].Source, settings[i].Reason
+			}
+		}
+		return "rule", ""
+	}
 
 	roots := []struct{ name, path string }{{"", game.SavePath}}
 	if extra, rootsErr := s.Daemon.Store.ListGameRoots(gameID); rootsErr == nil {
@@ -94,6 +120,11 @@ func (s *Server) handleGameSaveFiles(w http.ResponseWriter, r *http.Request) {
 		for _, f := range files {
 			f.Location = root.name
 			f.Excluded = rules.Match(f.Path)
+			if f.Excluded && root.name == "" {
+				f.Source, f.Reason = which(f.Path)
+			} else if f.Excluded {
+				f.Source = "rule"
+			}
 			out = append(out, f)
 		}
 	}
diff --git a/internal/api/routes_verify.go b/internal/api/routes_verify.go
index af9641ea..e3732368 100644
--- a/internal/api/routes_verify.go
+++ b/internal/api/routes_verify.go
@@ -31,7 +31,7 @@ func (s *Server) handleVerifySnapshots(w http.ResponseWriter, r *http.Request) {
 // handleCompareSnapshots says what changed from one of a game's snapshots to
 // another (snapshot.Manager.Compare).
 func (s *Server) handleCompareSnapshots(w http.ResponseWriter, r *http.Request) {
-	c, err := s.Daemon.Snapshots.Compare(chi.URLParam(r, "gameId"), chi.URLParam(r, "snapshotId"), chi.URLParam(r, "otherId"))
+	c, err := s.Daemon.Snapshots.Compare(chi.URLParam(r, "gameId"), s.snapshotID(r), s.resolveID(chi.URLParam(r, "otherId")))
 	if err != nil {
 		writeError(w, http.StatusBadRequest, err.Error())
 		return
diff --git a/internal/api/server.go b/internal/api/server.go
index 66a32562..dc7ad5e6 100644
--- a/internal/api/server.go
+++ b/internal/api/server.go
@@ -98,12 +98,20 @@ func (s *Server) peersPayload() map[string]any {
 			AppVersion    string `json:"appVersion,omitempty"`
 			BuildTimeMs   int64  `json:"buildTimeMs,omitempty"`
 			HasNewerBuild bool   `json:"hasNewerBuild,omitempty"`
+			// NeedsUpdate: its OpenSave keeps no save versions, so this
+			// device takes nothing from it until it is updated.
+			NeedsUpdate bool `json:"needsUpdate,omitempty"`
+			// Link is how fast this device has found its connection to it,
+			// and what kind of address it is reached on: what decides the
+			// order devices are synced in.
+			Link syncengine.LinkStat `json:"link"`
 			// What actually protects traffic with this device. Sent per peer
 			// rather than described once in the interface, because the answer
 			// differs per pairing and the reader cannot work out which case
 			// they are in from a general statement.
 			p2p.PeerProtection
-		}{Peer: p, PeerProtection: s.Daemon.P2P.PeerProtection(p)}
+		}{Peer: p, PeerProtection: s.Daemon.P2P.PeerProtection(p), NeedsUpdate: s.Daemon.P2P.Sync.PeerNeedsUpdate(p.ID),
+			Link: s.Daemon.P2P.Sync.Link(syncengine.Peer{ID: p.ID, Name: p.Name, Address: p.Address, Port: p.Port, IsWan: p.Address == "relay"})}
 		if b, ok := builds[p.ID]; ok {
 			entry.AppVersion = b.AppVersion
 			entry.BuildTimeMs = b.BuildTimeMs
@@ -559,18 +567,26 @@ func (s *Server) gamePayload(g store.Game) map[string]any {
 		// Whether the art cached for this game is explicit, so the client can
 		// blur it until someone asks to see it. An <img src> cannot read a
 		// response header, so it travels with the game rather than the image.
-		"coverExplicit":  s.CoverIsExplicit(coverKeyFor(g)),
-		"syncIgnore":     g.SyncIgnore,
-		"branches":       branches,
-		"createdAt":      g.CreatedAt,
-		"lastSyncedWith": lastSyncedWith,
+		"coverExplicit": s.CoverIsExplicit(coverKeyFor(g)),
+		"syncIgnore":    g.SyncIgnore,
+		// The files the game database names as this game's settings, left
+		// out of syncing unless syncDeviceSettings (daemon/devicesettings.go).
+		"deviceSettings":     s.Daemon.DeviceSettings(g),
+		"syncDeviceSettings": g.SyncDeviceSettings,
+		"branches":           branches,
+		"createdAt":          g.CreatedAt,
+		"lastSyncedWith":     lastSyncedWith,
 		// The save folder is not there — gone, moved, or on a drive not
 		// plugged in. Nothing is watched or synced for it until it is back.
 		"savePathMissing": daemon.SaveFolderMissing(g.SavePath),
-		"lastPlayedAt":    lastPlayed,
-		"playingSince":    playingSince,
-		"playtimeMs":      play.PlaytimeMs,
-		"playSessions":    play.Sessions,
+		// …and of those, the ones whose whole drive is absent here: the game
+		// lives on another device's D:\ or E:\, which is not a problem to
+		// warn about (daemon.SaveDriveMissing).
+		"saveDriveMissing": daemon.SaveFolderMissing(g.SavePath) && daemon.SaveDriveMissing(g.SavePath),
+		"lastPlayedAt":     lastPlayed,
+		"playingSince":     playingSince,
+		"playtimeMs":       play.PlaytimeMs,
+		"playSessions":     play.Sessions,
 		// Whether the game is installed on this device: "found", "not-found",
 		// or "" when there is no telling (daemon.InstallState). A game that is
 		// not here is one nobody plays here, which is why it shows no play.
diff --git a/internal/api/server_test.go b/internal/api/server_test.go
index f7a9ebd5..dfefa6a0 100644
--- a/internal/api/server_test.go
+++ b/internal/api/server_test.go
@@ -578,7 +578,11 @@ func TestDeleteSnapshotAndBranch(t *testing.T) {
 	ts.do(t, http.MethodPost, "/api/games", map[string]string{"name": "Del Game", "savePath": ts.saveDir})
 	waitInitialSnapshot(t, ts, "del-game")
 
-	// A manual second snapshot so we have one to delete.
+	// A manual second snapshot so we have one to delete — of a changed save:
+	// one of the same files would be the first snapshot, kept once.
+	if err := os.WriteFile(filepath.Join(ts.saveDir, "s.sav"), []byte("y"), 0o666); err != nil {
+		t.Fatal(err)
+	}
 	ts.do(t, http.MethodPost, "/api/games/del-game/snapshot", map[string]string{"comment": "manual"})
 	snaps, _ := ts.daemon.Store.ListSnapshots("del-game", "main")
 	if len(snaps) < 2 {
diff --git a/internal/cliapp/syncsummary.go b/internal/cliapp/syncsummary.go
index b5518341..ff13d852 100644
--- a/internal/cliapp/syncsummary.go
+++ b/internal/cliapp/syncsummary.go
@@ -101,6 +101,12 @@ func outcomeFromPeers(gameID string, peers map[string]peerSyncResult) syncOutcom
 			waiting = who + " is waiting for a folder to be chosen for it"
 		case "peer_holding":
 			waiting = who + " is holding it back: its save was emptied there"
+		case "peer_outdated_app":
+			waiting = who + " runs an older OpenSave without save versions — update it there"
+		case "waiting":
+			if waiting == "" {
+				waiting = "a newer version is on a device that is not online"
+			}
 		case "peer_missing":
 			if waiting == "" {
 				waiting = who + " does not track it"
diff --git a/internal/daemon/cloudsync.go b/internal/daemon/cloudsync.go
index 6b4dbbf5..70a55b01 100644
--- a/internal/daemon/cloudsync.go
+++ b/internal/daemon/cloudsync.go
@@ -401,7 +401,7 @@ func (d *Daemon) currentContentHash(game store.Game) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return watcher.ContentHash(m, game.SyncIgnore), nil
+	return watcher.ContentHash(m, d.IgnoreText(game)), nil
 }
 
 // CheckCloud reads the other devices' heads and acts on the ones ahead of
@@ -633,7 +633,7 @@ func (d *Daemon) pullFromCloud(game store.Game, fileName, deviceName string) err
 	}
 	// Another device's save: the files this device keeps for itself stay
 	// its own. See Manager.RestoreKeeping.
-	if _, err := d.Snapshots.RestoreKeeping(game.ID, snapID, ignore.Parse(game.SyncIgnore)); err != nil {
+	if _, err := d.Snapshots.RestoreKeeping(game.ID, snapID, ignore.Parse(d.IgnoreText(game))); err != nil {
 		return err
 	}
 	// What is on disk now is that snapshot, so record it as the baseline.
diff --git a/internal/daemon/daemon.go b/internal/daemon/daemon.go
index 306ce6ff..5324b5f2 100644
--- a/internal/daemon/daemon.go
+++ b/internal/daemon/daemon.go
@@ -38,6 +38,11 @@ type Options struct {
 	// DisableDiscovery skips UDP LAN discovery (tests pair peers manually
 	// and don't want broadcast traffic).
 	DisableDiscovery bool
+	// SyncEveryChange keeps and syncs every change made here as it comes,
+	// never holding a stretch of changes back as play when no session is seen
+	// (sessions.go, noteActivity). For tests of syncing itself, which change a
+	// save several times a minute and expect each change to travel.
+	SyncEveryChange bool
 }
 
 // Daemon is the assembled core. Access subsystems directly for operations
@@ -62,10 +67,17 @@ type Daemon struct {
 	// Games whose save folder is not there; see missing.go.
 	missingMu sync.Mutex
 	missing   map[string]bool
+	// relocateChecked is when each missing save folder was last looked for on
+	// another drive (missing.go).
+	relocateChecked map[string]time.Time
 
 	// Play sessions; see sessions.go.
 	sessions sessionState
 
+	// Device settings; see devicesettings.go.
+	devMu sync.Mutex
+	dev   devSettingsCache
+
 	// OnCloudOffers receives the saves from other devices' cloud backups
 	// that are waiting for an answer, whenever that list changes, and
 	// OnCloudPulled each one put in place without asking. See cloudsync.go.
@@ -80,6 +92,9 @@ type Daemon struct {
 	newGames   newGameState
 	// scanMu runs one save scan at a time. See ScanForSaves.
 	scanMu sync.Mutex
+	// mergeMu runs one merge of identical snapshots at a time
+	// (MergeSnapshotDuplicates).
+	mergeMu sync.Mutex
 
 	// uploads counts cloud mirrors still running, so Stop can wait for them
 	// rather than letting process exit truncate one.
@@ -145,6 +160,9 @@ func New(opts Options) (*Daemon, error) {
 
 	snaps := snapshot.New(s)
 	snaps.Log = log.Log
+	// Each new snapshot takes the place of older automatic ones it holds
+	// whole (snapshot/content.go).
+	snaps.PruneOnCreate = true
 
 	d := &Daemon{
 		Paths:     paths,
@@ -159,6 +177,13 @@ func New(opts Options) (*Daemon, error) {
 
 	d.initSessions()
 
+	// A game's device settings are left out of syncing (devicesettings.go).
+	d.P2P.Sync.DeviceSettingsFor = d.deviceSettingsFor
+	d.P2P.Sync.ObserveSettings = d.observeSettings
+	// A game being played here is synced when its session ends
+	// (syncengine/playing.go).
+	d.P2P.Sync.Playing = d.playingHere
+
 	// A restore or a branch switch rewrites a save folder; nothing syncing it
 	// may read it half-way (syncengine/settle.go).
 	snaps.WriteGate = d.P2P.Sync.Writing
@@ -215,7 +240,7 @@ func New(opts Options) (*Daemon, error) {
 			if err != nil {
 				return ""
 			}
-			return game.SyncIgnore
+			return d.IgnoreText(game)
 		},
 		GetLastManifestHash: func(gameID string) (string, error) {
 			game, err := s.GetGame(gameID)
@@ -226,10 +251,30 @@ func New(opts Options) (*Daemon, error) {
 		},
 		SetLastManifestHash: s.SetLastManifestHash,
 		CreateSnapshot: func(gameID string) error {
-			_, err := snaps.Create(gameID, "", true)
+			// While a game is played, the watcher only snapshots at a
+			// checkpoint (sessions.go), named so, and gone once the
+			// session's end is kept.
+			comment := ""
+			if d.playingHere(gameID) {
+				comment = checkpointComment
+			}
+			_, err := snaps.Create(gameID, comment, true)
 			return err
 		},
+		SyncWriting: func(gameID string) bool {
+			return d.P2P != nil && d.P2P.Sync != nil && d.P2P.Sync.BeingWritten(gameID)
+		},
+		SyncWritingNow: func(gameID string) bool {
+			return d.P2P != nil && d.P2P.Sync != nil && d.P2P.Sync.WritingNow(gameID)
+		},
+		HoldSnapshot: d.holdSnapshotWhilePlaying,
 		OnChanged: func(gameID string) {
+			// A change made here, by the game or the user: a new version of
+			// the save, recorded before anyone is offered it.
+			d.P2P.Sync.NoteLocalChange(gameID)
+			// How the files left out as its settings changed with it
+			// (detectsettings.go).
+			d.observeLocalSettings(gameID)
 			// An emptied save is noticed as it happens, and said on screen,
 			// even with no other device online to hold it back from.
 			_, _ = d.P2P.Sync.CheckHold(gameID, false)
@@ -266,6 +311,10 @@ func (d *Daemon) Start() error {
 	if err != nil {
 		return err
 	}
+	// Snapshots that hold the same files are kept once (snapshot/content.go).
+	// In the background, once things have settled: naming a snapshot taken
+	// before contents were recorded reads its whole archive.
+	time.AfterFunc(2*time.Minute, func() { d.MergeSnapshotDuplicates(false) })
 	// Switch games tracked under a made-up name get their real one, when an
 	// emulator here knows it by now.
 	d.nameSwitchGames()
@@ -276,6 +325,9 @@ func (d *Daemon) Start() error {
 	// then evicts entries it is about to want and starts re-reading saves.
 	delta.SetHashCacheBudgetForGames(len(games))
 
+	// Before anything is watched or compared.
+	d.adoptExclusionView(games)
+
 	for _, game := range games {
 		// Backfill cover art for games tracked before covers existed (or
 		// migrated from the JS app without one).
@@ -366,6 +418,29 @@ func (d *Daemon) Start() error {
 		}
 	})
 
+	// Find each game's settings by what they hold (detectsettings.go): a few
+	// minutes after start, then hourly for whatever is due.
+	d.P2P.GoSync(func(ctx context.Context) {
+		first := time.NewTimer(newGameFirstScan)
+		defer first.Stop()
+		select {
+		case <-ctx.Done():
+			return
+		case <-first.C:
+			d.detectSettingsDue(ctx)
+		}
+		ticker := time.NewTicker(time.Hour)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				d.detectSettingsDue(ctx)
+			}
+		}
+	})
+
 	// Look for newly installed games: a few minutes after start, then every
 	// hour. See newgames.go.
 	d.P2P.GoSync(func(ctx context.Context) {
@@ -985,6 +1060,40 @@ func (d *Daemon) checkSavePathShape(abs string) error {
 	return nil
 }
 
+// MergeSnapshotDuplicates keeps every game's identical snapshots once
+// (snapshot.Manager.MergeDuplicates), one game at a time. Returns how many
+// were merged and the bytes freed. all looks at every game; otherwise only at
+// games with snapshots whose content is not named yet.
+func (d *Daemon) MergeSnapshotDuplicates(all bool) (merged int, freed int64) {
+	d.mergeMu.Lock()
+	defer d.mergeMu.Unlock()
+	games, err := d.Store.ListGames()
+	if err != nil {
+		return 0, 0
+	}
+	for _, g := range games {
+		// Once per game: snapshots taken since are named as they are taken,
+		// and checked against the older ones then (PruneOnCreate). A game
+		// whose snapshots all have their content named has had its pass.
+		if unnamed, err := d.Store.SnapshotsWithoutContentHash(g.ID); !all && (err != nil || len(unnamed) == 0) {
+			continue
+		}
+		if n, f, err := d.Snapshots.MergeDuplicates(g.ID); err == nil {
+			merged += n
+			freed += f
+		}
+		// And steps on the way to a newer snapshot that holds them whole.
+		if n, f, err := d.Snapshots.PruneContained(g.ID); err == nil {
+			merged += n
+			freed += f
+		}
+	}
+	if merged > 0 {
+		d.Log.Log("info", fmt.Sprintf("snapshots cleaned up: %d identical or held whole by a newer one, %d MB freed", merged, freed>>20))
+	}
+	return merged, freed
+}
+
 // EnsureImportedSnapshot registers a snapshot restored from an .sscb
 // backup: creates the branch if missing and inserts the metadata row
 // unless that snapshot id is already known (idempotent re-imports).
diff --git a/internal/daemon/detectsettings.go b/internal/daemon/detectsettings.go
new file mode 100644
index 00000000..562206c4
--- /dev/null
+++ b/internal/daemon/detectsettings.go
@@ -0,0 +1,256 @@
+package daemon
+
+import (
+	"context"
+	"fmt"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/ignore"
+	"github.com/opensave/opensave/internal/store"
+)
+
+// Finding a game's settings by what they hold, and giving back what turns out
+// to be a save.
+//
+// Detection (presets/detectsettings.go) takes a file for settings by its
+// contents. Before anything is left out, and again at every later run, the
+// game's snapshots are asked how the file behaved (snapshot/history.go). And
+// as long as a file is left out, how it behaves is watched: in this device's
+// automatic snapshots, and in what every other device sends (observeSettings).
+// A settings file is changed now and then; a save changes with the sessions
+// played. One that changes along with the rest of the save in most of them is
+// a save after all: it syncs again, for good, whatever named it — this
+// detection or the game database — and the log says which and why.
+//
+// Errors cost little either way. A settings file taken for a save syncs, as
+// every file did before. A save taken for settings is still in every
+// snapshot, and comes back to syncing at the latest when it has been seen to
+// change with the save a few times.
+
+// detectorVersion moves when detection changes, so every game is judged again.
+const detectorVersion = "1"
+
+const (
+	observeMinMoved = 3 // changes along with the save before a file is given back
+	detectEvery     = 24 * time.Hour
+)
+
+func settingsDetectMark(gameID string) string { return "settings_detect:" + gameID }
+
+// detectSettingsDue runs detection for every game not judged under this
+// detector and folder yet, or not for a day. In the background, from Start.
+func (d *Daemon) detectSettingsDue(ctx context.Context) {
+	games, err := d.Store.ListGames()
+	if err != nil {
+		return
+	}
+	for _, game := range games {
+		if ctx.Err() != nil {
+			return
+		}
+		mark := d.Store.Mark(settingsDetectMark(game.ID))
+		want := detectorVersion + "|" + game.SavePath
+		if prev, at, ok := strings.Cut(mark, "@"); ok && prev == want {
+			if t, err := time.Parse(time.RFC3339, at); err == nil && time.Since(t) < detectEvery {
+				continue
+			}
+		}
+		d.DetectSettings(game)
+		_ = d.Store.SetMark(settingsDetectMark(game.ID), want+"@"+time.Now().UTC().Format(time.RFC3339))
+	}
+}
+
+// DetectSettings judges a game's files, records what is new, and re-asks the
+// snapshots about everything taken for settings so far.
+func (d *Daemon) DetectSettings(game store.Game) {
+	if d.Scanner == nil || game.SavePath == "" || SaveFolderMissing(game.SavePath) {
+		return
+	}
+	d.devMu.Lock()
+	db := d.devDBLocked()
+	d.devMu.Unlock()
+
+	known := map[string]store.SettingsFile{}
+	if verdicts, err := d.Store.SettingsFiles(game.ID); err == nil {
+		for _, v := range verdicts {
+			known[v.Path] = v
+		}
+	}
+	found := db.DetectSettings(game.Name, game.AppID, game.SavePath)
+	var ask []string
+	for _, f := range found {
+		if _, ok := known[f.Path]; !ok {
+			ask = append(ask, f.Path)
+		}
+	}
+	for p, v := range known {
+		if v.Verdict == store.VerdictSettings {
+			ask = append(ask, p)
+		}
+	}
+	saves := d.Snapshots.ChangesLikeASave(game.ID, ask)
+
+	changed := false
+	var taken, given []string
+	for _, f := range found {
+		if _, ok := known[f.Path]; ok {
+			continue
+		}
+		v := store.SettingsFile{GameID: game.ID, Path: f.Path, Verdict: store.VerdictSettings, Reason: f.Reason}
+		if why, isSave := saves[f.Path]; isSave {
+			v.Verdict, v.Reason = store.VerdictSave, why
+		} else {
+			taken = append(taken, f.Path)
+		}
+		if d.Store.SetSettingsFile(v) == nil {
+			changed = true
+		}
+	}
+	for p, v := range known {
+		if why, isSave := saves[p]; isSave && v.Verdict == store.VerdictSettings {
+			v.Verdict, v.Reason, v.AtMs = store.VerdictSave, why, 0
+			if d.Store.SetSettingsFile(v) == nil {
+				changed = true
+				given = append(given, p+" ("+why+")")
+			}
+		}
+	}
+	if !changed {
+		return
+	}
+	d.exclusionsChanged(game)
+	if len(taken) > 0 && !game.SyncDeviceSettings {
+		d.Log.Log("info", fmt.Sprintf("%q: %s %s this device's settings, not the save — each device keeps its own. "+
+			"Untick %s under the game's \"Files that shouldn't sync\" to sync %s after all.",
+			game.Name, strings.Join(taken, ", "), isAre(len(taken)), itThem(len(taken)), itThem(len(taken))))
+	}
+	if len(given) > 0 {
+		d.Log.Log("warn", fmt.Sprintf("%q: %s changed along with the save, so %s part of the save after all — syncing again.",
+			game.Name, strings.Join(given, ", "), itThey(len(given))))
+	}
+}
+
+// exclusionsChanged carries a game's recorded hashes over to its exclusion
+// list as it is now (adoptExclusionView), after detection changed it.
+func (d *Daemon) exclusionsChanged(game store.Game) {
+	d.forgetVerdicts(game.ID)
+	if fresh, err := d.Store.GetGame(game.ID); err == nil {
+		game = fresh
+	}
+	d.adoptExclusionView([]store.Game{game})
+}
+
+// observeSettings is told what a device holds of a game — another device's
+// manifest as it arrives, or this device's own at an automatic snapshot — and
+// keeps count, for every file left out as this game's settings, of how often
+// it changed when the rest of the save did. peerID is "" for this device.
+func (d *Daemon) observeSettings(game store.Game, peerID string, m delta.Manifest) {
+	if game.SyncDeviceSettings {
+		return
+	}
+	settings := d.deviceSettingsFor(game)
+	if len(settings) == 0 {
+		return
+	}
+	asSettings := ignore.Parse(strings.Join(settings, "\n"))
+	var watched []string
+	rest := delta.Manifest{Files: map[string]delta.FileEntry{}}
+	for p, f := range m.Files {
+		if asSettings.Match(p) {
+			watched = append(watched, p)
+		} else {
+			rest.Files[p] = f
+		}
+	}
+	if len(watched) == 0 {
+		return
+	}
+	user := ignore.Parse(game.SyncIgnore)
+	for p := range rest.Files {
+		if delta.NeverSynced(p) || (!user.Empty() && user.Match(p)) {
+			delete(rest.Files, p)
+		}
+	}
+	restHash := rest.ManifestHash()
+
+	// The rest of the save, as last seen from this device.
+	r := d.Store.GetSettingsObservation(game.ID, peerID, "")
+	saveMoved := r.LastHash != "" && r.LastHash != restHash
+	first := r.LastHash == ""
+	r.LastHash = restHash
+	_ = d.Store.SaveSettingsObservation(r)
+
+	var given []string
+	for _, p := range watched {
+		o := d.Store.GetSettingsObservation(game.ID, peerID, p)
+		h := m.Files[p].Hash
+		if saveMoved && o.LastHash != "" {
+			if o.LastHash != h {
+				o.Moved++
+			} else {
+				o.Still++
+			}
+		}
+		o.LastHash = h
+		_ = d.Store.SaveSettingsObservation(o)
+		if first || !saveMoved {
+			continue
+		}
+		moved, still := 0, 0
+		if all, err := d.Store.SettingsObservations(game.ID, p); err == nil {
+			for _, x := range all {
+				moved += x.Moved
+				still += x.Still
+			}
+		}
+		if moved >= observeMinMoved && moved > still {
+			why := fmt.Sprintf("changed in %d of the %d times the save changed", moved, moved+still)
+			if d.Store.SetSettingsFile(store.SettingsFile{GameID: game.ID, Path: p, Verdict: store.VerdictSave, Reason: why}) == nil {
+				given = append(given, p+" ("+why+")")
+			}
+		}
+	}
+	if len(given) > 0 {
+		d.exclusionsChanged(game)
+		d.Log.Log("warn", fmt.Sprintf("%q: %s changed along with the save, so %s part of the save after all — syncing again.",
+			game.Name, strings.Join(given, ", "), itThey(len(given))))
+	}
+}
+
+// observeLocalSettings is observeSettings for this device's own save, at an
+// automatic snapshot.
+func (d *Daemon) observeLocalSettings(gameID string) {
+	game, err := d.Store.GetGame(gameID)
+	if err != nil {
+		return
+	}
+	m, err := delta.BuildManifest(filepath.Clean(game.SavePath))
+	if err != nil {
+		return
+	}
+	d.observeSettings(game, "", m)
+}
+
+func isAre(n int) string {
+	if n == 1 {
+		return "is"
+	}
+	return "are"
+}
+
+func itThey(n int) string {
+	if n == 1 {
+		return "it is"
+	}
+	return "they are"
+}
+
+func itThem(n int) string {
+	if n == 1 {
+		return "it"
+	}
+	return "them"
+}
diff --git a/internal/daemon/devicesettings.go b/internal/daemon/devicesettings.go
new file mode 100644
index 00000000..6ef76c1c
--- /dev/null
+++ b/internal/daemon/devicesettings.go
@@ -0,0 +1,189 @@
+package daemon
+
+import (
+	"strings"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/presets"
+	"github.com/opensave/opensave/internal/store"
+	"github.com/opensave/opensave/internal/watcher"
+)
+
+// Device settings: the files the game database names as a game's settings
+// rather than its save are left out of syncing (presets/devicesettings.go),
+// through the game's exclusion list (syncengine.IgnoreText).
+//
+// Worked out once per run and kept: the database is read when first needed,
+// and a game's answer only changes when its name, App ID or folder does —
+// which makes it a different key here. A database update takes effect at the
+// next start, where adoptExclusionView carries every recorded hash over.
+
+type devSettingsCache struct {
+	loaded bool
+	db     *presets.DeviceSettings
+	// patterns: the game database's, by game name, App ID and folder.
+	patterns map[string][]string
+	// verdicts: what was found here (detectsettings.go), by game ID.
+	verdicts map[string][]store.SettingsFile
+}
+
+// devDB returns the game database for device settings, read when first
+// needed. Called with devMu held.
+func (d *Daemon) devDBLocked() *presets.DeviceSettings {
+	if !d.dev.loaded {
+		d.dev.db = d.Scanner.DeviceSettings()
+		d.dev.loaded = true
+	}
+	if d.dev.patterns == nil {
+		d.dev.patterns = map[string][]string{}
+	}
+	if d.dev.verdicts == nil {
+		d.dev.verdicts = map[string][]store.SettingsFile{}
+	}
+	return d.dev.db
+}
+
+// DeviceSetting is one entry of a game's device settings, for the window.
+type DeviceSetting struct {
+	Pattern string `json:"pattern"`
+	// Source: "database" (the game database names it), "detected" (found by
+	// what it holds), or "save" (shown to change with the save, so it syncs
+	// whatever named it).
+	Source string `json:"source"`
+	Reason string `json:"reason,omitempty"`
+}
+
+// deviceSettingsList is every entry, in the order they apply: the game
+// database's, what was detected here, then what was shown to be a save —
+// as "!" lines, which bring a file back whatever named it before.
+func (d *Daemon) deviceSettingsList(game store.Game) []DeviceSetting {
+	if d.Scanner == nil {
+		return nil
+	}
+	key := game.Name + "\x00" + game.AppID + "\x00" + game.SavePath
+	d.devMu.Lock()
+	defer d.devMu.Unlock()
+	db := d.devDBLocked()
+	pats, ok := d.dev.patterns[key]
+	if !ok {
+		pats = db.Patterns(game.Name, game.AppID, []string{game.SavePath})
+		d.dev.patterns[key] = pats
+	}
+	verdicts, ok := d.dev.verdicts[game.ID]
+	if !ok {
+		verdicts, _ = d.Store.SettingsFiles(game.ID)
+		d.dev.verdicts[game.ID] = verdicts
+	}
+	var out []DeviceSetting
+	for _, p := range pats {
+		out = append(out, DeviceSetting{Pattern: p, Source: "database"})
+	}
+	for _, v := range verdicts {
+		if v.Verdict == store.VerdictSettings {
+			out = append(out, DeviceSetting{Pattern: "/" + v.Path, Source: "detected", Reason: v.Reason})
+		}
+	}
+	for _, v := range verdicts {
+		if v.Verdict == store.VerdictSave {
+			out = append(out, DeviceSetting{Pattern: "!/" + v.Path, Source: "save", Reason: v.Reason})
+		}
+	}
+	return out
+}
+
+func (d *Daemon) deviceSettingsFor(game store.Game) []string {
+	list := d.deviceSettingsList(game)
+	out := make([]string, 0, len(list))
+	for _, s := range list {
+		out = append(out, s.Pattern)
+	}
+	return out
+}
+
+// forgetVerdicts drops a game's cached verdicts after they changed.
+func (d *Daemon) forgetVerdicts(gameID string) {
+	d.devMu.Lock()
+	delete(d.dev.verdicts, gameID)
+	d.devMu.Unlock()
+}
+
+// IgnoreText is the exclusion list a game is synced under (see
+// syncengine.Engine.IgnoreText).
+func (d *Daemon) IgnoreText(game store.Game) string {
+	if d.P2P == nil || d.P2P.Sync == nil {
+		return game.SyncIgnore
+	}
+	return d.P2P.Sync.IgnoreText(game)
+}
+
+// DeviceSettings returns a game's device settings, whether or not it is set
+// to sync them — what the window lists.
+func (d *Daemon) DeviceSettings(game store.Game) []DeviceSetting {
+	return d.deviceSettingsList(game)
+}
+
+// exclusionsMark records, per game, the exclusion list its recorded hashes
+// were last taken under, and the never-synced list of the build that took
+// them.
+func exclusionsMark(gameID string) string { return "exclusions:" + gameID }
+
+func exclusionsValue(text string) string {
+	return delta.NeverSyncedList + "\n--\n" + text
+}
+
+// NoteExclusions records that a game's hashes are now taken under its
+// current exclusion list — after the rules were changed and the hashes
+// rebased by whoever changed them.
+func (d *Daemon) NoteExclusions(game store.Game) {
+	_ = d.Store.SetMark(exclusionsMark(game.ID), exclusionsValue(d.IgnoreText(game)))
+}
+
+// adoptExclusionView carries every game's recorded hashes over to what the
+// game is excluded by now, when that changed without anyone writing a rule:
+// a build with a new delta.NeverSyncedList, or the game database naming
+// settings files. Each is re-taken only when the save is the one it was
+// recorded for, so a real change still counts.
+//
+// Without it, a build that starts excluding a file sees every save holding
+// one as changed: an auto-snapshot of a game nobody played, and a new version
+// of its own on every device at once, which the devices then disagree over.
+func (d *Daemon) adoptExclusionView(games []store.Game) {
+	neverSyncedDone := d.Store.Mark("never_synced") == delta.NeverSyncedList
+	for _, game := range games {
+		now := d.IgnoreText(game)
+		oldText, beforeNeverSynced := game.SyncIgnore, !neverSyncedDone
+		if prev := d.Store.Mark(exclusionsMark(game.ID)); prev != "" {
+			list, text, _ := strings.Cut(prev, "\n--\n")
+			if list == delta.NeverSyncedList && text == now {
+				continue
+			}
+			oldText, beforeNeverSynced = text, list != delta.NeverSyncedList
+		} else if !beforeNeverSynced && oldText == now {
+			_ = d.Store.SetMark(exclusionsMark(game.ID), exclusionsValue(now))
+			continue
+		}
+
+		extra, err := d.Store.GameRootPaths(game.ID)
+		if err != nil {
+			extra = nil
+		}
+		m, failures, err := delta.BuildMultiManifest(game.SavePath, extra)
+		if err != nil || len(failures) > 0 {
+			continue // unreadable now: tried again at the next start
+		}
+		was := watcher.ContentHash(m, oldText)
+		if beforeNeverSynced {
+			was = watcher.ContentHashBeforeNeverSynced(m, oldText)
+		}
+		if game.LastManifestHash != "" && game.LastManifestHash == was {
+			_ = d.Store.SetLastManifestHash(game.ID, watcher.ContentHash(m, now))
+		}
+		if d.P2P != nil && d.P2P.Sync != nil {
+			primary := m
+			primary.Extra = nil
+			d.P2P.Sync.AdoptExclusionView(game.ID, primary, oldText, beforeNeverSynced)
+		}
+		_ = d.Store.SetMark(exclusionsMark(game.ID), exclusionsValue(now))
+	}
+	_ = d.Store.SetMark("never_synced", delta.NeverSyncedList)
+}
diff --git a/internal/daemon/devicesettings_test.go b/internal/daemon/devicesettings_test.go
new file mode 100644
index 00000000..786900f0
--- /dev/null
+++ b/internal/daemon/devicesettings_test.go
@@ -0,0 +1,128 @@
+package daemon
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/ignore"
+	"github.com/opensave/opensave/internal/store"
+	"github.com/opensave/opensave/internal/watcher"
+)
+
+// When the game database starts naming a game's settings, the hash its last
+// snapshot recorded is carried over — only where the save is the one it was
+// recorded for — so the watcher does not take the new view for a change and
+// snapshot a game nobody played.
+func TestAdoptExclusionView_CarriesTheLastSnapshotHashOver(t *testing.T) {
+	d := newTestDaemon(t)
+	mk := func(id string, files map[string]string) store.Game {
+		dir := filepath.Join(t.TempDir(), id)
+		for name, data := range files {
+			if err := os.MkdirAll(dir, 0o777); err != nil {
+				t.Fatal(err)
+			}
+			if err := os.WriteFile(filepath.Join(dir, name), []byte(data), 0o666); err != nil {
+				t.Fatal(err)
+			}
+		}
+		g := store.Game{ID: id, Name: id, SavePath: dir, ActiveBranch: "main", MaxSnapshots: 5}
+		if err := d.Store.CreateGame(g); err != nil {
+			t.Fatal(err)
+		}
+		m, err := delta.BuildManifest(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := d.Store.SetLastManifestHash(id, watcher.ContentHash(m, "")); err != nil {
+			t.Fatal(err)
+		}
+		g, _ = d.Store.GetGame(id)
+		return g
+	}
+	same := mk("same", map[string]string{"slot1.sav": "a", "graphics.xml": "4k"})
+	changed := mk("changed", map[string]string{"slot1.sav": "a", "graphics.xml": "4k"})
+	// A real change since the last snapshot, not yet seen by the watcher.
+	if err := os.WriteFile(filepath.Join(changed.SavePath, "slot1.sav"), []byte("b"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	delta.InvalidateRoot(changed.SavePath)
+	_ = d.Store.SetMark("never_synced", delta.NeverSyncedList)
+
+	d.devMu.Lock()
+	d.dev = devSettingsCache{loaded: true, patterns: map[string][]string{}, verdicts: map[string][]store.SettingsFile{}}
+	for _, g := range []store.Game{same, changed} {
+		d.dev.patterns[g.Name+"\x00"+g.AppID+"\x00"+g.SavePath] = []string{"/graphics.xml"}
+	}
+	d.devMu.Unlock()
+
+	d.adoptExclusionView([]store.Game{same, changed})
+
+	hashNow := func(g store.Game) string {
+		m, _ := delta.BuildManifest(g.SavePath)
+		return watcher.ContentHash(m, d.IgnoreText(g))
+	}
+	if g, _ := d.Store.GetGame("same"); g.LastManifestHash != hashNow(g) {
+		t.Error("an unchanged save's last-snapshot hash was not carried over: the watcher will snapshot it")
+	}
+	if g, _ := d.Store.GetGame("changed"); g.LastManifestHash == hashNow(g) {
+		t.Error("a changed save's hash was carried over: its change would never be snapshotted")
+	}
+	if d.Store.Mark(exclusionsMark("same")) == "" {
+		t.Error("nothing recorded: this would run again at every start")
+	}
+}
+
+// A file taken for settings that another device's copy shows changing with
+// every change of the save is given back to syncing; one that stays put while
+// the save moves stays left out.
+func TestObserveSettings_GivesBackWhatChangesLikeASave(t *testing.T) {
+	d := newTestDaemon(t)
+	dir := t.TempDir()
+	game := store.Game{ID: "g", Name: "g", SavePath: dir, ActiveBranch: "main", MaxSnapshots: 5}
+	if err := d.Store.CreateGame(game); err != nil {
+		t.Fatal(err)
+	}
+	for _, p := range []string{"progress.cfg", "video.cfg"} {
+		if err := d.Store.SetSettingsFile(store.SettingsFile{GameID: "g", Path: p, Verdict: store.VerdictSettings, Reason: "test"}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	d.devMu.Lock()
+	d.dev = devSettingsCache{loaded: true, patterns: map[string][]string{}, verdicts: map[string][]store.SettingsFile{}}
+	d.devMu.Unlock()
+	excluded := func(p string) bool { return ignore.Parse(d.IgnoreText(game)).Match(p) }
+	if !excluded("progress.cfg") || !excluded("video.cfg") {
+		t.Fatal("setup: the detected files are not left out")
+	}
+
+	manifest := func(i int, video string) delta.Manifest {
+		return delta.Manifest{Files: map[string]delta.FileEntry{
+			"slot1.sav":    {Hash: fmt.Sprint("save", i)},
+			"progress.cfg": {Hash: fmt.Sprint("progress", i)},
+			"video.cfg":    {Hash: video},
+		}}
+	}
+	d.observeSettings(game, "peerA", manifest(0, "1080p"))
+	for i := 1; i <= 2; i++ {
+		d.observeSettings(game, "peerA", manifest(i, "1080p"))
+	}
+	if !excluded("progress.cfg") {
+		t.Fatal("given back after two changes: too eager")
+	}
+	d.observeSettings(game, "peerA", manifest(3, "1080p"))
+	if excluded("progress.cfg") {
+		t.Error("a file that changed with the save three times out of three is still left out")
+	}
+	if !excluded("video.cfg") {
+		t.Error("a settings file that stayed put was given back")
+	}
+	v, _ := d.Store.SettingsFiles("g")
+	for _, f := range v {
+		if f.Path == "progress.cfg" && f.Verdict != store.VerdictSave {
+			t.Errorf("verdict %q, want save", f.Verdict)
+		}
+	}
+}
diff --git a/internal/daemon/missing.go b/internal/daemon/missing.go
index aa083447..b55c7b1f 100644
--- a/internal/daemon/missing.go
+++ b/internal/daemon/missing.go
@@ -6,6 +6,8 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"strings"
+	"time"
 
 	"github.com/opensave/opensave/internal/delta"
 	"github.com/opensave/opensave/internal/logging"
@@ -34,10 +36,126 @@ func SaveFolderMissing(savePath string) bool {
 	return errors.Is(err, fs.ErrNotExist)
 }
 
+// SaveDriveMissing reports whether a save path lives on a drive this device
+// does not have at all — a D:\ or E:\ Steam library that exists on another
+// PC. That is where a game auto-tracked from a peer lands when the peer's
+// library is on a drive letter this machine lacks, and it is not something
+// to warn about: nothing was lost here, the game just isn't on this device.
+// Only a volume that is plainly absent counts; an unreachable network share
+// reports some other error and stays a warning.
+func SaveDriveMissing(savePath string) bool {
+	vol := filepath.VolumeName(savePath)
+	if vol == "" {
+		return false
+	}
+	_, err := os.Stat(vol + string(filepath.Separator))
+	return errors.Is(err, fs.ErrNotExist)
+}
+
+// The same save folder under another drive letter.
+//
+// A save inside a game's install folder — a Steam library, most often — sits
+// on whichever drive that library is on, and that differs between devices: D:
+// on one, E: on another. A game tracked on one device and auto-tracked on
+// another arrives with the first device's drive letter, and on the second the
+// folder is simply "missing" though it is right there on another drive.
+//
+// So a missing folder is looked for at the identical path on every other drive
+// letter. Exactly one match is adopted for this device, and said once. None, or
+// more than one (which would be a guess), is said once too, and the search is
+// not repeated more often than relocateRecheck: it touches every drive, and the
+// minute-long watch reconcile asks about missing folders constantly.
+const relocateRecheck = time.Hour
+
+// relocateCandidates lists the folders at savePath's exact path under the other
+// drive letters that exist here. Windows drive letters only.
+func relocateCandidates(savePath string) []string {
+	vol := filepath.VolumeName(savePath)
+	if len(vol) != 2 || vol[1] != ':' {
+		return nil
+	}
+	rest := savePath[2:]
+	own := strings.ToUpper(vol[:1])
+	var found []string
+	for c := 'C'; c <= 'Z'; c++ {
+		letter := string(c)
+		if letter == own {
+			continue
+		}
+		candidate := letter + ":" + rest
+		if !SaveFolderMissing(candidate) {
+			found = append(found, candidate)
+		}
+	}
+	return found
+}
+
+// relocateToOtherDrive switches a game whose save folder is missing to the same
+// path on another drive, when there is exactly one. Reports whether it did.
+func (d *Daemon) relocateToOtherDrive(gameID, name, savePath string) bool {
+	d.missingMu.Lock()
+	last, checked := d.relocateChecked[gameID]
+	if checked && time.Since(last) < relocateRecheck {
+		d.missingMu.Unlock()
+		return false
+	}
+	if d.relocateChecked == nil {
+		d.relocateChecked = map[string]time.Time{}
+	}
+	d.relocateChecked[gameID] = time.Now()
+	d.missingMu.Unlock()
+
+	if name == "" {
+		name = gameID
+	}
+	found := relocateCandidates(savePath)
+	switch {
+	case len(found) == 0:
+		if !checked {
+			d.Log.Log("info", fmt.Sprintf("the save folder of %q is not on another drive here either (looked for %s on every drive letter)",
+				name, logging.Quote(savePath[min(2, len(savePath)):])))
+		}
+		return false
+	case len(found) > 1:
+		if !checked {
+			d.Log.Log("warn", fmt.Sprintf("the save folder of %q is missing at %s, and the same path exists on %d other drives (%s) — not guessing which; set it with Edit",
+				name, logging.Quote(savePath), len(found), strings.Join(found, ", ")))
+		}
+		return false
+	}
+
+	newPath, err := d.ValidateSavePath(found[0])
+	if err != nil {
+		d.Log.Log("warn", fmt.Sprintf("the save folder of %q exists at %s, but it cannot be used: %v", name, logging.Quote(found[0]), err))
+		return false
+	}
+	game, err := d.Store.GetGame(gameID)
+	if err != nil || game.SavePath != savePath {
+		return false // changed meanwhile; the next reconcile sees the new path
+	}
+	game.SavePath = newPath
+	if err := d.Store.UpdateGame(game); err != nil {
+		d.Log.Log("warn", fmt.Sprintf("could not switch %q to %s: %v", name, logging.Quote(newPath), err))
+		return false
+	}
+	d.Log.Log("success", fmt.Sprintf("the save folder of %q was not at %s but is at %s on this device — using that",
+		name, logging.Quote(savePath), logging.Quote(newPath)))
+	d.missingMu.Lock()
+	delete(d.missing, gameID)
+	d.missingMu.Unlock()
+	if d.OnGameChanged != nil {
+		d.OnGameChanged(gameID)
+	}
+	return true
+}
+
 // noteMissing records whether a game's save folder is missing, and says so
 // once when that changes — not on every minute's check — so the log and the
 // screen follow the folder going and coming back.
 func (d *Daemon) noteMissing(gameID, name, savePath string, missing bool) {
+	if missing && d.relocateToOtherDrive(gameID, name, savePath) {
+		return // found under another drive letter and switched to it
+	}
 	d.missingMu.Lock()
 	was := d.missing[gameID]
 	if missing {
@@ -55,7 +173,10 @@ func (d *Daemon) noteMissing(gameID, name, savePath string, missing bool) {
 	if name == "" {
 		name = gameID
 	}
-	if missing {
+	if missing && SaveDriveMissing(savePath) {
+		d.Log.Log("info", fmt.Sprintf("the save folder of %q is on %s, which this device doesn't have (%s) — the game is skipped here "+
+			"and keeps syncing between the devices that have it", name, filepath.VolumeName(savePath), logging.Quote(savePath)))
+	} else if missing {
 		d.Log.Log("warn", fmt.Sprintf("the save folder of %q is not there (%s) — it is not watched or synced until it is back; "+
 			"OpenSave will not create it, since an empty folder in its place would read as every file deleted", name, logging.Quote(savePath)))
 	} else {
diff --git a/internal/daemon/missing_test.go b/internal/daemon/missing_test.go
new file mode 100644
index 00000000..0b5fdf6c
--- /dev/null
+++ b/internal/daemon/missing_test.go
@@ -0,0 +1,40 @@
+package daemon
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+// A save path on a drive letter this machine does not have is "not on this
+// device", not "folder gone": the UI informs rather than warns about it.
+func TestSaveDriveMissing(t *testing.T) {
+	existing := t.TempDir()
+	if SaveDriveMissing(filepath.Join(existing, "saves")) {
+		t.Error("a folder on an existing drive was reported as on a missing drive")
+	}
+	if SaveDriveMissing("relative/path") {
+		t.Error("a path without a volume was reported as on a missing drive")
+	}
+
+	if runtime.GOOS != "windows" {
+		return
+	}
+	// Find a drive letter that is not mounted.
+	for c := 'Z'; c >= 'D'; c-- {
+		root := string(c) + `:\`
+		if _, err := os.Stat(root); err == nil {
+			continue
+		}
+		p := string(c) + `:\SteamLibrary\steamapps\common\Game\save`
+		if !SaveDriveMissing(p) {
+			t.Errorf("%s: drive %c: is absent but was not reported missing", p, c)
+		}
+		if !SaveFolderMissing(p) {
+			t.Errorf("%s: SaveFolderMissing should still report the folder missing", p)
+		}
+		return
+	}
+	t.Skip("every drive letter D-Z is mounted")
+}
diff --git a/internal/daemon/relocate_test.go b/internal/daemon/relocate_test.go
new file mode 100644
index 00000000..152755fc
--- /dev/null
+++ b/internal/daemon/relocate_test.go
@@ -0,0 +1,82 @@
+package daemon
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"github.com/opensave/opensave/internal/store"
+)
+
+// freeDriveLetter returns a drive letter not mounted here, or "".
+func freeDriveLetter() string {
+	for c := 'Z'; c >= 'D'; c-- {
+		if _, err := os.Stat(string(c) + `:\`); err != nil {
+			return string(c)
+		}
+	}
+	return ""
+}
+
+// A game tracked elsewhere with its save on a drive this device lacks, while
+// the identical path exists on another drive here, is switched to it — once.
+func TestRelocateToOtherDrive(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("drive letters are a Windows thing")
+	}
+	free := freeDriveLetter()
+	if free == "" {
+		t.Skip("no free drive letter")
+	}
+	d := newTestDaemon(t)
+	real := filepath.Join(t.TempDir(), "SteamLibrary", "steamapps", "common", "Game", "save")
+	if err := os.MkdirAll(real, 0o777); err != nil {
+		t.Fatal(err)
+	}
+	elsewhere := free + ":" + real[2:] // same path, absent drive
+	if err := d.Store.CreateGame(store.Game{ID: "g", Name: "Game", SavePath: elsewhere, ActiveBranch: "main", AutoSync: true}); err != nil {
+		t.Fatal(err)
+	}
+
+	d.noteMissing("g", "Game", elsewhere, true)
+
+	g, err := d.Store.GetGame("g")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.EqualFold(g.SavePath, real) {
+		t.Fatalf("save path = %s, want it switched to %s", g.SavePath, real)
+	}
+	d.missingMu.Lock()
+	stillMissing := d.missing["g"]
+	d.missingMu.Unlock()
+	if stillMissing {
+		t.Error("the game is still marked missing after being found")
+	}
+}
+
+// Not found: said once, and not searched again on every reconcile.
+func TestRelocateNotFound_IsNotRepeated(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("drive letters are a Windows thing")
+	}
+	free := freeDriveLetter()
+	if free == "" {
+		t.Skip("no free drive letter")
+	}
+	d := newTestDaemon(t)
+	path := free + `:\NoSuchLibrary\steamapps\common\Nothing\save`
+	if err := d.Store.CreateGame(store.Game{ID: "n", Name: "Nothing", SavePath: path, ActiveBranch: "main", AutoSync: true}); err != nil {
+		t.Fatal(err)
+	}
+	if d.relocateToOtherDrive("n", "Nothing", path) {
+		t.Fatal("relocated to a folder that does not exist")
+	}
+	first := d.relocateChecked["n"]
+	d.relocateToOtherDrive("n", "Nothing", path)
+	if !d.relocateChecked["n"].Equal(first) {
+		t.Error("searched again straight away; it should wait relocateRecheck")
+	}
+}
diff --git a/internal/daemon/sessions.go b/internal/daemon/sessions.go
index 67775f99..738fde64 100644
--- a/internal/daemon/sessions.go
+++ b/internal/daemon/sessions.go
@@ -42,6 +42,25 @@ type sessionState struct {
 	list func() ([]sessions.Proc, error)
 
 	mu sync.Mutex
+	// checkpoint is when each playing game last had a snapshot taken while
+	// it was played (holdSnapshotWhilePlaying).
+	checkpoint map[string]time.Time
+	// programs: what programsIn found in each install folder.
+	programs map[string]programList
+	// active: stretches of changes no session accounts for (noteActivity);
+	// warned: games already said to change with no program seen.
+	active map[string]*activity
+	warned map[string]bool
+	// lastChange: when each game's save last changed here.
+	lastChange map[string]time.Time
+	// gap: the longest time seen between two changes of each game's save in
+	// a stretch (activity.maxGap).
+	gap map[string]time.Duration
+	// cpu: each process's processor time at the last poll, and when it last
+	// used any (activeProcesses).
+	cpu map[int]procSeen
+	// endedAt: when each game's last session ended (holdSnapshotWhilePlaying).
+	endedAt map[string]time.Time
 	// startHash is each playing game's save as it was when play began, to
 	// tell afterwards whether the session changed it.
 	startHash map[string]string
@@ -89,7 +108,9 @@ func (d *Daemon) PollSessions() {
 	if err != nil {
 		return
 	}
+	procs = d.activeProcesses(procs)
 	running := sessions.Running(procs, targets)
+	d.rememberInstallFolders(procs, targets, running)
 	d.notePrograms(procs, targets, learn, running)
 	d.sessions.tracker.Poll(running, time.Now())
 }
@@ -145,9 +166,16 @@ func (d *Daemon) sessionTargets() ([]sessions.Target, map[string]bool, error) {
 			}
 		}
 		dir, viaSteam := installFolder(g, byAppID, byFolder)
+		if dir == "" {
+			dir = d.knownInstallFolder(g.ID)
+		}
+		t.FolderNames = d.installFolderNames(g)
 		if dir != "" {
 			t.Dirs = append(t.Dirs, dir)
 		}
+		for _, dir := range t.Dirs {
+			t.Programs = append(t.Programs, d.programsIn(dir)...)
+		}
 		// Steam launches the games it has installed; the rest need a
 		// program to start them, and none was given.
 		if dir != "" && !viaSteam && exe == "" {
@@ -197,6 +225,9 @@ func (d *Daemon) InstallState(g store.Game) string {
 	if dir, _ := installFolder(g, byAppID, byFolder); dir != "" {
 		return InstallFound
 	}
+	if d.knownInstallFolder(g.ID) != "" {
+		return InstallFound
+	}
 	if g.AppID != "" && steam {
 		return InstallNotFound
 	}
@@ -362,9 +393,19 @@ func (d *Daemon) sessionStarted(gameID string, at time.Time) {
 }
 
 func (d *Daemon) sessionEnded(gameID string, started, ended time.Time) {
+	// The session's end keeps the save and syncs it; a stretch of changes
+	// it overlapped would otherwise hold the sync back for quietAfter more.
+	d.endActivity(gameID)
+	d.sessions.mu.Lock()
+	if d.sessions.endedAt == nil {
+		d.sessions.endedAt = map[string]time.Time{}
+	}
+	d.sessions.endedAt[gameID] = time.Now()
+	d.sessions.mu.Unlock()
 	d.sessions.mu.Lock()
 	startHash := d.sessions.startHash[gameID]
 	delete(d.sessions.startHash, gameID)
+	delete(d.sessions.checkpoint, gameID)
 	seen := d.sessions.seen[gameID]
 	delete(d.sessions.seen, gameID)
 	d.sessions.mu.Unlock()
@@ -397,6 +438,7 @@ func (d *Daemon) sessionEnded(gameID string, started, ended time.Time) {
 		d.Log.Log("warn", fmt.Sprintf("could not keep %q as it was left: %v", game.Name, err))
 		return
 	}
+	d.dropCheckpoints(game, started)
 	// Onward to the other devices, so it is there on the next one picked up.
 	go func() {
 		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
@@ -455,3 +497,504 @@ func (d *Daemon) endOpenSessions() {
 		}
 	}
 }
+
+// checkpointEvery is how often a game being played gets a snapshot of its
+// save: one per autosave pushed the snapshots worth keeping out, and none
+// until the session ends leaves nothing behind a crash.
+const checkpointEvery = 30 * time.Minute
+
+// holdSnapshotWhilePlaying tells the watcher not to snapshot a change to a
+// game being played — in a session, or changing with no session seen
+// (noteActivity) — unless checkpointEvery has passed since play began or
+// since the last checkpoint. The end of the session, or of the stretch of
+// changes, keeps the save as it was left.
+func (d *Daemon) holdSnapshotWhilePlaying(gameID string) bool {
+	// A game known to be closed: kept and synced at once.
+	if d.notBeingPlayed(gameID) {
+		return false
+	}
+	// In a session, its rules alone: a checkpoint every checkpointEvery from
+	// its start. Only a change no session accounts for is a stretch
+	// (noteActivity) — whose timer once ended inside a session, cleared the
+	// session's last checkpoint, and so let nearly every autosave after the
+	// first half hour through as one.
+	since := d.PlayingSince(gameID)
+	if since.IsZero() {
+		start, stretch := d.noteActivity(gameID)
+		if !stretch {
+			return false
+		}
+		since = start
+	}
+	d.sessions.mu.Lock()
+	defer d.sessions.mu.Unlock()
+	if d.sessions.checkpoint == nil {
+		d.sessions.checkpoint = map[string]time.Time{}
+	}
+	now := time.Now()
+	if !checkpointDue(since, d.sessions.checkpoint[gameID], now) {
+		return true
+	}
+	d.sessions.checkpoint[gameID] = now
+	return false
+}
+
+// checkpointDue says whether checkpointEvery has passed since play began, or
+// since the last checkpoint when there was one.
+func checkpointDue(since, last, now time.Time) bool {
+	if last.Before(since) {
+		last = since
+	}
+	return now.Sub(last) >= checkpointEvery
+}
+
+// Programs that come with a game without being it: installers, crash
+// reporters, redistributables, anti-cheat services. A process known only by
+// name is never matched by one of these — every game ships the same few.
+var notTheGame = []string{
+	"unins", "setup", "install", "crash", "report", "redist", "vcredist", "vc_redist", "dxsetup",
+	"directx", "prereq", "launcher", "easyanticheat", "eac", "battleye", "beservice", "update",
+	"patch", "config", "settings", "helper", "service", "cef", "webhelper", "dotnet", "physx",
+}
+
+// programsIn lists the programs in a game's install folder by file name,
+// lower-case, for a process known only by name (sessions.Proc.Name). Read at
+// most every installDirsFresh, three folders deep.
+func (d *Daemon) programsIn(dir string) []string {
+	d.sessions.mu.Lock()
+	if c, ok := d.sessions.programs[dir]; ok && time.Since(c.at) < installDirsFresh {
+		d.sessions.mu.Unlock()
+		return c.names
+	}
+	d.sessions.mu.Unlock()
+
+	var names []string
+	root := filepath.Clean(dir)
+	depth := strings.Count(root, string(filepath.Separator))
+	_ = filepath.WalkDir(root, func(p string, e os.DirEntry, err error) error {
+		if err != nil {
+			return nil
+		}
+		if e.IsDir() {
+			if strings.Count(filepath.Clean(p), string(filepath.Separator))-depth >= 3 {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		name := strings.ToLower(e.Name())
+		if !strings.HasSuffix(name, ".exe") || len(names) >= 64 {
+			return nil
+		}
+		for _, w := range notTheGame {
+			if strings.Contains(name, w) {
+				return nil
+			}
+		}
+		names = append(names, name)
+		return nil
+	})
+
+	d.sessions.mu.Lock()
+	if d.sessions.programs == nil {
+		d.sessions.programs = map[string]programList{}
+	}
+	d.sessions.programs[dir] = programList{at: time.Now(), names: names}
+	d.sessions.mu.Unlock()
+	return names
+}
+
+type programList struct {
+	at    time.Time
+	names []string
+}
+
+// quietAfter is how long a game's save must stay unchanged before a stretch
+// of changes no session accounts for is over: longer than the gap between
+// two autosaves of the games measured (Crimson Desert: up to six minutes).
+const quietAfter = 10 * time.Minute
+
+// activity is a stretch of changes to a game's save while no session of it
+// was seen: a game whose program cannot be told apart, or is not installed
+// where this device looks. It is handled as a session — not synced either
+// way, a checkpoint at most every checkpointEvery — and when the save has
+// stayed unchanged for quietAfter, it is kept and synced as a session's end
+// would be.
+type activity struct {
+	start, last time.Time
+	timer       *time.Timer
+	// changes counts the changes in it; a change within quietAfter of the
+	// one before, even one already sent, counts as a second.
+	changes int
+	// maxGap is the longest time between two of its changes, or the one
+	// remembered for the game: a game that autosaves every ten minutes must
+	// not have each autosave taken for the end of a stretch.
+	maxGap time.Duration
+}
+
+// firstChangeWait is how long a change on its own is held before it is kept
+// and synced: long enough that the next autosave of a game being played
+// arrives first (Crimson Desert: 3.6 minutes between its first two), so a
+// session not seen still sends one version, at its end. A game that saves
+// once, as it closes, reaches the other devices this much later — when its
+// session is not seen; the end of a session seen is sent at once.
+const firstChangeWait = 5 * time.Minute
+
+// maxQuiet caps how long a stretch can be held after its last change.
+const maxQuiet = 45 * time.Minute
+
+// quietFor is how long the save must stay unchanged to end the stretch: the
+// base wait, or twice the longest gap between the game's changes when that
+// is longer.
+func (a *activity) quietFor() time.Duration {
+	wait := quietAfter
+	if a.changes <= 1 {
+		wait = firstChangeWait
+	}
+	if w := 2 * a.maxGap; w > wait {
+		wait = w
+	}
+	if wait > maxQuiet {
+		wait = maxQuiet
+	}
+	return wait
+}
+
+// noteActivity records a change to a game's save made here (not by a sync)
+// and returns when the stretch it belongs to began, and whether it is held
+// as one. Every change is: a change on its own is kept and synced once the
+// save has stayed unchanged for firstChangeWait, a second within quietAfter
+// of the one before makes it a stretch that ends after quietAfter unchanged.
+// Only SyncEveryChange sends each change as it comes.
+func (d *Daemon) noteActivity(gameID string) (time.Time, bool) {
+	now := time.Now()
+	d.sessions.mu.Lock()
+	defer d.sessions.mu.Unlock()
+	if d.sessions.active == nil {
+		d.sessions.active = map[string]*activity{}
+	}
+	if d.sessions.lastChange == nil {
+		d.sessions.lastChange = map[string]time.Time{}
+	}
+	prev, seen := d.sessions.lastChange[gameID]
+	d.sessions.lastChange[gameID] = now
+	if d.opts.SyncEveryChange {
+		return now, false
+	}
+	a := d.sessions.active[gameID]
+	if a == nil {
+		a = &activity{start: now, changes: 1, maxGap: d.sessions.gap[gameID]}
+		if seen && now.Sub(prev) < maxQuiet && now.Sub(prev) < 2*(a.maxGap+quietAfter/2) {
+			a.start, a.changes = prev, 2
+			if g := now.Sub(prev); g > a.maxGap {
+				a.maxGap = g
+			}
+		}
+		d.sessions.active[gameID] = a
+		a.timer = time.AfterFunc(a.quietFor(), func() { d.activityQuiet(gameID) })
+	} else {
+		a.changes++
+		if g := now.Sub(a.last); g > a.maxGap {
+			a.maxGap = g
+		}
+		a.timer.Reset(a.quietFor())
+	}
+	if d.sessions.gap == nil {
+		d.sessions.gap = map[string]time.Duration{}
+	}
+	if a.maxGap > d.sessions.gap[gameID] {
+		d.sessions.gap[gameID] = a.maxGap
+	}
+	a.last = now
+	if a.changes == 2 && d.PlayingSince(gameID).IsZero() && !d.sessions.warned[gameID] {
+		if d.sessions.warned == nil {
+			d.sessions.warned = map[string]bool{}
+		}
+		d.sessions.warned[gameID] = true
+		name := gameID
+		if g, err := d.Store.GetGame(gameID); err == nil {
+			name = g.Name
+		}
+		go d.Log.Log("info", fmt.Sprintf("%q is changing its save, but its program is not seen running here — "+
+			"handled as being played: kept and synced once it has not changed for %s", name, spokenLength(quietAfter)))
+	}
+	return a.start, true
+}
+
+// changingNow reports a game in a stretch of changes (noteActivity).
+func (d *Daemon) changingNow(gameID string) bool {
+	d.sessions.mu.Lock()
+	defer d.sessions.mu.Unlock()
+	_, ok := d.sessions.active[gameID]
+	return ok
+}
+
+// endActivity forgets a stretch of changes; a session's end takes over.
+func (d *Daemon) endActivity(gameID string) {
+	d.sessions.mu.Lock()
+	if a := d.sessions.active[gameID]; a != nil {
+		a.timer.Stop()
+		delete(d.sessions.active, gameID)
+	}
+	d.sessions.mu.Unlock()
+}
+
+// activityQuiet ends a stretch of changes once the save has stayed unchanged
+// for quietAfter: the save as it was left is kept and goes to the other
+// devices, as at the end of a session.
+func (d *Daemon) activityQuiet(gameID string) {
+	d.sessions.mu.Lock()
+	a := d.sessions.active[gameID]
+	if a == nil {
+		d.sessions.mu.Unlock()
+		return
+	}
+	if wait := a.quietFor() - time.Since(a.last); wait > 0 {
+		a.timer.Reset(wait)
+		d.sessions.mu.Unlock()
+		return
+	}
+	delete(d.sessions.active, gameID)
+	if !d.PlayingSince(gameID).IsZero() {
+		// A session is open after all: its end keeps the save, and its
+		// checkpoints are its own.
+		d.sessions.mu.Unlock()
+		return
+	}
+	delete(d.sessions.checkpoint, gameID)
+	start, last, stretch := a.start, a.last, a.changes > 1
+	d.sessions.mu.Unlock()
+	game, err := d.Store.GetGame(gameID)
+	if err != nil {
+		return
+	}
+	if hash, err := d.currentContentHash(game); err == nil && hash != game.LastManifestHash {
+		comment := "Auto backup"
+		if last.Sub(start) >= shortestSession {
+			comment = fmt.Sprintf("After playing (%s)", spokenLength(last.Sub(start)))
+		}
+		if err := d.markSessionEnd(game, hash, start, comment); err != nil {
+			d.Log.Log("warn", fmt.Sprintf("could not keep %q as it was left: %v", game.Name, err))
+			return
+		}
+		d.dropCheckpoints(game, start)
+	}
+	d.P2P.Sync.NoteLocalChange(gameID)
+	if stretch {
+		d.Log.Log("info", fmt.Sprintf("%q has not changed its save for %s; sending it to your other devices",
+			game.Name, spokenLength(quietAfter)))
+	}
+	if d.OnGameChanged != nil {
+		d.OnGameChanged(gameID)
+	}
+	go func() {
+		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
+		defer cancel()
+		_, _ = d.P2P.SyncGame(ctx, gameID)
+	}()
+}
+
+// Games installed where no library lists them — a folder of the user's own,
+// "G:\Games\Crimson Desert" — are found by the name of their install folder:
+// the names the game database knows it by, and the game's own name when it
+// is specific enough not to be a folder of something else. Once a session
+// has been seen that way, the folder is remembered, so the game is known to
+// be installed and its programs can be told by name (programsIn).
+
+// genericFolders are names no game's folder is recognised by alone.
+var genericFolders = map[string]bool{
+	"games": true, "game": true, "steam": true, "steamapps": true, "common": true, "program files": true,
+	"program files x86": true, "bin": true, "bin64": true, "win64": true, "binaries": true, "launcher": true,
+	"data": true, "x64": true, "app": true, "client": true, "epic games": true, "gog games": true,
+}
+
+func (d *Daemon) installFolderNames(g store.Game) []string {
+	var names []string
+	add := func(n string) {
+		n = sessions.FolderName(n)
+		if n == "" || genericFolders[n] {
+			return
+		}
+		for _, have := range names {
+			if have == n {
+				return
+			}
+		}
+		names = append(names, n)
+	}
+	d.devMu.Lock()
+	db := d.devDBLocked()
+	d.devMu.Unlock()
+	for _, n := range db.InstallNames(g.Name, g.AppID) {
+		if len(sessions.FolderName(n)) >= 4 {
+			add(n)
+		}
+	}
+	// The game's own name only when it could hardly name anything else:
+	// two words, or one long one. "Rust" is a folder in every Rust toolchain.
+	if n := sessions.FolderName(g.Name); strings.Contains(n, " ") && len(n) >= 6 || len(n) >= 10 {
+		add(g.Name)
+	}
+	return names
+}
+
+func installFolderMark(gameID string) string { return "installdir:" + gameID }
+
+// knownInstallFolder is the install folder a session of the game was seen
+// running from, when it is still there.
+func (d *Daemon) knownInstallFolder(gameID string) string {
+	dir := d.Store.Mark(installFolderMark(gameID))
+	if dir == "" {
+		return ""
+	}
+	if info, err := os.Stat(dir); err != nil || !info.IsDir() {
+		return ""
+	}
+	return dir
+}
+
+// rememberInstallFolders records, for each game found running by the name of
+// its install folder, which folder that was.
+func (d *Daemon) rememberInstallFolders(procs []sessions.Proc, targets []sessions.Target, running map[string]int) {
+	for _, t := range targets {
+		pid, ok := running[t.GameID]
+		if !ok || len(t.Dirs) > 0 {
+			continue
+		}
+		for _, p := range procs {
+			if p.PID != pid {
+				continue
+			}
+			if dir := sessions.FolderOf(p, t); dir != "" && d.Store.Mark(installFolderMark(t.GameID)) != dir {
+				_ = d.Store.SetMark(installFolderMark(t.GameID), dir)
+			}
+		}
+	}
+}
+
+// playingHere says whether a game counts as being played on this device, for
+// the sync engine (syncengine/playing.go): a session is open, a stretch of
+// changes is (noteActivity) — or one is about to be: a change made here waits
+// out the watcher's debounce. A sync landing in that gap would otherwise
+// send the change on the moment before it is held.
+func (d *Daemon) playingHere(gameID string) bool {
+	if !d.PlayingSince(gameID).IsZero() || d.changingNow(gameID) {
+		return true
+	}
+	if d.notBeingPlayed(gameID) {
+		return false
+	}
+	return !d.opts.SyncEveryChange && d.Watcher != nil && d.Watcher.ChangePending(gameID)
+}
+
+// activeProcesses leaves out the processes doing nothing: next to no memory,
+// and no processor time since the last poll. A game leaves such a process
+// behind when it closes — a crash handler, a stub of the same name — and it
+// kept a session open for as long as it lingered, holding the save back
+// from syncing after the game was long gone.
+func (d *Daemon) activeProcesses(procs []sessions.Proc) []sessions.Proc {
+	return d.activeProcessesAt(procs, time.Now())
+}
+
+// procSeen is a process's processor time at the last poll, and when it was
+// last seen using any.
+type procSeen struct {
+	cpu  time.Duration
+	busy time.Time
+}
+
+// idleFor is how long a process may use no processor time at all before it
+// is no longer the game, however much memory it holds: what a game leaves
+// behind when it is closed or killed can keep its memory, and kept a session
+// open with it. A game being played uses some all the time — a paused one
+// still draws its menu.
+const idleFor = time.Minute
+
+// busyCPU is the processor time between two polls that counts as using any.
+const busyCPU = 20 * time.Millisecond
+
+func (d *Daemon) activeProcessesAt(procs []sessions.Proc, now time.Time) []sessions.Proc {
+	d.sessions.mu.Lock()
+	defer d.sessions.mu.Unlock()
+	prev := d.sessions.cpu
+	next := make(map[int]procSeen, len(procs))
+	out := procs[:0:0]
+	for _, p := range procs {
+		before, seen := prev[p.PID]
+		if sessions.Idle(p, before.cpu, seen) {
+			if p.Measured {
+				next[p.PID] = procSeen{cpu: p.CPU, busy: before.busy}
+			}
+			continue
+		}
+		if !p.Measured {
+			out = append(out, p) // cannot be told: counted
+			continue
+		}
+		s := procSeen{cpu: p.CPU, busy: before.busy}
+		if !seen || p.CPU-before.cpu >= busyCPU {
+			s.busy = now // new, or working
+		}
+		next[p.PID] = s
+		if now.Sub(s.busy) > idleFor {
+			continue // holding memory, doing nothing
+		}
+		out = append(out, p)
+	}
+	d.sessions.cpu = next
+	return out
+}
+
+// sessionEndGrace is how long after a session ends a change to its save is
+// still the session's — the game writing as it closes — and sent at once.
+const sessionEndGrace = 2 * time.Minute
+
+// recognised reports a game whose sessions this device has seen: one has
+// been recorded, or the folder it ran from is known. While such a game is not
+// running, a change to its save is not play, and is sent at once.
+func (d *Daemon) recognised(gameID string) bool {
+	if d.knownInstallFolder(gameID) != "" {
+		return true
+	}
+	st, err := d.Store.PlayStatsFor(gameID)
+	return err == nil && st.Sessions > 0
+}
+
+// notBeingPlayed reports a change that is certainly not play: no session is
+// open, and either one ended moments ago, or the game is one whose sessions
+// are seen here.
+func (d *Daemon) notBeingPlayed(gameID string) bool {
+	if !d.PlayingSince(gameID).IsZero() {
+		return false
+	}
+	d.sessions.mu.Lock()
+	ended, ok := d.sessions.endedAt[gameID]
+	d.sessions.mu.Unlock()
+	if ok && time.Since(ended) < sessionEndGrace {
+		return true
+	}
+	return d.recognised(gameID)
+}
+
+// checkpointComment names the snapshots taken while a game is played.
+const checkpointComment = "Checkpoint while playing"
+
+// dropCheckpoints removes a session's checkpoints once the save as the
+// session left it is kept: they were there in case the session never got to
+// end — a crash, the power going — and after it did, they are only clutter
+// between one session's snapshot and the next. Pinned ones stay.
+func (d *Daemon) dropCheckpoints(game store.Game, since time.Time) {
+	snaps, err := d.Store.ListSnapshots(game.ID, game.ActiveBranch)
+	if err != nil {
+		return
+	}
+	for _, s := range snaps {
+		if !s.IsSystemAuto || s.Pinned || s.Comment != checkpointComment {
+			continue
+		}
+		at, err := time.Parse(time.RFC3339Nano, s.Timestamp)
+		if err != nil || at.Before(since) {
+			continue
+		}
+		_, _ = d.Snapshots.DeleteSnapshot(game.ID, s.ID)
+	}
+}
diff --git a/internal/daemon/sessions_test.go b/internal/daemon/sessions_test.go
index 362c27c7..1548a89f 100644
--- a/internal/daemon/sessions_test.go
+++ b/internal/daemon/sessions_test.go
@@ -193,3 +193,228 @@ func TestSpokenLength(t *testing.T) {
 		}
 	}
 }
+
+// While a game is played, a change to its save is snapshotted once per
+// checkpointEvery, not at every autosave; and it is not synced either way.
+func TestSessionHoldsSnapshotsAndSyncWhilePlaying(t *testing.T) {
+	d := newTestDaemon(t)
+	g, _ := sessionGame(t, d, "autosaving")
+	d.sessions.list = func() ([]sessions.Proc, error) { return []sessions.Proc{{PID: 7, Exe: g.ExePath}}, nil }
+	d.PollSessions()
+	if d.PlayingSince(g.ID).IsZero() {
+		t.Fatal("setup: not playing")
+	}
+	if !d.holdSnapshotWhilePlaying(g.ID) {
+		t.Error("an autosave right after the game started was snapshotted")
+	}
+	if !d.P2P.Sync.PlayingHere(g.ID) {
+		t.Error("the sync engine does not know the game is being played")
+	}
+	// Checkpoints: half an hour after play began, then half an hour after
+	// the last one.
+	start := time.Now()
+	for _, c := range []struct {
+		last  time.Time
+		after time.Duration
+		want  bool
+	}{
+		{time.Time{}, 10 * time.Minute, false},
+		{time.Time{}, checkpointEvery, true},
+		{start.Add(40 * time.Minute), 50 * time.Minute, false},
+		{start.Add(40 * time.Minute), 70 * time.Minute, true},
+	} {
+		if got := checkpointDue(start, c.last, start.Add(c.after)); got != c.want {
+			t.Errorf("checkpoint due %v after start (last %v) = %v, want %v", c.after, c.last, got, c.want)
+		}
+	}
+}
+
+// A game whose program is not seen: a change is held, not synced, and kept
+// once the save has stayed unchanged for firstChangeWait; a second soon after
+// makes it a stretch, which ends after quietAfter unchanged.
+func TestActivityWithoutASessionIsHandledAsPlay(t *testing.T) {
+	d := newTestDaemon(t)
+	g, dir := sessionGame(t, d, "unseen")
+	if !d.holdSnapshotWhilePlaying(g.ID) {
+		t.Fatal("a single change was not held")
+	}
+	d.sessions.mu.Lock()
+	if q := d.sessions.active[g.ID].quietFor(); q != firstChangeWait {
+		t.Errorf("a single change waits %v, want %v", q, firstChangeWait)
+	}
+	d.sessions.mu.Unlock()
+	if !d.holdSnapshotWhilePlaying(g.ID) {
+		t.Fatal("a second change soon after was not held")
+	}
+	d.sessions.mu.Lock()
+	if q := d.sessions.active[g.ID].quietFor(); q != quietAfter {
+		t.Errorf("a stretch waits %v, want %v", q, quietAfter)
+	}
+	d.sessions.mu.Unlock()
+	if !d.changingNow(g.ID) || !d.P2P.Sync.PlayingHere(g.ID) {
+		t.Fatal("a game changing its save is not handled as being played")
+	}
+	before := len(snapComments(t, d, g.ID))
+	if err := os.WriteFile(filepath.Join(dir, "slot1.sav"), []byte("left like this"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+
+	// Quiet for longer than quietAfter.
+	d.sessions.mu.Lock()
+	d.sessions.active[g.ID].last = time.Now().Add(-quietAfter - time.Second)
+	d.sessions.mu.Unlock()
+	d.activityQuiet(g.ID)
+	if d.changingNow(g.ID) || d.P2P.Sync.PlayingHere(g.ID) {
+		t.Error("still handled as being played after the save went quiet")
+	}
+	if got := snapComments(t, d, g.ID); len(got) != before+1 {
+		t.Errorf("snapshots %q: want one more, keeping the save as it was left", got)
+	}
+}
+
+// The names a game's install folder is recognised by: never a generic folder,
+// and the game's own name only when it could hardly be another folder's.
+func TestInstallFolderNames(t *testing.T) {
+	d := newTestDaemon(t)
+	for name, want := range map[string]bool{"Crimson Desert": true, "Rust": false, "Games": false, "Satisfactory": true} {
+		got := d.installFolderNames(store.Game{ID: name, Name: name})
+		if (len(got) > 0) != want {
+			t.Errorf("%q: folder names %v, want some = %v", name, got, want)
+		}
+	}
+}
+
+// A process doing nothing does not keep a session open; once a game's
+// sessions are seen here, a change while it is closed is sent at once.
+func TestSessionEndsWhenOnlyAStubIsLeft(t *testing.T) {
+	d := newTestDaemon(t)
+	g, _ := sessionGame(t, d, "stub")
+	running := sessions.Proc{PID: 9, Exe: g.ExePath, Measured: true, Memory: 2 << 30, CPU: time.Second}
+	d.sessions.list = func() ([]sessions.Proc, error) { return []sessions.Proc{running}, nil }
+	d.PollSessions()
+	if d.PlayingSince(g.ID).IsZero() {
+		t.Fatal("setup: the game was not seen running")
+	}
+	// The game closed; a stub of the same program lingers, doing nothing.
+	stub := sessions.Proc{PID: 9, Exe: g.ExePath, Measured: true, Memory: 800 << 10, CPU: time.Second}
+	d.sessions.list = func() ([]sessions.Proc, error) { return []sessions.Proc{stub}, nil }
+	d.PollSessions()
+	since := d.PlayingSince(g.ID)
+	d.sessions.tracker.Finish(g.ID, since.Add(time.Hour)) // past the grace the tracker allows
+	d.PollSessions()
+	if !d.PlayingSince(g.ID).IsZero() {
+		t.Fatal("a lingering stub keeps the session open")
+	}
+	if d.holdSnapshotWhilePlaying(g.ID) {
+		t.Error("a change right after the session ended was held")
+	}
+	if !d.recognised(g.ID) {
+		t.Error("a game with a recorded session is not recognised")
+	}
+}
+
+// A session's checkpoints are gone once the save it left is kept: one
+// snapshot per session, not one per half hour.
+func TestSessionEndDropsItsCheckpoints(t *testing.T) {
+	d := newTestDaemon(t)
+	g, dir := sessionGame(t, d, "checkpointed")
+	d.sessions.list = func() ([]sessions.Proc, error) { return []sessions.Proc{{PID: 3, Exe: g.ExePath}}, nil }
+	d.PollSessions()
+	since := d.PlayingSince(g.ID)
+	if since.IsZero() {
+		t.Fatal("setup: not playing")
+	}
+	for i, data := range []string{"half an hour in", "an hour in"} {
+		if err := os.WriteFile(filepath.Join(dir, "slot1.sav"), []byte(data), 0o666); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := d.Snapshots.Create(g.ID, checkpointComment, true); err != nil {
+			t.Fatalf("checkpoint %d: %v", i, err)
+		}
+	}
+	if err := os.WriteFile(filepath.Join(dir, "slot1.sav"), []byte("as it was left"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	d.sessions.tracker.Finish(g.ID, since.Add(90*time.Minute))
+
+	got := snapComments(t, d, g.ID)
+	if len(got) != 1 || got[0] != "After playing (1 h 30 min)" {
+		t.Errorf("snapshots = %q, want only the session's", got)
+	}
+}
+
+// In a session, a change is the session's: no stretch is started alongside
+// it, whose timer once cleared the session's last checkpoint and let nearly
+// every autosave after the first half hour through as a new one.
+func TestSessionChangesStartNoStretch(t *testing.T) {
+	d := newTestDaemon(t)
+	g, _ := sessionGame(t, d, "in-session")
+	d.sessions.list = func() ([]sessions.Proc, error) { return []sessions.Proc{{PID: 4, Exe: g.ExePath}}, nil }
+	d.PollSessions()
+	if d.PlayingSince(g.ID).IsZero() {
+		t.Fatal("setup: not playing")
+	}
+	for i := 0; i < 3; i++ {
+		if !d.holdSnapshotWhilePlaying(g.ID) {
+			t.Fatalf("autosave %d in a fresh session was snapshotted", i)
+		}
+	}
+	if d.changingNow(g.ID) {
+		t.Error("a stretch runs alongside the session")
+	}
+}
+
+// A game that autosaves every ten minutes keeps its stretch open: the wait
+// is twice the longest gap seen, within bounds.
+func TestStretchWaitAdaptsToAutosaves(t *testing.T) {
+	for _, c := range []struct {
+		a    activity
+		want time.Duration
+	}{
+		{activity{changes: 1}, firstChangeWait},
+		{activity{changes: 2, maxGap: time.Minute}, quietAfter},
+		{activity{changes: 3, maxGap: 10 * time.Minute}, 20 * time.Minute},
+		{activity{changes: 1, maxGap: 10 * time.Minute}, 20 * time.Minute},
+		{activity{changes: 4, maxGap: time.Hour}, maxQuiet},
+	} {
+		if got := c.a.quietFor(); got != c.want {
+			t.Errorf("%d changes, gap %v: wait %v, want %v", c.a.changes, c.a.maxGap, got, c.want)
+		}
+	}
+}
+
+// What a killed game leaves behind can keep its memory; it is no longer the
+// game once it has used no processor time for a minute. One still working —
+// a paused game draws its menu — stays.
+func TestActiveProcesses_HoldingMemoryDoingNothing(t *testing.T) {
+	d := newTestDaemon(t)
+	t0 := time.Now()
+	left := func(cpu time.Duration) sessions.Proc {
+		return sessions.Proc{PID: 1, Exe: `G:\Games\Crimson Desert\bin64\CrimsonDesert.exe`, Measured: true, Memory: 2 << 30, CPU: cpu}
+	}
+	playing := func(cpu time.Duration) sessions.Proc {
+		return sessions.Proc{PID: 2, Exe: `G:\Games\Other\game.exe`, Measured: true, Memory: 2 << 30, CPU: cpu}
+	}
+	count := func(ps []sessions.Proc) map[int]bool {
+		m := map[int]bool{}
+		for _, p := range ps {
+			m[p.PID] = true
+		}
+		return m
+	}
+	got := count(d.activeProcessesAt([]sessions.Proc{left(5 * time.Second), playing(time.Second)}, t0))
+	if !got[1] || !got[2] {
+		t.Fatalf("first seen: %v, want both", got)
+	}
+	got = count(d.activeProcessesAt([]sessions.Proc{left(5 * time.Second), playing(2 * time.Second)}, t0.Add(30*time.Second)))
+	if !got[1] || !got[2] {
+		t.Errorf("half a minute in: %v, want both", got)
+	}
+	got = count(d.activeProcessesAt([]sessions.Proc{left(5 * time.Second), playing(3 * time.Second)}, t0.Add(70*time.Second)))
+	if got[1] {
+		t.Error("a process doing nothing for over a minute still counts as the game")
+	}
+	if !got[2] {
+		t.Error("a game using the processor was dropped")
+	}
+}
diff --git a/internal/daemon/verify.go b/internal/daemon/verify.go
index ebf57cbd..47552baf 100644
--- a/internal/daemon/verify.go
+++ b/internal/daemon/verify.go
@@ -64,6 +64,11 @@ func (d *Daemon) VerifySnapshots(ctx context.Context, gameID string, pace time.D
 				}
 				was := snap.Problem
 				if err := d.Snapshots.Verify(snap); err != nil {
+					if _, gerr := d.Store.GetSnapshot(snap.ID); gerr != nil {
+						// Removed while this ran (a newer snapshot holding
+						// the same files took its place): not damaged, gone.
+						continue
+					}
 					report.Damaged = append(report.Damaged, DamagedSnapshot{
 						GameID: g.ID, GameName: g.Name, SnapshotID: snap.ID, Timestamp: snap.Timestamp, Problem: err.Error(),
 					})
diff --git a/internal/delta/hashcache.go b/internal/delta/hashcache.go
index 5ee3b611..61adcda8 100644
--- a/internal/delta/hashcache.go
+++ b/internal/delta/hashcache.go
@@ -4,6 +4,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"sort"
 	"strings"
 	"sync"
 	"time"
@@ -64,11 +65,19 @@ type cacheEntry struct {
 // the old behaviour.
 const (
 	// cacheMinBytes is the floor, and the value a small library gets.
-	cacheMinBytes = 64 << 20
+	//
+	// A budget, not an allocation: the cache only ever holds entries for
+	// files that exist, so a small library never comes near it. It has to
+	// cover the rare library with one enormous save, though — a game that
+	// writes every map chunk as its own file (Project Zomboid: ~240k files,
+	// ~53MB by entryCost) filled a 64MB budget on its own, and the cache then
+	// evicted and re-read that save on every pass, which is the churn it
+	// exists to prevent.
+	cacheMinBytes = 256 << 20
 	// cacheMaxCeiling is the most this will ever hold, however many games are
 	// tracked. A cache is meant to replace repeated reading, not to become
 	// the memory problem it was added to fix.
-	cacheMaxCeiling = 384 << 20
+	cacheMaxCeiling = 512 << 20
 	// cachePerGameBytes is how much the budget grows per tracked game.
 	//
 	// From measurement rather than taste: a game's manifest costs roughly
@@ -77,7 +86,10 @@ const (
 	// library and is not enough for someone tracking 350 games — the cache
 	// sits at its cap and evicts entries it is about to want again, which
 	// quietly reinstates the repeated reading it exists to remove.
-	cachePerGameBytes = 512 << 10
+	//
+	// Doubled alongside the floor so a large library still gets more than a
+	// small one (350 games -> 350MB).
+	cachePerGameBytes = 1 << 20
 )
 
 // cacheMaxBytes is the current budget. Not a constant: it scales with how many
@@ -193,6 +205,12 @@ func FileEntryFor(path string) (FileEntry, error) {
 	return cachedHashFile(path, info)
 }
 
+// FileEntryForInfo is FileEntryFor with the FileInfo a directory walk already
+// has, so a cache hit costs no syscall at all.
+func FileEntryForInfo(path string, info os.FileInfo) (FileEntry, error) {
+	return cachedHashFile(path, info)
+}
+
 func cachedHashFile(path string, info os.FileInfo) (FileEntry, error) {
 	key := cacheKeyFor(path)
 	stamp := cacheStamp{size: info.Size(), mtimeNs: info.ModTime().UnixNano()}
@@ -302,13 +320,10 @@ func evictLocked() {
 	for k, v := range hashCache.entries {
 		all = append(all, aged{k, v.usedAt})
 	}
-	// Partial selection would be faster, but this runs only on overflow and
-	// clarity is worth more here than the microseconds.
-	for i := 1; i < len(all); i++ {
-		for j := i; j > 0 && all[j].at.Before(all[j-1].at); j-- {
-			all[j], all[j-1] = all[j-1], all[j]
-		}
-	}
+	// sort.Slice, not an insertion sort: map iteration order is random, so
+	// insertion sort was quadratic in the entry count — ~10^10 comparisons for
+	// a quarter-million-file save, on every overflow.
+	sort.Slice(all, func(i, j int) bool { return all[i].at.Before(all[j].at) })
 	target := cacheMaxBytes * 3 / 4
 	for _, a := range all {
 		if hashCache.bytes <= target {
diff --git a/internal/delta/manifest.go b/internal/delta/manifest.go
index f70016e5..c4f195bc 100644
--- a/internal/delta/manifest.go
+++ b/internal/delta/manifest.go
@@ -10,7 +10,9 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"sort"
 	"strconv"
@@ -28,6 +30,32 @@ import (
 // peer as a real file (and cascade into name.opensave.tmp.opensave.tmp).
 const TmpSuffix = ".opensave.tmp"
 
+// NeverSynced reports whether a file in a save folder is never part of the
+// save, on any device.
+//
+// remotecache.vdf is Steam's record of what Steam Cloud has uploaded for a
+// game, kept beside the game's files in Steam's userdata folder — which is
+// where a game whose saves live in Steam Cloud is tracked. It describes this
+// device's Steam, not the save: syncing it hands Steam another machine's
+// bookkeeping. And that folder is under Program Files, where writing needs
+// rights OpenSave does not have, so the copy failed with "access denied" on
+// every pass, from every device.
+//
+// Left out where decisions are made — what to sync, what a version is, what
+// a snapshot holds — and NOT from the manifest a device builds and serves: a
+// device on an older build would read the gap as the file deleted, and
+// delete its own copy (the same reasoning as exclusion rules,
+// syncengine/ignore.go).
+func NeverSynced(rel string) bool {
+	return strings.EqualFold(path.Base(rel), "remotecache.vdf")
+}
+
+// NeverSyncedList names what NeverSynced leaves out. It goes into the tag of
+// a version's hash, so changing the list re-takes the hash quietly on every
+// device, like changing a game's exclusion rules — instead of every device
+// calling the same save a new version of its own at once.
+const NeverSyncedList = "remotecache.vdf"
+
 // staleTmpAge is how old a leftover TmpSuffix file must be before the
 // manifest walk garbage-collects it. Generous enough that a patch actively
 // writing its temp file is never deleted out from under it.
@@ -217,9 +245,17 @@ func HashFile(path string) (FileEntry, error) {
 		}
 	}
 
+	wholeHash := hex.EncodeToString(whole.Sum(nil))
+	// A single-block file's block hash is the whole-file hash; share the one
+	// string. Most save files are single-block, and on a save of a quarter
+	// million of them the duplicate hex strings cost ~20MB per manifest.
+	if len(blocks) == 1 {
+		blocks[0].Hash = wholeHash
+	}
+
 	return FileEntry{
 		Size:      info.Size(),
-		Hash:      hex.EncodeToString(whole.Sum(nil)),
+		Hash:      wholeHash,
 		Blocks:    blocks,
 		BlockSize: blockSize,
 		MtimeMs:   Milli(info.ModTime().UnixMilli()),
@@ -263,7 +299,20 @@ func buildManifest(root string) (Manifest, error) {
 	}
 
 	var dirs []string
-	err = filepath.Walk(root, func(path string, walkInfo os.FileInfo, walkErr error) error {
+	// WalkDir, not Walk: Walk issues an extra Lstat per entry, which on
+	// Windows opens every file and dominated the cost of a warm build of a
+	// large save (13-19s vs ~1.2s for 240k files). WalkDir takes size and
+	// mtime from the directory listing itself.
+	err = filepath.WalkDir(root, func(path string, d fs.DirEntry, walkErr error) error {
+		var walkInfo os.FileInfo
+		if walkErr == nil && path != root {
+			// Reparse points are rejected below from the listing's type bits,
+			// before Info is asked for anything.
+			if d.Type()&(os.ModeSymlink|os.ModeIrregular) != 0 {
+				return nil
+			}
+			walkInfo, walkErr = d.Info()
+		}
 		if walkErr != nil {
 			// The save folder itself could not be read — its listing refused,
 			// or the folder gone since it was looked at. That is not an empty
@@ -282,7 +331,7 @@ func buildManifest(root string) (Manifest, error) {
 			// skipped instead of failing the whole manifest — one
 			// unreadable directory must not abort every sync of the game.
 			if os.IsPermission(walkErr) {
-				if walkInfo != nil && walkInfo.IsDir() {
+				if d != nil && d.IsDir() {
 					return filepath.SkipDir
 				}
 				return nil
@@ -341,7 +390,6 @@ func buildManifest(root string) (Manifest, error) {
 			}
 			return nil
 		}
-
 		entry, err := cachedHashFile(path, walkInfo)
 		if err != nil {
 			// Deleted after the walk listed it and before it could be read.
diff --git a/internal/delta/neversynced_test.go b/internal/delta/neversynced_test.go
new file mode 100644
index 00000000..1bcf8c21
--- /dev/null
+++ b/internal/delta/neversynced_test.go
@@ -0,0 +1,17 @@
+package delta
+
+import "testing"
+
+func TestNeverSynced(t *testing.T) {
+	for p, want := range map[string]bool{
+		"remotecache.vdf":        true,
+		"remote/RemoteCache.VDF": true,
+		"remote/save.sav":        false,
+		"remotecache.vdf.bak":    false,
+		"myremotecache.vdf":      false,
+	} {
+		if got := NeverSynced(p); got != want {
+			t.Errorf("NeverSynced(%q) = %v, want %v", p, got, want)
+		}
+	}
+}
diff --git a/internal/delta/patch.go b/internal/delta/patch.go
index 7976fc45..98e0104b 100644
--- a/internal/delta/patch.go
+++ b/internal/delta/patch.go
@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/opensave/opensave/internal/fsx"
+	"github.com/opensave/opensave/internal/owntouch"
 )
 
 // BlockSource supplies the bytes for one block index, either from the
@@ -77,7 +78,11 @@ func replaceWithRetry(tmpPath, filePath string) error {
 			delay *= 2
 		}
 		clearReadOnlyIfSet(filePath)
+		// Marked before the rename, so the watcher's event finds it already
+		// known as a change OpenSave made (owntouch).
+		owntouch.Mark(filePath)
 		if err = renameFile(tmpPath, filePath); err == nil {
+			owntouch.Settled(filePath)
 			return nil
 		}
 	}
diff --git a/internal/owntouch/owntouch.go b/internal/owntouch/owntouch.go
new file mode 100644
index 00000000..e63987a5
--- /dev/null
+++ b/internal/owntouch/owntouch.go
@@ -0,0 +1,137 @@
+// Package owntouch remembers which paths in save folders OpenSave itself has
+// just written or removed, so the watcher can tell those changes apart from
+// ones a game or a person made.
+//
+// Without it every change OpenSave applied — a file pulled from a peer, a
+// deletion a peer asked for, the mtimes "keep mine" touches — looked to the
+// watcher exactly like the game saving. A peer deleting files one request at
+// a time over several minutes produced a full auto-snapshot every couple of
+// seconds, each of a save that was neither the old one nor the new one, and
+// those filled the retention budget and pushed out the snapshots that mattered.
+package owntouch
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"time"
+)
+
+// window is how long a mark counts. Long enough to cover the watcher's
+// debounce and a slow burst of events behind it; short enough that the same
+// path changed again by a game a little later is not mistaken for ours.
+const window = 2 * time.Minute
+
+var (
+	mu     sync.Mutex
+	marked = map[string]mark{}
+)
+
+// mark is when OpenSave touched a path, whether it removed it, and - once
+// OpenSave is done with a file (Settled) - the time and size it left it with.
+type mark struct {
+	at      time.Time
+	removed bool
+	settled bool
+	mod     time.Time
+	size    int64
+}
+
+func key(path string) string {
+	p := filepath.Clean(path)
+	if runtime.GOOS == "windows" {
+		p = strings.ToLower(p)
+	}
+	return p
+}
+
+// Mark records that OpenSave itself just created, wrote, removed or re-dated
+// path. Its parent folder is marked too: creating or removing an entry is an
+// event on the folder as well.
+func Mark(path string) { record(path, false) }
+
+// MarkRemoved records that OpenSave itself is about to remove path. Only a
+// path marked this way counts as OpenSave's when it is found gone: a file
+// OpenSave has just written and a game or a person then deleted is their
+// change, and has to sync and be snapshotted as one.
+func MarkRemoved(path string) { record(path, true) }
+
+// Settled records the time and size OpenSave has left a file it marked with,
+// once it is done with it (written, renamed into place, re-dated). From then on
+// the file is OpenSave's only while it still has exactly those: a game saving
+// over it a moment later - inside any slack a time comparison would need - is
+// the game's change.
+func Settled(path string) {
+	fi, err := os.Lstat(path)
+	if err != nil || fi.IsDir() {
+		return
+	}
+	k := key(path)
+	mu.Lock()
+	defer mu.Unlock()
+	if m, ok := marked[k]; ok {
+		m.settled, m.mod, m.size = true, fi.ModTime(), fi.Size()
+		marked[k] = m
+	}
+}
+
+func record(path string, removed bool) {
+	if path == "" {
+		return
+	}
+	now := time.Now()
+	mu.Lock()
+	defer mu.Unlock()
+	marked[key(path)] = mark{at: now, removed: removed}
+	marked[key(filepath.Dir(path))] = mark{at: now}
+	if len(marked) > 50000 {
+		for k, m := range marked {
+			if now.Sub(m.at) > window {
+				delete(marked, k)
+			}
+		}
+	}
+}
+
+// slack is how much later than its mark a file OpenSave wrote may be dated:
+// re-dating to now happens just after marking, and some filesystems keep
+// times to two seconds.
+const slack = 2 * time.Second
+
+// Recent reports whether the change just seen at path is one OpenSave made:
+// the path was marked within the window, and nothing has written it since.
+//
+// The second half matters as much as the first. A game often saves straight
+// after a sync — the same file OpenSave has just put there — and a mark alone
+// would take that save for the sync's for two minutes: no snapshot of it, and
+// no new version to hand to anyone. Whatever OpenSave leaves behind is dated
+// no later than its mark (a pulled file keeps the time it was written to its
+// temporary name, or is given the peer's; re-dating sets now), so a file
+// dated later was written by someone else. Once OpenSave is done with a file
+// (Settled) the test is exact: the time and size it left, or not ours - the
+// slack would take a game's save within two seconds of a pull for the pull's.
+// A path that is gone is OpenSave's
+// only if OpenSave removed it (MarkRemoved): one it wrote and someone then
+// deleted is theirs. A folder carries no save of its own, so its mark is
+// enough.
+func Recent(path string) bool {
+	mu.Lock()
+	m, ok := marked[key(path)]
+	mu.Unlock()
+	if !ok || time.Since(m.at) > window {
+		return false
+	}
+	fi, err := os.Lstat(path)
+	if err != nil {
+		return m.removed
+	}
+	if fi.IsDir() {
+		return true
+	}
+	if m.settled {
+		return fi.ModTime().Equal(m.mod) && fi.Size() == m.size
+	}
+	return !fi.ModTime().After(m.at.Add(slack))
+}
diff --git a/internal/owntouch/owntouch_test.go b/internal/owntouch/owntouch_test.go
new file mode 100644
index 00000000..824f04a4
--- /dev/null
+++ b/internal/owntouch/owntouch_test.go
@@ -0,0 +1,132 @@
+package owntouch
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestMarkAndRecent(t *testing.T) {
+	dir := t.TempDir()
+	file := filepath.Join(dir, "sub", "slot1.sav")
+	if Recent(file) {
+		t.Fatal("unmarked path reported as ours")
+	}
+	Mark(file)
+	if err := os.MkdirAll(filepath.Dir(file), 0o777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(file, []byte("pulled"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if !Recent(file) {
+		t.Error("marked path not reported as ours")
+	}
+	if !Recent(filepath.Dir(file)) {
+		t.Error("the parent folder of a marked path not reported as ours")
+	}
+	if Recent(filepath.Join(dir, "sub", "slot2.sav")) {
+		t.Error("a sibling of a marked path reported as ours")
+	}
+}
+
+// A game saving over a file OpenSave has just written is the game's change,
+// however soon after.
+func TestRecent_AWriteAfterTheMarkIsNotOurs(t *testing.T) {
+	dir := t.TempDir()
+	file := filepath.Join(dir, "slot1.sav")
+	Mark(file)
+	if err := os.WriteFile(file, []byte("pulled"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if !Recent(file) {
+		t.Fatal("the file OpenSave wrote is not reported as ours")
+	}
+	// The game saves a little later.
+	later := time.Now().Add(5 * time.Second)
+	if err := os.WriteFile(file, []byte("played"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chtimes(file, later, later); err != nil {
+		t.Fatal(err)
+	}
+	if Recent(file) {
+		t.Error("a save written after OpenSave's was taken for OpenSave's")
+	}
+}
+
+// A file OpenSave has just written, then deleted by a game or a person, is
+// their deletion: it has to sync and be snapshotted. Only a path OpenSave
+// removed itself is its own when found gone.
+func TestRecent_AFileDeletedAfterOurWriteIsNotOurs(t *testing.T) {
+	dir := t.TempDir()
+	pulled := filepath.Join(dir, "pulled.sav")
+	Mark(pulled)
+	if err := os.WriteFile(pulled, []byte("from the peer"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Remove(pulled); err != nil {
+		t.Fatal(err)
+	}
+	if Recent(pulled) {
+		t.Error("a file deleted by someone else after OpenSave wrote it was taken for OpenSave's")
+	}
+
+	removed := filepath.Join(dir, "removed.sav")
+	if err := os.WriteFile(removed, []byte("old"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	MarkRemoved(removed)
+	if err := os.Remove(removed); err != nil {
+		t.Fatal(err)
+	}
+	if !Recent(removed) {
+		t.Error("a file OpenSave removed was not reported as its own")
+	}
+}
+
+// A game saving over a file OpenSave has just pulled is the game's change
+// however soon it comes - inside the slack a time comparison allows too.
+func TestRecent_AWriteRightAfterASettledPullIsNotOurs(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "progress.sav")
+	Mark(file)
+	if err := os.WriteFile(file, []byte("from A"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	Settled(file)
+	if !Recent(file) {
+		t.Fatal("the file OpenSave pulled is not reported as ours")
+	}
+	// Same size, a moment later: only the time tells them apart.
+	time.Sleep(20 * time.Millisecond)
+	if err := os.WriteFile(file, []byte("from B"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if Recent(file) {
+		t.Error("a save written straight after the pull was taken for OpenSave's")
+	}
+}
+
+// A pull renames a file into place, then re-dates it to the peer's time. In
+// between, and before it is Settled again, the file is still OpenSave's.
+func TestRecent_ARedatedPullBeforeItSettlesAgainIsOurs(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "auto.sav")
+	Mark(file)
+	if err := os.WriteFile(file, []byte("pulled"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	Settled(file)
+	Mark(file)
+	peers := time.Now().Add(-time.Hour)
+	if err := os.Chtimes(file, peers, peers); err != nil {
+		t.Fatal(err)
+	}
+	if !Recent(file) {
+		t.Error("a pulled file re-dated to the peer's time was taken for someone else's change")
+	}
+	Settled(file)
+	if !Recent(file) {
+		t.Error("a pulled file settled at the peer's time was taken for someone else's change")
+	}
+}
diff --git a/internal/p2p/deletion_race_test.go b/internal/p2p/deletion_race_test.go
index 09f46f32..952fcb19 100644
--- a/internal/p2p/deletion_race_test.go
+++ b/internal/p2p/deletion_race_test.go
@@ -58,7 +58,7 @@ func (a *deviceA) FetchManifest(ctx context.Context, peer syncengine.Peer, gameI
 	if err != nil {
 		return syncengine.ManifestResponse{}, err
 	}
-	return syncengine.ManifestResponse{Manifest: m, ActiveBranch: "main", Proto: syncengine.ProtoMultiRoot}, nil
+	return syncengine.ManifestResponse{Manifest: m, ActiveBranch: "main", Proto: syncengine.ProtoVersions}, nil
 }
 
 func (a *deviceA) FetchBlocks(ctx context.Context, peer syncengine.Peer, ref syncengine.FileRef, blockIndices []int, blockSize int) ([]syncengine.BlockData, error) {
@@ -159,7 +159,7 @@ func writeFile(t *testing.T, path, body string) {
 // deleteRequest is one of A's deletion requests arriving at B's LAN route.
 func (f *raceFixture) deleteRequest(t *testing.T, rel, root string) {
 	t.Helper()
-	body, _ := json.Marshal(map[string]string{"relPath": rel, "root": root})
+	body, _ := json.Marshal(map[string]any{"relPath": rel, "root": root, "versioned": true})
 	r := httptest.NewRequest(http.MethodPost, "/api/p2p/delete-file/"+f.gameID, bytes.NewReader(body))
 	r.RemoteAddr = "198.51.100.7:40000" // not a paired device's address: no lineage refresh races the test
 	rc := chi.NewRouteContext()
@@ -172,6 +172,26 @@ func (f *raceFixture) deleteRequest(t *testing.T, rel, root string) {
 	}
 }
 
+// A deletion asked for by an OpenSave that keeps no save versions is refused:
+// its old file comparison reads whatever its copy lacks as deleted.
+func TestADeletionFromAnOldBuildIsRefused(t *testing.T) {
+	f := newRaceFixture(t, false)
+	body, _ := json.Marshal(map[string]any{"relPath": "slot1.sav"})
+	r := httptest.NewRequest(http.MethodPost, "/api/p2p/delete-file/"+f.gameID, bytes.NewReader(body))
+	r.RemoteAddr = "198.51.100.7:40000"
+	rc := chi.NewRouteContext()
+	rc.URLParams.Add("gameId", f.gameID)
+	r = r.WithContext(context.WithValue(r.Context(), chi.RouteCtxKey, rc))
+	w := httptest.NewRecorder()
+	f.e.handleDeleteFile(w, r)
+	if w.Code != http.StatusConflict {
+		t.Errorf("a deletion from an old build: %d %s, want %d", w.Code, w.Body.String(), http.StatusConflict)
+	}
+	if !f.has(f.bDir, "slot1.sav") {
+		t.Error("the file was deleted on an old build's word")
+	}
+}
+
 func (f *raceFixture) has(dir, rel string) bool {
 	_, err := os.Stat(filepath.Join(dir, filepath.FromSlash(rel)))
 	return err == nil
@@ -212,7 +232,7 @@ func TestASyncBetweenTwoRelayedDeletionsIsNotAConflict(t *testing.T) {
 	os.Remove(filepath.Join(f.a.dir, "slot1.sav"))
 	os.Remove(filepath.Join(f.a.dir, "slot2.sav"))
 
-	body, _ := json.Marshal(map[string]string{"relPath": "slot1.sav"})
+	body, _ := json.Marshal(map[string]any{"relPath": "slot1.sav", "versioned": true})
 	if code, out := w.serveDeleteFile("/delete-file/"+f.gameID, body, "node_unknown"); code != 200 {
 		t.Fatalf("relayed delete: %d %v", code, out)
 	}
diff --git a/internal/p2p/engine.go b/internal/p2p/engine.go
index b1777e2e..83e68ea7 100644
--- a/internal/p2p/engine.go
+++ b/internal/p2p/engine.go
@@ -9,11 +9,15 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io/fs"
 	"net/http"
+	"os"
+	"path/filepath"
 	"strings"
 	"sync"
 	"time"
 
+	"github.com/opensave/opensave/internal/delta"
 	"github.com/opensave/opensave/internal/e2ee"
 	"github.com/opensave/opensave/internal/p2p/discovery"
 	"github.com/opensave/opensave/internal/p2p/pairing"
@@ -119,6 +123,20 @@ type Engine struct {
 	// that a device has gone away — see offlineStrikes.
 	pingMu     sync.Mutex
 	pingMisses map[string]int
+	// probeMu serializes PingPairedPeers rounds.
+	probeMu sync.Mutex
+	// probingLinks: peers whose connection is being speed-tested now
+	// (probeLinkSoon).
+	probeLinksMu sync.Mutex
+	probingLinks map[string]bool
+	// When each game last got a safety snapshot before a peer's deletions
+	// (snapshotBeforePeerDeletions).
+	peerDeleteSnapMu sync.Mutex
+	peerDeleteSnap   map[string]time.Time
+	// answered holds the peers that answered a probe during this run, and
+	// startedMs is when the run began (heardThisRun).
+	answered  map[string]bool
+	startedMs int64
 
 	// Lineage refreshes already running after a peer-applied deletion, keyed
 	// by game+peer. Deletions arrive one file at a time, and each one leaves
@@ -283,6 +301,7 @@ func New(s *store.Store, snaps *snapshot.Manager, logf func(level, msg string))
 		Log:       logf,
 		ctx:       ctx,
 		cancel:    cancel,
+		startedMs: time.Now().UnixMilli(),
 	}
 	e.Wan = newWanClient(e)
 	e.RelayHost = NewRelayHost(logf)
@@ -379,9 +398,34 @@ func (e *Engine) clearPingMisses(peerID string) {
 	delete(e.pingMisses, peerID)
 }
 
+// noteAnswered records that a peer answered a probe during this run.
+func (e *Engine) noteAnswered(peerID string) {
+	e.pingMu.Lock()
+	defer e.pingMu.Unlock()
+	if e.answered == nil {
+		e.answered = map[string]bool{}
+	}
+	e.answered[peerID] = true
+}
+
+// heardThisRun reports whether there is any sign of a peer since this engine
+// started: a probe it answered, or a sighting (discovery, a request from it,
+// the relay) recent enough to postdate the start.
+func (e *Engine) heardThisRun(p store.Peer) bool {
+	e.pingMu.Lock()
+	answered := e.answered[p.ID]
+	e.pingMu.Unlock()
+	return answered || p.LastSeenMs >= e.startedMs
+}
+
 // PingPairedPeers probes every paired LAN peer and updates their
 // online/offline status.
 func (e *Engine) PingPairedPeers(ctx context.Context) {
+	// One probe round at a time: the presence loop and a sync starting can
+	// both ask, and two overlapping rounds would count one outage as two
+	// misses toward offlineStrikes.
+	e.probeMu.Lock()
+	defer e.probeMu.Unlock()
 	peers, err := e.Store.ListPeers()
 	if err != nil {
 		return
@@ -414,11 +458,19 @@ func (e *Engine) PingPairedPeers(ctx context.Context) {
 		// failed cannot lose anything: a device that really has gone is
 		// declared offline a few probes later, and the only cost is a sync
 		// attempt that fails the way it would have anyway.
+		//
+		// That patience is for a device seen during this run. A status
+		// carried over from the previous run is not evidence of anything: a
+		// PC switched off overnight came back "online" from the database on
+		// every start, and stayed so until three probes had failed. A device
+		// not heard from since this start goes offline on its first miss.
 		newStatus := p.Status
 		if ok {
 			e.clearPingMisses(p.ID)
+			e.noteAnswered(p.ID)
 			newStatus = "online"
-		} else if e.notePingMiss(p.ID) >= offlineStrikes {
+			e.probeLinkSoon(p)
+		} else if misses := e.notePingMiss(p.ID); misses >= offlineStrikes || !e.heardThisRun(p) {
 			newStatus = "offline"
 		}
 
@@ -434,6 +486,35 @@ func (e *Engine) PingPairedPeers(ctx context.Context) {
 	}
 }
 
+// probeLinkSoon tests the connection to a peer that just answered, in the
+// background, when nothing has measured it for a while
+// (syncengine/linkspeed.go). One test per peer at a time.
+func (e *Engine) probeLinkSoon(p store.Peer) {
+	if e.Sync == nil || p.Address == "relay" {
+		return
+	}
+	e.probeLinksMu.Lock()
+	if e.probingLinks == nil {
+		e.probingLinks = map[string]bool{}
+	}
+	if e.probingLinks[p.ID] {
+		e.probeLinksMu.Unlock()
+		return
+	}
+	e.probingLinks[p.ID] = true
+	e.probeLinksMu.Unlock()
+	go func() {
+		defer func() {
+			e.probeLinksMu.Lock()
+			delete(e.probingLinks, p.ID)
+			e.probeLinksMu.Unlock()
+		}()
+		if e.Sync.ProbeLinkIfDue(context.Background(), syncengine.Peer{ID: p.ID, Name: p.Name, Address: p.Address, Port: p.Port}) {
+			e.notifyPeerUpdate()
+		}
+	}()
+}
+
 // ErrNoPeersOnline is a sync with no other device to sync with: none of the
 // paired devices answered. Not a failure of anything — the save goes over when
 // one is back — which is why it is told apart from errors that are.
@@ -542,6 +623,8 @@ func (e *Engine) StartResyncLoop() {
 	stop := e.stopRetry
 	e.pendingMu.Unlock()
 
+	e.startPresenceLoop(stop)
+
 	// Tracked on the engine's lifecycle so a shutdown lands between ticks
 	// rather than half-way through a transfer.
 	e.GoSync(func(ctx context.Context) {
@@ -571,6 +654,43 @@ func (e *Engine) StartResyncLoop() {
 	})
 }
 
+// presenceInterval is how often paired LAN peers are probed just to know
+// whether they are there.
+//
+// Probing used to happen only as a side effect: before the 60-second
+// reconcile, before a retry, before a manual sync. The reconcile runs on the
+// same goroutine as the sync-everything pass it starts, so while a big
+// library synced the next probe waited minutes, and with offlineStrikes
+// that was minutes more before a device that had gone showed as gone. Peers
+// seen by UDP broadcast hid this; one reached over a VPN such as Tailscale,
+// which does not carry broadcasts, depends on probes alone.
+//
+// 10s makes "back" show within ~10s and "gone" within ~30s. A probe is one
+// small GET per peer. A var only so a test can shorten it.
+var presenceInterval = 10 * time.Second
+
+// startPresenceLoop probes paired peers on its own ticker, independent of
+// whatever sync work is running. It stops with the resync loop.
+func (e *Engine) startPresenceLoop(stop chan struct{}) {
+	e.GoSync(func(ctx context.Context) {
+		// At once, then on the ticker: until the first round, every peer
+		// shows whatever the previous run left in the database.
+		e.PingPairedPeers(ctx)
+		ticker := time.NewTicker(presenceInterval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-stop:
+				return
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				e.PingPairedPeers(ctx)
+			}
+		}
+	})
+}
+
 // reconcileAllGames refreshes peer status and re-syncs every game — the
 // periodic backstop against missed sync triggers.
 func (e *Engine) reconcileAllGames(ctx context.Context) {
@@ -602,6 +722,13 @@ func (e *Engine) retryPendingResyncs(ctx context.Context) {
 		if ctx.Err() != nil {
 			return // shutting down; the failsafe picks these up next start
 		}
+		// A save folder that is not here cannot be synced, and retrying it
+		// every tick only logs the same failure. Dropped from the queue; the
+		// periodic reconcile picks it up once the folder is back.
+		if g, err := e.Store.GetGame(id); err == nil && saveFolderAbsent(g.SavePath) {
+			e.ClearPendingResync(id)
+			continue
+		}
 		e.Log("info", fmt.Sprintf("retrying interrupted sync for %s", id))
 		results, err := e.Sync.SyncGame(ctx, id, online)
 		if err == nil {
@@ -616,6 +743,22 @@ func (e *Engine) retryPendingResyncs(ctx context.Context) {
 	}
 }
 
+// saveFolderAbsent reports whether a game's save folder is not on this device
+// — the same test as daemon.SaveFolderMissing, which this package cannot
+// import. For a single-file save it is the folder the file goes in that
+// counts: the file itself may not have been written yet.
+func saveFolderAbsent(savePath string) bool {
+	if savePath == "" {
+		return false
+	}
+	folder := savePath
+	if isFile, err := delta.ResolveLocalSaveFilePath(savePath); err == nil && isFile {
+		folder = filepath.Dir(savePath)
+	}
+	_, err := os.Stat(folder)
+	return errors.Is(err, fs.ErrNotExist)
+}
+
 // SyncAllGames syncs every tracked game (used when a peer comes online).
 func (e *Engine) SyncAllGames(ctx context.Context) {
 	if e.Pause.Paused() {
@@ -633,6 +776,12 @@ func (e *Engine) SyncAllGames(ctx context.Context) {
 		if !g.AutoSync {
 			continue
 		}
+		// Not on this device (a drive it doesn't have, a folder that is
+		// gone): nothing to sync, and it is shown as such on screen. Trying
+		// anyway logged an error for it against every peer, every pass.
+		if saveFolderAbsent(g.SavePath) {
+			continue
+		}
 		results, err := e.Sync.SyncGame(ctx, g.ID, online)
 		if errors.Is(err, syncengine.ErrHeld) {
 			continue // said once, when it was held; asked about on screen
diff --git a/internal/p2p/manifest_gzip_test.go b/internal/p2p/manifest_gzip_test.go
new file mode 100644
index 00000000..cca409cf
--- /dev/null
+++ b/internal/p2p/manifest_gzip_test.go
@@ -0,0 +1,63 @@
+package p2p
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/p2p/syncengine"
+)
+
+// A large manifest goes over the wire gzipped, and an ordinary Go client —
+// which is what every OpenSave version uses — reads it without knowing.
+func TestManifestIsCompressedAndReadable(t *testing.T) {
+	m := delta.Manifest{Files: map[string]delta.FileEntry{}}
+	for i := 0; i < 5000; i++ {
+		sum := sha256.Sum256([]byte(fmt.Sprint(i))) // random-looking, like real hashes
+		h := hex.EncodeToString(sum[:])
+		m.Files[fmt.Sprintf("Saves/world/map_%d_%d.bin", i/100, i%100)] = delta.FileEntry{
+			Size: 3500, Hash: h, BlockSize: 65536, Blocks: []delta.Block{{Index: 0, Hash: h, Length: 3500}},
+		}
+	}
+	resp := syncengine.ManifestResponse{Manifest: m, ActiveBranch: "main"}
+
+	var wire atomic.Int64
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cw := &countingResponse{ResponseWriter: w}
+		jsonOKCompressed(cw, r, resp)
+		wire.Store(cw.n)
+	}))
+	defer srv.Close()
+
+	req, _ := http.NewRequest(http.MethodGet, srv.URL, nil)
+	var got syncengine.ManifestResponse
+	if err := doJSONWith(lanBulkClient, req, &got); err != nil {
+		t.Fatal(err)
+	}
+	if len(got.Manifest.Files) != len(m.Files) || got.Manifest.ManifestHash() != m.ManifestHash() {
+		t.Fatalf("manifest came back with %d files and a different hash", len(got.Manifest.Files))
+	}
+
+	plain := httptest.NewRecorder()
+	jsonOK(plain, resp)
+	if wire.Load()*2 > int64(plain.Body.Len()) {
+		t.Errorf("compressed %d bytes vs %d plain — expected at least 2x smaller", wire.Load(), plain.Body.Len())
+	}
+	t.Logf("manifest of %d files: %d bytes plain, %d on the wire", len(m.Files), plain.Body.Len(), wire.Load())
+}
+
+type countingResponse struct {
+	http.ResponseWriter
+	n int64
+}
+
+func (c *countingResponse) Write(b []byte) (int, error) {
+	n, err := c.ResponseWriter.Write(b)
+	c.n += int64(n)
+	return n, err
+}
diff --git a/internal/p2p/missing_skip_test.go b/internal/p2p/missing_skip_test.go
new file mode 100644
index 00000000..b5de9828
--- /dev/null
+++ b/internal/p2p/missing_skip_test.go
@@ -0,0 +1,28 @@
+package p2p
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSaveFolderAbsent(t *testing.T) {
+	dir := t.TempDir()
+	if saveFolderAbsent(dir) {
+		t.Error("an existing folder was reported absent")
+	}
+	if !saveFolderAbsent(filepath.Join(dir, "gone")) {
+		t.Error("a missing folder was not reported absent")
+	}
+	// A single-file save not written yet: its folder is what counts.
+	file := filepath.Join(dir, "save.sav")
+	if err := os.WriteFile(file, []byte("x"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if saveFolderAbsent(file) {
+		t.Error("an existing single-file save was reported absent")
+	}
+	if saveFolderAbsent("") {
+		t.Error("an empty path was reported absent")
+	}
+}
diff --git a/internal/p2p/presence_test.go b/internal/p2p/presence_test.go
new file mode 100644
index 00000000..aca90a06
--- /dev/null
+++ b/internal/p2p/presence_test.go
@@ -0,0 +1,125 @@
+package p2p
+
+import (
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"strconv"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/internal/snapshot"
+	"github.com/opensave/opensave/internal/store"
+)
+
+func waitUntil(d time.Duration, cond func() bool) bool {
+	deadline := time.Now().Add(d)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return true
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	return cond()
+}
+
+// A PC switched off since the last run is in the database as "online". It must
+// not stay so for the three probes a device seen this run is allowed: the
+// first unanswered probe after start says it is not there.
+func TestPresence_StaleOnlineFromLastRunClearsAtOnce(t *testing.T) {
+	old := presenceInterval
+	presenceInterval = time.Hour // only the probe at start
+	t.Cleanup(func() { presenceInterval = old })
+
+	s, err := store.Open(filepath.Join(t.TempDir(), "opensave.db"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { s.Close() })
+	if err := s.EnsureDefaultSettings(t.TempDir(), t.TempDir()); err != nil {
+		t.Fatal(err)
+	}
+	// Nothing listens here: the PC is off.
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	port := ln.Addr().(*net.TCPAddr).Port
+	ln.Close()
+	lastRun := time.Now().Add(-10 * time.Hour).UnixMilli()
+	if err := s.UpsertPeer(store.Peer{ID: "node_off", Name: "Switched off", Address: "127.0.0.1", Port: port, Status: "online", LastSeenMs: lastRun}); err != nil {
+		t.Fatal(err)
+	}
+
+	e := New(s, snapshot.New(s), func(string, string) {})
+	stop := make(chan struct{})
+	start := time.Now()
+	e.startPresenceLoop(stop)
+	t.Cleanup(func() { close(stop); e.cancel(); e.bgSyncWG.Wait() })
+
+	if !waitUntil(5*time.Second, func() bool {
+		p, err := s.GetPeer("node_off")
+		return err == nil && p.Status == "offline"
+	}) {
+		t.Fatal("a peer online only in the last run's record was still online after the first probe")
+	}
+	t.Logf("shown offline %.1fs after start", time.Since(start).Seconds())
+}
+
+// A peer's online state follows it on its own ticker, whatever else the
+// engine is doing — not only when a sync or the minute-long reconcile happens
+// to probe. A peer reached over a VPN gets no UDP broadcasts, so without this
+// its indicator lagged by minutes.
+func TestPresenceLoop_FollowsPeer(t *testing.T) {
+	old := presenceInterval
+	presenceInterval = 50 * time.Millisecond
+	t.Cleanup(func() { presenceInterval = old })
+
+	var up atomic.Bool
+	up.Store(true)
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !up.Load() {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"status":"ok","appVersion":"2.4.0"}`))
+	}))
+	t.Cleanup(srv.Close)
+	host, portStr, _ := net.SplitHostPort(srv.Listener.Addr().String())
+	port, _ := strconv.Atoi(portStr)
+
+	s, err := store.Open(filepath.Join(t.TempDir(), "opensave.db"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { s.Close() })
+	if err := s.EnsureDefaultSettings(t.TempDir(), t.TempDir()); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.UpsertPeer(store.Peer{ID: "node_vpn", Name: "Over VPN", Address: host, Port: port, Status: "offline"}); err != nil {
+		t.Fatal(err)
+	}
+
+	e := New(s, snapshot.New(s), func(string, string) {})
+	stop := make(chan struct{})
+	e.startPresenceLoop(stop)
+	t.Cleanup(func() { close(stop); e.cancel(); e.bgSyncWG.Wait() })
+
+	status := func() string {
+		p, err := s.GetPeer("node_vpn")
+		if err != nil {
+			return ""
+		}
+		return p.Status
+	}
+	if !waitUntil(3*time.Second, func() bool { return status() == "online" }) {
+		t.Fatalf("peer answering pings still %q", status())
+	}
+	up.Store(false)
+	if !waitUntil(3*time.Second, func() bool { return status() == "offline" }) {
+		t.Fatalf("peer no longer answering still %q", status())
+	}
+}
diff --git a/internal/p2p/routes.go b/internal/p2p/routes.go
index 67d11a5c..79a48b0c 100644
--- a/internal/p2p/routes.go
+++ b/internal/p2p/routes.go
@@ -1,7 +1,9 @@
 package p2p
 
 import (
+	"compress/gzip"
 	"context"
+	"crypto/rand"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -11,12 +13,14 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"sync/atomic"
 	"time"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/opensave/opensave/internal/delta"
 	"github.com/opensave/opensave/internal/logging"
+	"github.com/opensave/opensave/internal/owntouch"
 	"github.com/opensave/opensave/internal/p2p/pairing"
 	"github.com/opensave/opensave/internal/p2p/syncengine"
 	"github.com/opensave/opensave/internal/store"
@@ -40,18 +44,48 @@ func (e *Engine) RegisterRoutes(r chi.Router) {
 		r.Get("/api/p2p/games", e.handlePeerGameList)
 		r.Post("/api/p2p/sync-event/{gameId}", e.handleSyncEvent)
 		r.Get("/api/p2p/app-binary", e.handleAppBinary)
+		r.Get("/api/p2p/speedtest", e.handleSpeedTest)
 
 		// What moves save data, or starts a sync: refused while paused.
 		r.Group(func(r chi.Router) {
 			r.Use(e.refuseWhilePaused)
 			r.Get("/api/p2p/manifest/{gameId}", e.handleManifest)
 			r.Post("/api/p2p/blocks/{gameId}", e.handleBlocks)
+			r.Post("/api/p2p/files/{gameId}", e.handleFileBatch)
 			r.Post("/api/p2p/delete-file/{gameId}", e.handleDeleteFile)
 			r.Get("/api/sync/trigger/{gameId}", e.handleSyncTrigger)
 		})
 	})
 }
 
+// speedTestMax bounds one speed test, and speedTestData is what it sends:
+// random, so nothing along the way can compress it into a better figure.
+const speedTestMax = 8 << 20
+
+var speedTestData = func() []byte {
+	b := make([]byte, 1<<20)
+	_, _ = rand.Read(b)
+	return b
+}()
+
+// handleSpeedTest sends n bytes (at most speedTestMax) for a peer to time:
+// how it learns how close this device is (syncengine/linkspeed.go).
+func (e *Engine) handleSpeedTest(w http.ResponseWriter, r *http.Request) {
+	n, _ := strconv.Atoi(r.URL.Query().Get("n"))
+	if n <= 0 || n > speedTestMax {
+		n = speedTestMax
+	}
+	w.Header().Set("Content-Type", "application/octet-stream")
+	w.Header().Set("Content-Length", strconv.Itoa(n))
+	for n > 0 {
+		chunk := min(n, len(speedTestData))
+		if _, err := w.Write(speedTestData[:chunk]); err != nil {
+			return
+		}
+		n -= chunk
+	}
+}
+
 func clientIP(r *http.Request) string {
 	host, _, err := net.SplitHostPort(r.RemoteAddr)
 	if err != nil {
@@ -678,6 +712,11 @@ func (e *Engine) handleManifest(w http.ResponseWriter, r *http.Request) {
 		jsonError(w, http.StatusNotFound, syncengine.HeldMessage)
 		return
 	}
+	// Being played here: synced when the session ends (syncengine/playing.go).
+	if e.Sync.PlayingHere(game.ID) {
+		jsonError(w, http.StatusServiceUnavailable, syncengine.PlayingMessage)
+		return
+	}
 	// Never describe a save a sync here is writing: part-way through, it is a
 	// mixture no device holds, and the asker would judge it as a save that had
 	// moved (syncengine/settle.go). Held still while it is read; a write in
@@ -720,11 +759,25 @@ func (e *Engine) handleManifest(w http.ResponseWriter, r *http.Request) {
 		ActiveBranch:      game.ActiveBranch,
 		Proto:             ServedProto(),
 		DeletionConfirmed: e.Sync.DeletionConfirmed(game.ID),
+		Version:           e.Sync.LocalVersion(game, manifest),
+		Versions:          true,
 	}
 	if latest, err := e.Snapshots.LatestSnapshot(gameID, ""); err == nil {
-		resp.LatestSnapshot = &syncengine.SnapshotInfo{ID: latest.ID, Timestamp: latest.Timestamp, Comment: latest.Comment}
-	}
-	jsonOK(w, resp)
+		resp.LatestSnapshot = &syncengine.SnapshotInfo{ID: latest.ID, Timestamp: latest.Timestamp, Comment: latest.Comment, ContentHash: latest.ContentHash}
+	}
+	// The asker already holds the state both devices last agreed on and says
+	// so; if this device still holds it too, "unchanged" is the whole answer.
+	// For a save of a quarter-million files the full manifest is tens of MB
+	// of JSON, and the minute-long reconcile used to fetch it from every peer
+	// every minute to learn that nothing had changed. Single-location games
+	// only: extra locations carry their own bases (see RootHash).
+	if ifHash := r.URL.Query().Get("ifHash"); ifHash != "" && len(manifest.Extra) == 0 &&
+		manifest.ManifestHash() == ifHash {
+		resp.Manifest = delta.Manifest{}
+		resp.Unchanged = true
+		resp.ManifestHash = ifHash
+	}
+	jsonOKCompressed(w, r, resp)
 }
 
 // holdForServing holds the game's save still for a manifest to be served from
@@ -787,11 +840,141 @@ func (e *Engine) handleBlocks(w http.ResponseWriter, r *http.Request) {
 	jsonOK(w, map[string]any{"blocks": encodeBlocks(blocks, wantsGzip(body.Encodings))})
 }
 
+// Bounds on one batch request, so a peer cannot make this device read and
+// hold an unbounded amount in a single response. The requester stays well
+// under them (syncengine.batchMaxFiles / batchMaxBytes).
+const (
+	fileBatchMaxFiles = 1024
+	fileBatchMaxBytes = 16 << 20
+)
+
+// handleFileBatch serves the blocks of many files of one save location in a
+// single response: handleBlocks for a list. One round trip per file was what
+// made a save of many small files take hours; see syncengine.ProtoBatchFiles.
+//
+// A file that cannot be read is reported in its own entry rather than failing
+// the batch, so the requester can say which one it was.
+func (e *Engine) handleFileBatch(w http.ResponseWriter, r *http.Request) {
+	gameID := chi.URLParam(r, "gameId")
+	var body struct {
+		Root      string                         `json:"root"`
+		Files     []syncengine.FileBlocksRequest `json:"files"`
+		Encodings []string                       `json:"encodings"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil || len(body.Files) == 0 {
+		jsonError(w, http.StatusBadRequest, "files are required")
+		return
+	}
+	if len(body.Files) > fileBatchMaxFiles {
+		jsonError(w, http.StatusBadRequest, fmt.Sprintf("at most %d files per batch", fileBatchMaxFiles))
+		return
+	}
+	game, err := e.trackedGameForPeer(gameID)
+	if err != nil {
+		jsonError(w, http.StatusNotFound, "Game not found.")
+		return
+	}
+	base, ok := e.resolveServeRoot(gameID, game, body.Root)
+	if !ok {
+		jsonError(w, http.StatusNotFound, "This device has no save location named "+strconv.Quote(body.Root)+" for that game.")
+		return
+	}
+	singleFile, _ := delta.ResolveLocalSaveFilePath(base)
+
+	// What the answer will hold: each file's requested blocks at its block
+	// size, but never more than the file itself. Counting every block at full
+	// size took 256 files of 4 KB for 16 MB — the whole limit — and refused
+	// any batch of small files with a few larger ones among them, which failed
+	// whole pulls of saves made of many small files.
+	var requested int64
+	for _, f := range body.Files {
+		bs := f.BlockSize
+		if bs <= 0 {
+			bs = 64 * 1024
+		}
+		n := int64(len(f.BlockIndices)) * int64(bs)
+		if f.RelPath != "" && delta.IsSafePath(base, f.RelPath) {
+			p := delta.LocalNameFor(base, f.RelPath)
+			if singleFile {
+				p = base
+			}
+			if info, err := os.Stat(p); err == nil && info.Size() < n {
+				n = info.Size()
+			}
+		}
+		requested += n
+	}
+	if requested > fileBatchMaxBytes {
+		jsonError(w, http.StatusBadRequest, "batch too large")
+		return
+	}
+	gz := wantsGzip(body.Encodings)
+
+	out := make([]syncengine.FileBlocks, 0, len(body.Files))
+	for _, f := range body.Files {
+		entry := syncengine.FileBlocks{RelPath: f.RelPath}
+		if f.RelPath == "" || !delta.IsSafePath(base, f.RelPath) {
+			entry.Error = "invalid path"
+			out = append(out, entry)
+			continue
+		}
+		// Resolved, not joined — see handleBlocks.
+		fullPath := delta.LocalNameFor(base, f.RelPath)
+		if singleFile {
+			fullPath = base
+		}
+		blocks, err := delta.ReadBlocks(fullPath, f.BlockIndices, f.BlockSize)
+		if err != nil {
+			entry.Error = "read blocks failed: " + err.Error()
+		} else {
+			entry.Blocks = encodeBlocks(blocks, gz)
+		}
+		out = append(out, entry)
+	}
+	jsonOK(w, map[string]any{"files": out})
+}
+
+// peerDeleteSnapshotEvery is how far apart two safety snapshots before a
+// peer's deletions may be: one for a burst of deletions, however many files
+// it removes one request at a time.
+const peerDeleteSnapshotEvery = 10 * time.Minute
+
+// snapshotBeforePeerDeletions takes a safety snapshot of a game before the
+// first of a burst of deletions a peer asks for.
+func (e *Engine) snapshotBeforePeerDeletions(gameID string, r *http.Request) {
+	if e.Snapshots == nil {
+		return
+	}
+	e.peerDeleteSnapMu.Lock()
+	if e.peerDeleteSnap == nil {
+		e.peerDeleteSnap = map[string]time.Time{}
+	}
+	if last, ok := e.peerDeleteSnap[gameID]; ok && time.Since(last) < peerDeleteSnapshotEvery {
+		e.peerDeleteSnapMu.Unlock()
+		return
+	}
+	e.peerDeleteSnap[gameID] = time.Now()
+	e.peerDeleteSnapMu.Unlock()
+
+	asker := "another device"
+	if peer, ok := e.peerByAddress(clientIP(r)); ok {
+		asker = peer.Name
+	}
+	if _, err := e.Snapshots.CreateBeforeReplacing(gameID, fmt.Sprintf("Before %s deleted files here", asker)); err != nil {
+		e.Log("warn", fmt.Sprintf("safety snapshot before %s's deletions failed: %v", asker, err))
+	}
+}
+
 func (e *Engine) handleDeleteFile(w http.ResponseWriter, r *http.Request) {
 	gameID := chi.URLParam(r, "gameId")
 	var body struct {
 		RelPath string `json:"relPath"`
 		Root    string `json:"root"`
+		// Versioned: the asking device keeps save versions. One that does
+		// not decides deletions by the old file comparison, which reads
+		// whatever its copy lacks as deleted — the deletions that emptied
+		// whole saves when a copy had arrived only half-way.
+		Versioned bool `json:"versioned"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&body); err != nil || body.RelPath == "" {
 		jsonError(w, http.StatusBadRequest, "relPath is required.")
@@ -803,6 +986,19 @@ func (e *Engine) handleDeleteFile(w http.ResponseWriter, r *http.Request) {
 		jsonError(w, http.StatusNotFound, "Game not found.")
 		return
 	}
+	if e.Sync.PlayingHere(game.ID) {
+		jsonError(w, http.StatusServiceUnavailable, syncengine.PlayingMessage)
+		return
+	}
+	if game.AutoSync && !body.Versioned {
+		from := clientIP(r)
+		if p, ok := e.peerByAddress(from); ok {
+			from = p.Name
+		}
+		e.Sync.RefusedOldBuildDelete(game.Name, from)
+		jsonError(w, http.StatusConflict, syncengine.OldBuildDeleteMessage)
+		return
+	}
 	base, ok := e.resolveServeRoot(gameID, game, body.Root)
 	if !ok {
 		jsonError(w, http.StatusNotFound, "This device has no save location named "+strconv.Quote(body.Root)+" for that game.")
@@ -821,6 +1017,11 @@ func (e *Engine) handleDeleteFile(w http.ResponseWriter, r *http.Request) {
 	_ = os.Chmod(full, 0o666)
 	deleting := time.Now()
 	if info, statErr := os.Stat(full); statErr == nil {
+		// One copy of the save before a peer starts deleting from it, not
+		// one per file: the watcher no longer snapshots changes OpenSave
+		// applies (owntouch), and it was those, one every few seconds of a
+		// half-deleted save, that had been the only copy before.
+		e.snapshotBeforePeerDeletions(game.ID, r)
 		// What is removed is remembered, so a sync of this device's own that
 		// lands before the rest of the batch does not take it for a change
 		// made here (syncengine/peerdeleted.go).
@@ -829,6 +1030,8 @@ func (e *Engine) handleDeleteFile(w http.ResponseWriter, r *http.Request) {
 			entry, _ = delta.FileEntryFor(full)
 		}
 		// Empty dirs only, for a folder, like rmdirSync.
+		// A change made at a peer's request, not by the game (owntouch).
+		owntouch.MarkRemoved(full)
 		if os.Remove(full) == nil && (info.IsDir() || entry.Hash != "") {
 			e.Sync.NotePeerDeletion(game.ID, body.Root, body.RelPath, entry, info.IsDir())
 		}
@@ -939,6 +1142,10 @@ func (e *Engine) handleSyncEvent(w http.ResponseWriter, r *http.Request) {
 		if e.Sync.Progress.OnSyncError != nil {
 			e.Sync.Progress.OnSyncError(gameID, ev)
 		}
+		// A sync here waiting for this peer to finish need not wait on.
+		if peer, ok := e.peerByAddress(clientIP(r)); ok {
+			e.Sync.NotePeerPulled(gameID, peer.ID)
+		}
 	}
 	jsonOK(w, map[string]any{"success": true})
 }
@@ -979,6 +1186,32 @@ func progressEventFromMap(data map[string]any) syncengine.ProgressEvent {
 	return ev
 }
 
+// jsonOKCompressed is jsonOK, gzipped when the asker accepts it.
+//
+// For manifests. A save of a few hundred thousand files makes a manifest of
+// tens of MB of JSON, which over a VPN did not arrive inside a peer's request
+// timeout — so the peer never learned what this device holds, every sync of
+// that game failed, and the two devices never converged. Manifest JSON (paths
+// and hex hashes) compresses several times over. Go's HTTP client asks for
+// gzip and unpacks it on its own unless told not to, so every version of
+// OpenSave reads these, including ones that predate this.
+func jsonOKCompressed(w http.ResponseWriter, r *http.Request, v any) {
+	if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
+		jsonOK(w, v)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Content-Encoding", "gzip")
+	w.Header().Add("Vary", "Accept-Encoding")
+	w.WriteHeader(http.StatusOK)
+	zw, err := gzip.NewWriterLevel(w, gzip.BestSpeed)
+	if err != nil {
+		return
+	}
+	_ = json.NewEncoder(zw).Encode(v)
+	_ = zw.Close()
+}
+
 func jsonOK(w http.ResponseWriter, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusOK)
@@ -1019,7 +1252,7 @@ func (e *Engine) resolveServeRoot(gameID string, game store.Game, root string) (
 // argument — the read still happens concurrently.
 var servedProto atomic.Int64
 
-func init() { servedProto.Store(syncengine.ProtoMultiRoot) }
+func init() { servedProto.Store(syncengine.ProtoVersions) }
 
 // ServedProto reports the protocol revision advertised to peers.
 func ServedProto() int { return int(servedProto.Load()) }
@@ -1040,6 +1273,8 @@ func SetServedProto(v int) int {
 // freshly-pushed files deliberately stay out of it (see persistLineage), so
 // deleting one here would pull it back instead of propagating the delete.
 func (e *Engine) peerFinishedPulling(gameID string, peer syncengine.Peer, data map[string]any) {
+	// A sync here waiting for this peer before slower ones goes on.
+	defer e.Sync.NotePeerPulled(gameID, peer.ID)
 	// Recorded first, and synchronously: the peer said exactly which files it
 	// wrote, so the lineage can be updated now rather than after a manifest
 	// round trip. That round trip is what left a window in which deleting a
diff --git a/internal/p2p/syncengine/batch.go b/internal/p2p/syncengine/batch.go
new file mode 100644
index 00000000..c1b1ff23
--- /dev/null
+++ b/internal/p2p/syncengine/batch.go
@@ -0,0 +1,301 @@
+package syncengine
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/owntouch"
+)
+
+// Pulling many small files, many to a request.
+//
+// A pull used to cost one HTTP round trip per file, one after the other.
+// Measured on a Windows LAN, that came to ~200ms a file, of which writing the
+// file (temp, fsync, rename) was ~2ms — so a save made of a quarter-million
+// small files took half a day to move 50MB. Batching removes the round trips;
+// the writing is the same as before, file by file, and just as safe: each
+// file still goes through a PatchWriter (temp file, content verified against
+// the manifest's hash, fsync, rename) and gets the peer's mtime.
+
+const (
+	// batchFileMaxBytes is the most a file may need fetched to go into a
+	// batch. Anything bigger keeps the per-file path, which already fetches
+	// its blocks in parallel and has nothing to gain here.
+	batchFileMaxBytes = 256 << 10
+	// batchMaxFiles and batchMaxBytes bound one request, well inside what the
+	// responder accepts (p2p.fileBatchMaxFiles / fileBatchMaxBytes).
+	batchMaxFiles = 256
+	batchMaxBytes = 4 << 20
+	// batchMaxClaimed bounds a request as the responder measures it: every
+	// block requested at the file's full block size, whatever the file's real
+	// size. 256 files of 4 KB claim 16 MB that way, which is the responder's
+	// whole limit (p2p.fileBatchMaxBytes), so any file of two blocks in such
+	// a batch had it refused as "batch too large" — and the whole pull with
+	// it, over and over, on saves of many small files. Kept under the limit
+	// with room to spare.
+	batchMaxClaimed = 12 << 20
+)
+
+// claimedBytes is what a file costs a batch as the responder counts it.
+func claimedBytes(f delta.FileEntry, indices []int) int64 {
+	bs := f.BlockSize
+	if bs <= 0 {
+		bs = 64 * 1024
+	}
+	return int64(len(indices)) * int64(bs)
+}
+
+// batchWorkers is how many batch requests are in flight at once: enough to
+// keep the link and the responder's disk busy while this side writes. A var
+// only so a test can make the order deterministic.
+var batchWorkers = 4
+
+type batchJob struct {
+	relPath   string
+	localPath string
+	remote    delta.FileEntry
+	indices   []int
+}
+
+// changedBytes is how much of a file a pull has to fetch.
+func changedBytes(f delta.FileEntry, indices []int) int64 {
+	var n int64
+	for _, idx := range indices {
+		if idx >= 0 && idx < len(f.Blocks) {
+			n += int64(f.Blocks[idx].Length)
+		}
+	}
+	return n
+}
+
+// groupBatches splits jobs into requests of at most batchMaxFiles files,
+// batchMaxBytes of blocks, and batchMaxClaimed as the responder counts them.
+func groupBatches(jobs []batchJob) [][]batchJob {
+	var out [][]batchJob
+	var cur []batchJob
+	var curBytes, curClaimed int64
+	for _, j := range jobs {
+		b := changedBytes(j.remote, j.indices)
+		c := claimedBytes(j.remote, j.indices)
+		if len(cur) > 0 && (len(cur) >= batchMaxFiles || curBytes+b > batchMaxBytes || curClaimed+c > batchMaxClaimed) {
+			out = append(out, cur)
+			cur, curBytes, curClaimed = nil, 0, 0
+		}
+		cur = append(cur, j)
+		curBytes += b
+		curClaimed += c
+	}
+	if len(cur) > 0 {
+		out = append(out, cur)
+	}
+	return out
+}
+
+// pullBatched fetches and writes jobs, many files per request. It returns the
+// paths it wrote, including on error, since those files are on disk.
+func (e *Engine) pullBatched(ctx context.Context, batcher BatchFetcher, peer Peer, gameID, root string,
+	jobs []batchJob, throttle *throttler, tracker *progressTracker, onProgress func(force bool)) ([]string, error) {
+
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	var (
+		mu       sync.Mutex
+		written  []string
+		firstErr error
+	)
+	fail := func(err error) {
+		mu.Lock()
+		if firstErr == nil {
+			firstErr = err
+			cancel()
+		}
+		mu.Unlock()
+	}
+
+	batches := groupBatches(jobs)
+	workers := batchWorkers
+	if workers > len(batches) {
+		workers = len(batches)
+	}
+	work := make(chan []batchJob)
+	var wg sync.WaitGroup
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for batch := range work {
+				done, err := e.pullOneBatch(ctx, batcher, peer, gameID, root, batch, throttle, tracker, onProgress)
+				mu.Lock()
+				written = append(written, done...)
+				mu.Unlock()
+				if err != nil {
+					fail(err)
+					return
+				}
+				// One line per batch instead of one per file: a quarter-million
+				// "file updated" lines rotated every other message out of the log.
+				if len(done) > 0 {
+					e.Log("info", fmt.Sprintf("%d files updated (%s … %s)", len(done), done[0], done[len(done)-1]))
+				}
+				onProgress(true)
+			}
+		}()
+	}
+	for _, b := range batches {
+		select {
+		case work <- b:
+		case <-ctx.Done():
+		}
+	}
+	close(work)
+	wg.Wait()
+
+	mu.Lock()
+	defer mu.Unlock()
+	if firstErr != nil {
+		return written, firstErr
+	}
+	return written, ctx.Err()
+}
+
+// pullOneBatch fetches one batch and writes its files.
+func (e *Engine) pullOneBatch(ctx context.Context, batcher BatchFetcher, peer Peer, gameID, root string,
+	batch []batchJob, throttle *throttler, tracker *progressTracker, onProgress func(force bool)) ([]string, error) {
+
+	req := make([]FileBlocksRequest, len(batch))
+	for i, j := range batch {
+		req[i] = FileBlocksRequest{RelPath: j.relPath, BlockIndices: j.indices, BlockSize: j.remote.BlockSize}
+	}
+
+	var resp []FileBlocks
+	var err error
+	const maxAttempts = 3
+	for attempt := 1; attempt <= maxAttempts; attempt++ {
+		resp, err = batcher.FetchFileBatch(ctx, peer, gameID, root, req)
+		if err == nil || errors.Is(err, ErrBatchUnsupported) {
+			break
+		}
+		// Refused as too large — a responder that counts or limits batches
+		// differently: the same request will be refused again, so it goes
+		// as two halves instead. Asking again unchanged failed the whole pull.
+		if strings.Contains(err.Error(), "batch too large") && len(batch) > 1 {
+			half := len(batch) / 2
+			first, err := e.pullOneBatch(ctx, batcher, peer, gameID, root, batch[:half], throttle, tracker, onProgress)
+			if err != nil {
+				return first, err
+			}
+			second, err := e.pullOneBatch(ctx, batcher, peer, gameID, root, batch[half:], throttle, tracker, onProgress)
+			return append(first, second...), err
+		}
+		e.Log("warn", fmt.Sprintf("batch fetch attempt %d/%d of %d files failed: %v", attempt, maxAttempts, len(batch), err))
+		if attempt < maxAttempts {
+			select {
+			case <-ctx.Done():
+				return nil, ctx.Err()
+			case <-time.After(time.Duration(attempt) * time.Second):
+			}
+		}
+	}
+	if errors.Is(err, ErrBatchUnsupported) {
+		return e.pullBatchOneByOne(ctx, peer, gameID, root, batch, throttle, tracker, onProgress)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("fetch batch of %d files: %w", len(batch), err)
+	}
+
+	byPath := make(map[string]FileBlocks, len(resp))
+	for _, f := range resp {
+		byPath[f.RelPath] = f
+	}
+
+	var written []string
+	for _, j := range batch {
+		if ctx.Err() != nil {
+			return written, ctx.Err()
+		}
+		f, ok := byPath[j.relPath]
+		if !ok {
+			return written, fmt.Errorf("fetch blocks for %s: %s did not send it", j.relPath, peer.Name)
+		}
+		if f.Error != "" {
+			return written, fmt.Errorf("fetch blocks for %s: %s", j.relPath, f.Error)
+		}
+		if err := writeBatchedFile(j, f.Blocks); err != nil {
+			return written, err
+		}
+		written = append(written, j.relPath)
+		n := changedBytes(j.remote, j.indices)
+		tracker.add(n)
+		onProgress(false)
+		throttle.wait(ctx, n)
+	}
+	return written, nil
+}
+
+// writeBatchedFile writes one file from its fetched blocks, exactly as
+// pullFile does: unchanged blocks from the copy on disk, the rest from the
+// peer, verified and renamed into place by the PatchWriter.
+func writeBatchedFile(j batchJob, blocks []BlockData) error {
+	writer, err := delta.NewPatchWriter(j.localPath, j.remote)
+	if err != nil {
+		return fmt.Errorf("patch %s: %w", j.relPath, err)
+	}
+	committed := false
+	defer func() {
+		if !committed {
+			writer.Abort()
+		}
+	}()
+	incoming := make(map[int]bool, len(j.indices))
+	for _, idx := range j.indices {
+		incoming[idx] = true
+	}
+	if err := writer.SeedUnchanged(j.localPath, incoming); err != nil {
+		return fmt.Errorf("patch %s: %w", j.relPath, err)
+	}
+	for _, b := range blocks {
+		if err := writer.WriteBlock(b.Index, b.Data); err != nil {
+			return fmt.Errorf("patch %s: %w", j.relPath, err)
+		}
+	}
+	if err := writer.Commit(); err != nil {
+		return fmt.Errorf("patch %s: %w", j.relPath, err)
+	}
+	committed = true
+	if j.remote.MtimeMs > 0 {
+		mtime := time.UnixMilli(int64(j.remote.MtimeMs))
+		owntouch.Mark(j.localPath) // see pullFiles: re-dated, then Settled
+		_ = os.Chtimes(j.localPath, mtime, mtime)
+		owntouch.Settled(j.localPath)
+	}
+	return nil
+}
+
+// pullBatchOneByOne is the fallback when the transport cannot batch for this
+// peer after all: the same files, through the per-file path.
+func (e *Engine) pullBatchOneByOne(ctx context.Context, peer Peer, gameID, root string,
+	batch []batchJob, throttle *throttler, tracker *progressTracker, onProgress func(force bool)) ([]string, error) {
+
+	var written []string
+	for _, j := range batch {
+		if err := e.pullFile(ctx, peer, FileRef{GameID: gameID, Root: root, RelPath: j.relPath}, j.localPath,
+			j.remote, j.indices, throttle, tracker, onProgress); err != nil {
+			return written, err
+		}
+		if j.remote.MtimeMs > 0 {
+			mtime := time.UnixMilli(int64(j.remote.MtimeMs))
+			owntouch.Mark(j.localPath)
+			_ = os.Chtimes(j.localPath, mtime, mtime)
+			owntouch.Settled(j.localPath)
+		}
+		written = append(written, j.relPath)
+	}
+	return written, nil
+}
diff --git a/internal/p2p/syncengine/batch_test.go b/internal/p2p/syncengine/batch_test.go
new file mode 100644
index 00000000..c468cf6c
--- /dev/null
+++ b/internal/p2p/syncengine/batch_test.go
@@ -0,0 +1,237 @@
+package syncengine
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/snapshot"
+)
+
+// batchTransport is fakeTransport plus batched fetches, answering manifests
+// with a chosen protocol revision.
+type batchTransport struct {
+	*fakeTransport
+	proto int
+
+	mu         sync.Mutex
+	batchCalls int
+	blockCalls int
+	failPath   string // answered with a per-file error
+	// maxFiles, when set, refuses bigger batches the way a responder that
+	// counts differently does.
+	maxFiles int
+}
+
+func (b *batchTransport) FetchManifest(ctx context.Context, peer Peer, gameID string, q ManifestQuery) (ManifestResponse, error) {
+	resp, err := b.fakeTransport.FetchManifest(ctx, peer, gameID, q)
+	resp.Proto = b.proto
+	return resp, err
+}
+
+func (b *batchTransport) FetchBlocks(ctx context.Context, peer Peer, ref FileRef, idx []int, bs int) ([]BlockData, error) {
+	b.mu.Lock()
+	b.blockCalls++
+	b.mu.Unlock()
+	return b.fakeTransport.FetchBlocks(ctx, peer, ref, idx, bs)
+}
+
+func (b *batchTransport) FetchFileBatch(ctx context.Context, peer Peer, gameID, root string, files []FileBlocksRequest) ([]FileBlocks, error) {
+	b.mu.Lock()
+	b.batchCalls++
+	b.mu.Unlock()
+	if len(files) > batchMaxFiles {
+		return nil, fmt.Errorf("batch of %d files is over the limit", len(files))
+	}
+	if b.maxFiles > 0 && len(files) > b.maxFiles {
+		return nil, fmt.Errorf(`peer returned 400: {"error":"batch too large"}`)
+	}
+	out := make([]FileBlocks, 0, len(files))
+	for _, f := range files {
+		if f.RelPath == b.failPath {
+			out = append(out, FileBlocks{RelPath: f.RelPath, Error: "read blocks failed: locked"})
+			continue
+		}
+		blocks, err := b.fakeTransport.FetchBlocks(ctx, peer, FileRef{GameID: gameID, Root: root, RelPath: f.RelPath}, f.BlockIndices, f.BlockSize)
+		if err != nil {
+			out = append(out, FileBlocks{RelPath: f.RelPath, Error: err.Error()})
+			continue
+		}
+		out = append(out, FileBlocks{RelPath: f.RelPath, Blocks: blocks})
+	}
+	return out, nil
+}
+
+func setupBatchEngine(t *testing.T, proto int) (*engineEnv, *batchTransport) {
+	t.Helper()
+	env := setupEngine(t)
+	bt := &batchTransport{fakeTransport: env.transport, proto: proto}
+	env.engine = New(env.store, snapshot.New(env.store), bt)
+	return env, bt
+}
+
+// A save of many small files, plus one big one, in two folders.
+func writeManyFiles(t *testing.T, dir string, n int) {
+	t.Helper()
+	for i := 0; i < n; i++ {
+		write(t, dir, fmt.Sprintf("map/chunk_%04d.bin", i), strings.Repeat(fmt.Sprintf("chunk %d;", i), 20))
+	}
+	big := bytes.Repeat([]byte("large save data "), (512<<10)/16) // 512KB: per-file path
+	if err := os.WriteFile(filepath.Join(dir, "big.sav"), big, 0o666); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func assertSameTree(t *testing.T, want, got string) {
+	t.Helper()
+	wm, err := delta.BuildManifest(want)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gm, err := delta.BuildManifest(got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if wm.ManifestHash() != gm.ManifestHash() {
+		t.Fatalf("trees differ: %d files remote, %d local", len(wm.Files), len(gm.Files))
+	}
+	for p, f := range wm.Files {
+		if gm.Files[p].MtimeMs != f.MtimeMs {
+			t.Errorf("%s: mtime %d, want the peer's %d", p, gm.Files[p].MtimeMs, f.MtimeMs)
+			break
+		}
+	}
+}
+
+func TestPull_BatchesSmallFiles(t *testing.T) {
+	env, bt := setupBatchEngine(t, ProtoBatchFiles)
+	writeManyFiles(t, env.remoteDir, 600)
+
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil {
+		t.Fatalf("SyncWithPeer: %v", err)
+	}
+	if res.Status != "updated" || res.Direction != "pull" {
+		t.Fatalf("result = %+v, want updated/pull", res)
+	}
+	assertSameTree(t, env.remoteDir, env.localDir)
+
+	// 600 one-block files: each claims a 64 KB block as the responder counts,
+	// so 192 fit a request (batchMaxClaimed) -> 4 requests; the big file alone
+	// goes block by block.
+	if bt.batchCalls != 4 {
+		t.Errorf("batch requests = %d, want 4", bt.batchCalls)
+	}
+	if bt.blockCalls != 1 {
+		t.Errorf("per-file block requests = %d, want 1 (the big file)", bt.blockCalls)
+	}
+	if len(env.transport.syncEvents) == 0 || env.transport.syncEvents[len(env.transport.syncEvents)-1] != "sync-complete" {
+		t.Errorf("sync did not complete: %v", env.transport.syncEvents)
+	}
+}
+
+// A peer that predates batching is asked file by file, as before.
+func TestPull_OldPeerIsNotBatched(t *testing.T) {
+	env, bt := setupBatchEngine(t, ProtoMultiRoot)
+	writeManyFiles(t, env.remoteDir, 50)
+
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatalf("SyncWithPeer: %v", err)
+	}
+	assertSameTree(t, env.remoteDir, env.localDir)
+	if bt.batchCalls != 0 {
+		t.Errorf("batch requests to an old peer = %d, want 0", bt.batchCalls)
+	}
+	if bt.blockCalls != 51 {
+		t.Errorf("per-file requests = %d, want 51", bt.blockCalls)
+	}
+}
+
+// One file the peer cannot read fails the pull and names the file, like the
+// per-file path does; nothing half-written is left in its place.
+func TestPull_BatchFileErrorNamesTheFile(t *testing.T) {
+	env, bt := setupBatchEngine(t, ProtoBatchFiles)
+	writeManyFiles(t, env.remoteDir, 10)
+	bt.failPath = "map/chunk_0003.bin"
+
+	_, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err == nil || !strings.Contains(err.Error(), "map/chunk_0003.bin") {
+		t.Fatalf("err = %v, want one naming map/chunk_0003.bin", err)
+	}
+	if _, statErr := os.Stat(filepath.Join(env.localDir, "map", "chunk_0003.bin")); statErr == nil {
+		t.Error("the file the peer could not serve was written anyway")
+	}
+	leftovers, _ := filepath.Glob(filepath.Join(env.localDir, "map", "*"+delta.TmpSuffix))
+	if len(leftovers) > 0 {
+		t.Errorf("temp files left behind: %v", leftovers)
+	}
+}
+
+func TestGroupBatches_RespectsLimits(t *testing.T) {
+	var jobs []batchJob
+	for i := 0; i < 600; i++ {
+		jobs = append(jobs, batchJob{relPath: fmt.Sprint(i),
+			remote:  delta.FileEntry{Blocks: []delta.Block{{Index: 0, Length: 32 << 10}}},
+			indices: []int{0}})
+	}
+	for _, b := range groupBatches(jobs) {
+		var n int64
+		for _, j := range b {
+			n += changedBytes(j.remote, j.indices)
+		}
+		if len(b) > batchMaxFiles || n > batchMaxBytes {
+			t.Errorf("batch of %d files / %d bytes is over the limit", len(b), n)
+		}
+	}
+}
+
+// Many small files, some of several blocks: what the responder counts —
+// every block at full block size — must stay under its limit too. 256 files
+// of one 64 KB block already claim 16 MB, and each file of more blocks tipped
+// a batch over it: "batch too large", and the whole pull failed.
+func TestGroupBatches_StaysUnderWhatTheResponderCounts(t *testing.T) {
+	var jobs []batchJob
+	for i := 0; i < 2000; i++ {
+		blocks := []delta.Block{{Index: 0, Length: 4 << 10}}
+		indices := []int{0}
+		if i%10 == 0 {
+			blocks = []delta.Block{{Index: 0, Length: 64 << 10}, {Index: 1, Length: 64 << 10}, {Index: 2, Length: 10 << 10}}
+			indices = []int{0, 1, 2}
+		}
+		jobs = append(jobs, batchJob{relPath: fmt.Sprint(i),
+			remote: delta.FileEntry{BlockSize: 64 << 10, Blocks: blocks}, indices: indices})
+	}
+	for _, b := range groupBatches(jobs) {
+		var claimed int64
+		for _, j := range b {
+			claimed += claimedBytes(j.remote, j.indices)
+		}
+		if claimed > 16<<20 {
+			t.Errorf("a batch of %d files claims %d bytes, over the responder's limit", len(b), claimed)
+		}
+	}
+}
+
+// A responder that refuses a batch as too large gets it again in halves,
+// rather than the pull failing.
+func TestPull_ABatchRefusedAsTooLargeIsSplit(t *testing.T) {
+	env, bt := setupBatchEngine(t, ProtoBatchFiles)
+	bt.maxFiles = 10
+	for i := 0; i < 40; i++ {
+		write(t, env.remoteDir, fmt.Sprintf("chunks/c%02d.bin", i), fmt.Sprint("chunk ", i))
+	}
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatalf("sync failed though the batches could be split: %v", err)
+	}
+	for i := 0; i < 40; i++ {
+		if _, err := os.Stat(filepath.Join(env.localDir, "chunks", fmt.Sprintf("c%02d.bin", i))); err != nil {
+			t.Fatalf("c%02d.bin did not arrive", i)
+		}
+	}
+}
diff --git a/internal/p2p/syncengine/changedsince_test.go b/internal/p2p/syncengine/changedsince_test.go
new file mode 100644
index 00000000..59c57c10
--- /dev/null
+++ b/internal/p2p/syncengine/changedsince_test.go
@@ -0,0 +1,93 @@
+package syncengine
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/owntouch"
+)
+
+// A pull that finished and whose file the game then saved over is not a
+// pull that stopped part-way: resuming it would replace the new save with
+// the peer's.
+func TestChangedSincePulled(t *testing.T) {
+	dir := t.TempDir()
+	pulled := filepath.Join(dir, "progress.sav")
+	pull := func(content string) {
+		owntouch.Mark(pulled)
+		if err := os.WriteFile(pulled, []byte(content), 0o666); err != nil {
+			t.Fatal(err)
+		}
+		owntouch.Settled(pulled)
+	}
+	build := func() delta.Manifest {
+		m, err := delta.BuildManifest(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return m
+	}
+	d := Decision{FilesToPull: []string{"progress.sav"}}
+
+	pull("from A")
+	remote := build()
+
+	// The game saves straight after the pull.
+	time.Sleep(20 * time.Millisecond)
+	if err := os.WriteFile(pulled, []byte("from B"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	delta.InvalidateRoot(dir)
+	if !changedSincePulled(dir, build(), remote, d) {
+		t.Error("a save written over a finished pull was taken for the pull stopping part-way")
+	}
+
+	// The pull wrote an old copy and stopped: still as OpenSave left it.
+	pull("old")
+	delta.InvalidateRoot(dir)
+	if changedSincePulled(dir, build(), remote, d) {
+		t.Error("a pull that stopped part-way was taken for a change made here")
+	}
+
+	// A file the pull did not touch was the same on both sides when it was
+	// planned; differing now, it was changed here meanwhile.
+	if !changedSincePulled(dir, build(), remote, Decision{}) {
+		t.Error("a file the pull did not touch, changed since, was taken for the pull stopping part-way")
+	}
+}
+
+// A file the pull did not touch, deleted here the moment the pull is done
+// (TestFullSyncFlow on CI): a deletion made here, not a part of the pull to
+// fetch again.
+func TestChangedSincePulled_AnUntouchedFileDeleted(t *testing.T) {
+	dir := t.TempDir()
+	for name, content := range map[string]string{"slot1.sav": "from B", "config/video.ini": "fullscreen=1"} {
+		full := filepath.Join(dir, filepath.FromSlash(name))
+		if err := os.MkdirAll(filepath.Dir(full), 0o777); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(full, []byte(content), 0o666); err != nil {
+			t.Fatal(err)
+		}
+	}
+	owntouch.Mark(filepath.Join(dir, "slot1.sav"))
+	owntouch.Settled(filepath.Join(dir, "slot1.sav"))
+	remote, err := delta.BuildManifest(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Remove(filepath.Join(dir, "config", "video.ini")); err != nil {
+		t.Fatal(err)
+	}
+	delta.InvalidateRoot(dir)
+	fresh, err := delta.BuildManifest(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !changedSincePulled(dir, fresh, remote, Decision{FilesToPull: []string{"slot1.sav"}}) {
+		t.Error("a file deleted here right after the pull was taken for the pull stopping part-way, and would be fetched back")
+	}
+}
diff --git a/internal/p2p/syncengine/conflict.go b/internal/p2p/syncengine/conflict.go
index 66293b5a..742c35f8 100644
--- a/internal/p2p/syncengine/conflict.go
+++ b/internal/p2p/syncengine/conflict.go
@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/owntouch"
 	"github.com/opensave/opensave/internal/snapshot"
 )
 
@@ -41,9 +42,12 @@ func (e *Engine) ResolveConflict(ctx context.Context, gameID, peerID, resolution
 		// see both sides still differing from the stale base and re-conflict
 		// immediately — the "keep looping" bug.
 		e.Log("info", fmt.Sprintf("conflict on %q resolved: keep LOCAL — our version becomes the shared state", gameID))
-		if err := e.markResolvedLocal(gameID, peer); err != nil {
+		if err := e.markResolvedLocal(ctx, gameID, peer); err != nil {
 			return "", err
 		}
+		// This device's version becomes newer than both, so the peer — and any
+		// device holding either side's — takes it whole (version.go).
+		e.versionAfterResolution(gameID, conflict.remoteVersion, true)
 		e.clearConflict(gameID)
 		e.Transport.TriggerPeerPull(peer, gameID)
 		return "", nil
@@ -63,8 +67,18 @@ func (e *Engine) ResolveConflict(ctx context.Context, gameID, peerID, resolution
 			e.Log("warn", fmt.Sprintf("safety snapshot before keep-remote failed: %v", err))
 		}
 		if err := e.overwriteLocalWithRemote(ctx, gameID, peer, remoteData, "Resolved conflict: Overwrite with remote"); err != nil {
+			// This device's own version is given up either way; what is still
+			// missing comes from the peer like any outdated device's would.
+			e.discardLocalVersion(gameID, remoteData.Version)
 			return "", err
 		}
+		if game, err := e.Store.GetGame(gameID); err == nil {
+			if now, err := delta.BuildManifest(game.SavePath); err == nil && sameFiles(now, remoteData.Manifest) {
+				e.versionAfterResolution(gameID, remoteData.Version, false)
+			} else {
+				e.discardLocalVersion(gameID, remoteData.Version)
+			}
+		}
 		e.markResolvedConverged(gameID, peer)
 		e.clearConflict(gameID)
 		return "", nil
@@ -103,9 +117,10 @@ func (e *Engine) ResolveConflict(ctx context.Context, gameID, peerID, resolution
 		// From here it is "keep mine": this device's version becomes the
 		// shared one, and the peer is asked to take it — which, if it has
 		// changes of its own, asks the person there in turn.
-		if err := e.markResolvedLocal(gameID, peer); err != nil {
+		if err := e.markResolvedLocal(ctx, gameID, peer); err != nil {
 			return "", err
 		}
+		e.versionAfterResolution(gameID, conflict.remoteVersion, true)
 		e.clearConflict(gameID)
 		e.Transport.TriggerPeerPull(peer, gameID)
 		return created, nil
@@ -160,7 +175,20 @@ func (e *Engine) markResolvedConverged(gameID string, peer Peer) {
 // unambiguously the newest, so the sync direction is a push. The peer
 // then receives our version (its own engine snapshots first, or re-raises
 // its own conflict if it has newer unsynced work of its own).
-func (e *Engine) markResolvedLocal(gameID string, peer Peer) error {
+//
+// The lineage, though, must say only what both sides verifiably hold. It used
+// to record every local file as shared (persistLineage(local, local)) before
+// the peer had pulled any of it. When that pull did not finish — a large save,
+// a request that timed out, a device that went away — the next sync found
+// those files "shared before and missing on the peer now", read that as the
+// peer having deleted them, and deleted them HERE: on the device whose version
+// the person had just chosen to keep. A save of a quarter-million files lost
+// tens of thousands that way, and the folders differing again re-raised the
+// conflict, inviting the same answer. Now the record is the intersection of
+// the two saves as they stand; a file only this device has is new to the
+// peer, not deleted by it, and goes over on the next sync. If the peer cannot
+// be asked, nothing is recorded as shared, so nothing can be read as deleted.
+func (e *Engine) markResolvedLocal(ctx context.Context, gameID string, peer Peer) error {
 	game, err := e.Store.GetGame(gameID)
 	if err != nil {
 		return err
@@ -170,7 +198,18 @@ func (e *Engine) markResolvedLocal(gameID string, peer Peer) error {
 	if err != nil {
 		return err
 	}
-	e.persistLineage(gameID, peer.ID, local, local)
+	var files, dirs []string
+	if remote, err := e.peerStateForResolution(ctx, gameID, peer); err == nil {
+		files, dirs = IntersectLineage(local, remote.Manifest)
+	} else {
+		e.Log("info", fmt.Sprintf("could not read %s's save while resolving %q (%v) — nothing is recorded as shared, so nothing it lacks is taken as deleted", peer.Name, gameID, err))
+	}
+	rules := e.rulesFor(gameID)
+	files = filterPathList(files, rules)
+	dirs = filterPathList(dirs, rules)
+	if err := e.Store.SetSyncState(gameID, peer.ID, files, dirs); err != nil {
+		e.Log("warn", fmt.Sprintf("persist sync lineage failed: %v", err))
+	}
 	_ = e.Store.SetAgreedHash(gameID, peer.ID, local.ManifestHash())
 	_ = e.Store.SetLastManifestHash(gameID, e.contentHashOf(gameID, local, game.SavePath))
 	e.recordSynced(gameID, peer.ID)
@@ -197,14 +236,18 @@ func (e *Engine) touchSaveMtimes(gameID, root string) {
 		return
 	}
 	if !info.IsDir() {
+		owntouch.Mark(root)
 		_ = os.Chtimes(root, now, now)
+		owntouch.Settled(root)
 		return
 	}
 	_ = filepath.Walk(root, func(path string, fi os.FileInfo, walkErr error) error {
 		if walkErr != nil || fi.IsDir() {
 			return nil
 		}
+		owntouch.Mark(path)
 		_ = os.Chtimes(path, now, now)
+		owntouch.Settled(path)
 		return nil
 	})
 }
@@ -337,6 +380,7 @@ func (e *Engine) overwriteLocalWithRemote(ctx context.Context, gameID string, pe
 		}
 		full := filepath.Join(game.SavePath, filepath.FromSlash(relPath))
 		_ = os.Chmod(full, 0o666)
+		owntouch.MarkRemoved(full)
 		_ = os.Remove(full)
 	}
 
diff --git a/internal/p2p/syncengine/devicesettings_test.go b/internal/p2p/syncengine/devicesettings_test.go
new file mode 100644
index 00000000..c9691e07
--- /dev/null
+++ b/internal/p2p/syncengine/devicesettings_test.go
@@ -0,0 +1,94 @@
+package syncengine
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/ignore"
+	"github.com/opensave/opensave/internal/store"
+)
+
+func withDeviceSettings(env *engineEnv, patterns ...string) {
+	env.engine.DeviceSettingsFor = func(store.Game) []string { return patterns }
+}
+
+// Each device keeps its own graphics settings: a differing settings file is
+// not a difference, in either direction — until the game is set to sync them.
+func TestDeviceSettings_KeptPerDevice(t *testing.T) {
+	env := setupEngine(t)
+	withDeviceSettings(env, "/graphics.xml")
+	write(t, env.localDir, "slot1.sav", "progress")
+	write(t, env.localDir, "graphics.xml", "4k ultra")
+	write(t, env.remoteDir, "slot1.sav", "progress")
+	write(t, env.remoteDir, "graphics.xml", "720p low")
+
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil || res.Status != "in_sync" {
+		t.Fatalf("status = %q, %v; want in_sync", res.Status, err)
+	}
+	if got, _ := os.ReadFile(filepath.Join(env.localDir, "graphics.xml")); string(got) != "4k ultra" {
+		t.Errorf("this device's settings became %q", got)
+	}
+	if got, _ := os.ReadFile(filepath.Join(env.remoteDir, "graphics.xml")); string(got) != "720p low" {
+		t.Errorf("the peer's settings became %q", got)
+	}
+
+	// A save change still travels; the settings beside it still do not.
+	write(t, env.remoteDir, "slot1.sav", "more progress")
+	res, err = env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil || res.Status != "updated" {
+		t.Fatalf("status = %q, %v; want updated", res.Status, err)
+	}
+	if got, _ := os.ReadFile(filepath.Join(env.localDir, "slot1.sav")); string(got) != "more progress" {
+		t.Errorf("the save did not arrive: %q", got)
+	}
+	if got, _ := os.ReadFile(filepath.Join(env.localDir, "graphics.xml")); string(got) != "4k ultra" {
+		t.Errorf("this device's settings became %q", got)
+	}
+
+	game, _ := env.store.GetGame("game1")
+	game.SyncDeviceSettings = true
+	if env.engine.IgnoreText(game) != game.SyncIgnore {
+		t.Error("set to sync its settings, the game still leaves them out")
+	}
+}
+
+// The game's own rules come after the settings, so "!" brings one back.
+func TestDeviceSettings_OwnRuleCanBringOneBack(t *testing.T) {
+	env := setupEngine(t)
+	withDeviceSettings(env, "/graphics.xml", "/input.cfg")
+	game, _ := env.store.GetGame("game1")
+	game.SyncIgnore = "!input.cfg"
+	rules := env.engine.IgnoreText(game)
+	if r := ignore.Parse(rules); !r.Match("graphics.xml") || r.Match("input.cfg") {
+		t.Errorf("rules %q: graphics.xml excluded %v, input.cfg excluded %v", rules, r.Match("graphics.xml"), r.Match("input.cfg"))
+	}
+}
+
+// Leaving the settings out where they were synced before is not a change of
+// the save: the recorded hash is re-taken, no version is named.
+func TestDeviceSettings_StartingToLeaveThemOutIsNotAVersion(t *testing.T) {
+	env := setupEngine(t)
+	game, _ := env.store.GetGame("game1")
+	game.AutoSync = true
+	_ = env.store.UpdateGame(game)
+	write(t, env.localDir, "slot1.sav", "progress")
+	write(t, env.localDir, "graphics.xml", "4k ultra")
+	env.engine.NoteLocalChange("game1")
+	before, _ := env.store.GetGameVersion("game1")
+
+	withDeviceSettings(env, "/graphics.xml")
+	m, _ := delta.BuildManifest(env.localDir)
+	env.engine.AdoptExclusionView("game1", m, "", false)
+	env.engine.NoteLocalChange("game1")
+	after, _ := env.store.GetGameVersion("game1")
+	if after.Vector != before.Vector {
+		t.Fatalf("version moved from %s to %s", before.Vector, after.Vector)
+	}
+	if after.Hash != env.engine.versionHashOf("game1", m) {
+		t.Error("the recorded hash was not re-taken")
+	}
+}
diff --git a/internal/p2p/syncengine/engine.go b/internal/p2p/syncengine/engine.go
index e3b02b00..379885b9 100644
--- a/internal/p2p/syncengine/engine.go
+++ b/internal/p2p/syncengine/engine.go
@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/owntouch"
 	"github.com/opensave/opensave/internal/snapshot"
 	"github.com/opensave/opensave/internal/store"
 	"github.com/opensave/opensave/internal/syncpause"
@@ -27,6 +28,17 @@ type Conflict struct {
 	RemoteStats SideStats  `json:"remoteStats"`
 	DiffFiles   []DiffFile `json:"diffFiles"` // capped; DiffTotal is the real count
 	DiffTotal   int        `json:"diffTotal"`
+	// The uncapped counts by kind. DiffFiles stops at 100, and counting from
+	// it told someone choosing "keep theirs" on a save of a quarter-million
+	// files that it would remove a few dozen, when it removed tens of
+	// thousands: every OnlyLocal file goes.
+	OnlyLocalTotal  int `json:"onlyLocalTotal"`
+	OnlyRemoteTotal int `json:"onlyRemoteTotal"`
+	ChangedTotal    int `json:"changedTotal"`
+
+	// remoteVersion is the peer's version when the conflict was found, which
+	// the answer is recorded against (version.go).
+	remoteVersion *VersionInfo
 }
 
 // SideStats summarises one side's save state for the conflict UI.
@@ -73,7 +85,7 @@ type Result struct {
 	// back until told whether that was meant (hold.go). peer_busy: the peer was
 	// still writing the game for a sync of its own; this one runs again when it
 	// has finished (settle.go).
-	Status    string `json:"status"` // in_sync | updated | updated_bidirectional | deletions_synced | triggered_peer_pull | conflict | peer_missing | peer_awaiting_folder | peer_holding | peer_busy
+	Status    string `json:"status"` // in_sync | updated | updated_bidirectional | deletions_synced | triggered_peer_pull | conflict | peer_missing | peer_awaiting_folder | peer_holding | peer_busy | unwritable
 	Direction string `json:"direction"`
 	PeerID    string `json:"peerId,omitempty"`
 	PeerName  string `json:"peerName,omitempty"`
@@ -119,6 +131,8 @@ type Engine struct {
 	// on this device right now (settle.go).
 	settleMu sync.Mutex
 	gates    map[string]*saveGate
+	// writtenAt is when a sync last stopped writing each game (BeingWritten).
+	writtenAt map[string]time.Time
 	// servedMu guards served: the save states this device recently handed
 	// each peer, per game (served.go).
 	servedMu sync.Mutex
@@ -136,6 +150,36 @@ type Engine struct {
 	// rootConflicts holds divergences in a game's EXTRA save locations, keyed
 	// by game and location so several can wait on a decision at once.
 	rootConflicts map[string]*RootConflict
+
+	// linkMu guards links, the measured connection to each peer, and
+	// pullWaits, syncs waiting for a close peer to finish taking a save
+	// (linkspeed.go).
+	linkMu    sync.Mutex
+	links     map[string]store.PeerLink
+	pullWaits map[string]chan struct{}
+	// versionMu serialises reading and writing the save versions
+	// (version.go); pullingNow is the games this run is pulling a newer
+	// version of, and loggedOnce the last "waiting" line said per game/peer.
+	versionMu  sync.Mutex
+	pullingNow map[string]bool
+	loggedOnce map[string]string
+	// peerVersions: per peer, whether its OpenSave keeps save versions
+	// (PeerNeedsUpdate).
+	peerVersions map[string]bool
+	// DeviceSettingsFor returns the exclusion patterns for a game's device
+	// settings (presets/devicesettings.go), set by the daemon; nil leaves
+	// them syncing. See IgnoreText.
+	DeviceSettingsFor func(store.Game) []string
+	// ObserveSettings is told what another device holds of a game, as its
+	// manifest arrives, to watch how the files left out as the game's
+	// settings change (daemon/detectsettings.go). Set by the daemon.
+	ObserveSettings func(game store.Game, peerID string, remote delta.Manifest)
+	// Playing reports whether a game is being played on this device
+	// (playing.go). Set by the daemon from its play sessions.
+	Playing func(gameID string) bool
+	// unwritable: games whose save folder could not be written, and until
+	// when pulls into it wait (version.go).
+	unwritable map[string]time.Time
 }
 
 // New creates an Engine.
@@ -235,6 +279,10 @@ func (e *Engine) SyncGame(ctx context.Context, gameID string, onlinePeers []Peer
 	if e.Paused != nil && e.Paused() {
 		return nil, ErrPaused
 	}
+	// A game being played here is synced when the session ends (playing.go).
+	if e.PlayingHere(gameID) {
+		return nil, ErrPlayingHere
+	}
 	// A save emptied here is not synced until someone says whether that was
 	// meant (hold.go).
 	if held, err := e.CheckHold(gameID, false); err == nil && held {
@@ -303,13 +351,32 @@ func (e *Engine) SyncGame(ctx context.Context, gameID string, onlinePeers []Peer
 
 	results := map[string]Result{}
 	busy := false
-	for _, peer := range onlinePeers {
+	// Fastest connections first (linkspeed.go): a device that is behind takes
+	// the newer save from the closest device holding it, and a newer save
+	// here reaches the close devices before the slow ones.
+	onlinePeers = e.OrderBySpeed(onlinePeers)
+	for i, peer := range onlinePeers {
 		// Hard per-peer cap: a wedged transport must never hold
 		// activeSyncs forever (which would silently block every future
 		// sync of this game until an app restart).
 		peerCtx, cancel := context.WithTimeout(ctx, perPeerSyncTimeout)
 		res, err := e.SyncWithPeer(peerCtx, gameID, peer)
 		cancel()
+		// A close device asked to take this save is given the time to, before
+		// a much slower one is asked: it gets it at full speed, and the slow
+		// one can then take it from whichever device is nearest to it.
+		if err == nil && res.Status == "triggered_peer_pull" && e.waitsForClose(onlinePeers, i) {
+			var size int64
+			if game, gerr := e.Store.GetGame(gameID); gerr == nil {
+				if m, merr := e.ReadManifest(ctx, gameID, game.SavePath); merr == nil {
+					for _, f := range m.Files {
+						size += f.Size
+					}
+				}
+			}
+			e.Log("info", fmt.Sprintf("waiting for %s (close by) to take %s before the slower devices", peer.Name, gameID))
+			e.waitForPull(ctx, gameID, peer, size)
+		}
 		if err == nil && res.Status == "peer_busy" {
 			// Nothing was compared, so nothing is stamped, and it is not a
 			// failure: the peer was writing this game for a sync of its own.
@@ -334,7 +401,7 @@ func (e *Engine) SyncGame(ctx context.Context, gameID string, onlinePeers []Peer
 		// divergence from the NEXT sync, causing the peer to silently overwrite
 		// its own changes instead of detecting the conflict and asking.
 		switch res.Status {
-		case "conflict", "error":
+		case "conflict", "error", versionStatusWaiting, StatusPeerOutdatedApp, "unwritable", "peer_playing":
 		case "peer_missing", "peer_awaiting_folder", "peer_holding":
 			// The two devices talked and finished, which is what the
 			// per-device stamp has always recorded. But nothing of THIS game
@@ -397,15 +464,34 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 	if err != nil {
 		return Result{}, err
 	}
-	e.Log("info", fmt.Sprintf("syncing %q with %q (%s)", game.Name, peer.Name,
-		map[bool]string{true: "WAN relay", false: "direct LAN"}[peer.Wan()]))
-
-	// 1. Fetch remote manifest + branch info.
 	isFile, _ := delta.ResolveLocalSaveFilePath(game.SavePath)
-	remoteData, err := e.Transport.FetchManifest(ctx, peer, gameID, ManifestQuery{
+	query := ManifestQuery{
 		Name: game.Name, SavePath: game.SavePath, IsFile: isFile,
 		AppID: game.AppID, CoverURL: game.CoverURL,
-	})
+	}
+
+	// 0. Nothing changed on either side since they last agreed: say so and
+	// stop, without moving either manifest. Quietly, too — this is the
+	// answer for nearly every game on every periodic pass.
+	res, done, prefetched := e.quickInSync(ctx, gameID, game, peer, query)
+	if done {
+		return res, nil
+	}
+
+	// Said once there is something to do (after the in-sync check below),
+	// not here: a line per game per peer per periodic pass, most of them
+	// followed by "already in sync", rotated everything else out of the log
+	// within hours.
+	syncingLine := fmt.Sprintf("syncing %q with %q (%s)", game.Name, peer.Name,
+		map[bool]string{true: "WAN relay", false: "direct LAN"}[peer.Wan()])
+
+	// 1. Fetch remote manifest + branch info.
+	var remoteData ManifestResponse
+	if prefetched != nil {
+		remoteData = *prefetched
+	} else {
+		remoteData, err = e.Transport.FetchManifest(ctx, peer, gameID, query)
+	}
 	if err != nil {
 		// The peer simply isn't tracking this game (they untracked it, or
 		// never had it). That's a stable state, not a transient network
@@ -421,6 +507,12 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 		// The peer is still writing this game for a sync of its own, and
 		// would otherwise have described a save half-way between two states
 		// (settle.go). SyncGame asks again once it has finished.
+		// Being played there: its save comes when the session ends, sent by
+		// that device. Before isSettling, whose message this one contains.
+		if isPlaying(err) {
+			e.logOnce(gameID+"|playing|"+peer.ID, fmt.Sprintf("%s is playing %q right now; its save follows when the session ends", peer.Name, game.Name))
+			return Result{Status: "peer_playing", PeerID: peer.ID, PeerName: peer.Name}, nil
+		}
 		if isSettling(err) {
 			e.Log("info", fmt.Sprintf("%s is still applying a sync of %q; asking again when it has finished", peer.Name, game.Name))
 			return Result{Status: "peer_busy", PeerID: peer.ID, PeerName: peer.Name}, nil
@@ -436,6 +528,19 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 		return Result{}, fmt.Errorf("fetch remote manifest: %w", err)
 	}
 
+	// 1b. A peer whose OpenSave keeps no save versions decides by the old file
+	// comparison: whatever its copy lacks, it reads as deleted, and a copy that
+	// arrived half-way spreads its gaps that way. Nothing is taken from it
+	// until it is updated — it can still take from here, and its deletions are
+	// refused where they arrive (handleDeleteFile).
+	e.notePeerApp(peer.ID, remoteData.KeepsVersions())
+	if game.AutoSync && !remoteData.KeepsVersions() {
+		e.logOnce(gameID+"|"+peer.ID, fmt.Sprintf(
+			"%q: %s runs an OpenSave without save versions — nothing is taken from it until it is updated",
+			game.Name, peer.Name))
+		return Result{Status: StatusPeerOutdatedApp, PeerID: peer.ID, PeerName: peer.Name}, nil
+	}
+
 	// 2. Branch alignment: local follows the remote's active branch.
 	//
 	// From the switch until the peer's save has been pulled into the branch,
@@ -495,18 +600,48 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 	// would propagate a deletion of the very file being protected.
 	ignoreRules := e.rulesFor(gameID)
 	unfilteredLocal, unfilteredRemote := localManifest, remoteData.Manifest
-	if !ignoreRules.Empty() {
-		localManifest = filterManifest(localManifest, ignoreRules)
-		remoteData.Manifest = filterManifest(remoteData.Manifest, ignoreRules)
+	if e.ObserveSettings != nil && len(unfilteredRemote.Files) > 0 {
+		e.ObserveSettings(game, peer.ID, unfilteredRemote)
 	}
+	localManifest = filterManifest(localManifest, ignoreRules)
+	remoteData.Manifest = filterManifest(remoteData.Manifest, ignoreRules)
 
-	// 3. Existing unresolved conflict blocks further syncing.
+	// 3. Existing unresolved conflict blocks further syncing — unless a
+	// version newer than both sides of it has turned up since (version.go):
+	// it was settled elsewhere, by an answer or "Use this save everywhere",
+	// and the question here no longer stands.
 	e.mu.Lock()
-	if existing := e.activeConflicts[gameID]; existing != nil {
-		e.mu.Unlock()
-		return Result{Status: "conflict", PeerID: peer.ID, PeerName: peer.Name}, nil
-	}
+	existing := e.activeConflicts[gameID]
 	e.mu.Unlock()
+	if existing != nil {
+		if remoteData.Version == nil || !game.AutoSync || !e.supersedes(gameID, *remoteData.Version) {
+			return Result{Status: "conflict", PeerID: peer.ID, PeerName: peer.Name}, nil
+		}
+		e.Log("info", fmt.Sprintf("%q: %s holds a version newer than both sides of the open conflict — taking it instead of asking",
+			game.Name, peer.Name))
+		e.clearConflict(gameID)
+	}
+
+	// 3b. The versions decide, when both devices keep them (version.go): who
+	// is behind takes the newer save whole, from a device that holds it, and
+	// nothing is ever taken from — or deleted on the word of — a device that
+	// is behind. Left to the file comparison below only when the two hold
+	// the same save, or both saves predate versions.
+	//
+	// Not while this device is fetching back a save someone emptied here and
+	// asked to have put back (hold.go): the emptying was recorded as a version
+	// of its own, and by versions this device would be the newer one, with
+	// nothing to fetch. The hold's own rules bring the files back.
+	fetchingBack := false
+	if h, ok, err := e.Store.GetDeletionHold(gameID); err == nil && ok && h.State == store.HoldFetching {
+		fetchingBack = true
+	}
+	if remoteData.Version != nil && game.AutoSync && !fetchingBack {
+		if res, handled, err := e.syncByVersion(ctx, game, peer, localManifest, unfilteredLocal, remoteData); handled {
+			releaseAligned()
+			return res, err
+		}
+	}
 
 	// 4. Conflict detection (lineage + skew-tolerant mtimes).
 	lastSyncMs := e.lastSyncTimeMs(peer.ID)
@@ -582,7 +717,7 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 	// side's UNFILTERED state still hashes to the base then that side has not
 	// changed since, whichever view the base was written in, and its filtered
 	// hash is the same fact expressed in today's terms.
-	if !ignoreRules.Empty() && agreedHash != "" {
+	if agreedHash != "" {
 		switch agreedHash {
 		case unfilteredLocal.ManifestHash():
 			agreedHash = localManifest.ManifestHash()
@@ -607,10 +742,9 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 	// path is still recorded as shared. Leave it there and the decision reads
 	// "we both had this and now I do not" — and propagates a deletion of the
 	// file the rule exists to protect.
-	if rules := e.rulesFor(gameID); !rules.Empty() {
-		lineageFiles = filterLineage(lineageFiles, rules)
-		lineageDirs = filterLineage(lineageDirs, rules)
-	}
+	rules := ignoreRules
+	lineageFiles = filterLineage(lineageFiles, rules)
+	lineageDirs = filterLineage(lineageDirs, rules)
 
 	// A side that is merely behind the other has not diverged from it, whatever
 	// the clocks and the base say (OnlyBehind).
@@ -634,17 +768,14 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 	// Excluded paths are filtered out for the same reason the lineage is: a
 	// rule change must never be able to reach across and remove a peer's file.
 	deleted := e.recordedDeletions(gameID, delta.PrimaryRoot)
-	if rules := e.rulesFor(gameID); !rules.Empty() {
-		for path := range deleted {
-			if rules.Match(path) {
-				delete(deleted, path)
-			}
+	for path := range deleted {
+		if excluded(rules, path) {
+			delete(deleted, path)
 		}
 	}
 	decision := ComputeWithDeletions(localManifest, remoteData.Manifest, lineageFiles, lineageDirs, agreedHash, deleted)
 
 	if !decision.HasChanges() {
-		e.Log("success", fmt.Sprintf("%q already in sync with %q", game.Name, peer.Name))
 		e.persistLineage(gameID, peer.ID, localManifest, remoteData.Manifest)
 		// Both sides verifiably identical: this is a convergence point.
 		_ = e.Store.SetAgreedHash(gameID, peer.ID, localManifest.ManifestHash())
@@ -663,8 +794,23 @@ func (e *Engine) SyncWithPeer(ctx context.Context, gameID string, peer Peer) (Re
 		// are read through the gate — so the branch hold goes first.
 		releaseAligned()
 		e.syncExtraRoots(ctx, gameID, game, peer, remoteData)
+		// Share the peer's latest history entry here too. A pull does this;
+		// a sync that found both sides already identical used to skip it,
+		// so a game whose saves were identical everywhere from the start
+		// (Steam Cloud, a copied folder) showed a snapshot on the device
+		// that tracked it and none on any other. The local files ARE the
+		// peer's at this point, so the mirror describes the same content.
+		// Once per peer snapshot: recordMirrorSnapshot skips an id it has.
+		//
+		// Last, after the other locations have synced: it zips every location,
+		// and taking that time before them delayed their sync for nothing.
+		if remoteData.LatestSnapshot != nil && len(localManifest.Files) > 0 && !e.hasSnapshot(gameID, game) {
+			e.recordMirrorSnapshot(gameID, game, peer, *remoteData.LatestSnapshot,
+				fmt.Sprintf("Synced from peer: %s (%s)", peer.Name, remoteData.LatestSnapshot.Comment))
+		}
 		return Result{Status: "in_sync", Direction: "none"}, nil
 	}
+	e.Log("info", syncingLine)
 
 	if emptiedUnconfirmed(remoteData.Manifest.Files, decision, remoteData.DeletionConfirmed) {
 		e.Log("info", fmt.Sprintf("%q holds none of %q's save files now, and has not confirmed deleting them — keeping this device's copies",
@@ -897,10 +1043,9 @@ func (e *Engine) persistLineage(gameID, peerID string, local, remote delta.Manif
 	// RefreshLineage rebuilds this from unfiltered manifests, so without a
 	// filter here an excluded file would be written back in — and the next
 	// sync would read it as shared, then as deleted, and propagate that.
-	if rules := e.rulesFor(gameID); !rules.Empty() {
-		files = filterPathList(files, rules)
-		dirs = filterPathList(dirs, rules)
-	}
+	rules := e.rulesFor(gameID)
+	files = filterPathList(files, rules)
+	dirs = filterPathList(dirs, rules)
 	if err := e.Store.SetSyncState(gameID, peerID, files, dirs); err != nil {
 		e.Log("warn", fmt.Sprintf("persist sync lineage failed: %v", err))
 	}
@@ -1032,9 +1177,7 @@ func (e *Engine) AddConfirmedLineageForRoot(gameID, peerID, root string, files [
 	// Excluded paths must not enter the record of what both sides hold, for
 	// the same reason persistLineage filters them: the next sync would read an
 	// excluded file as shared, then as deleted, and propagate that.
-	if rules := e.rulesFor(gameID); !rules.Empty() {
-		merged = filterPathList(merged, rules)
-	}
+	merged = filterPathList(merged, e.rulesFor(gameID))
 	sort.Strings(merged)
 	if err := e.Store.SetSyncStateForRoot(gameID, peerID, root, merged, existingDirs); err != nil {
 		e.Log("warn", fmt.Sprintf("recording confirmed lineage failed: %v", err))
@@ -1260,6 +1403,10 @@ func (e *Engine) registerConflict(gameID string, peer Peer, localManifest delta.
 	// Capture comparison data while we hold both manifests, so the UI can
 	// show which side is further along and exactly what differs.
 	diffs := diffManifests(localManifest, remoteData.Manifest)
+	counts := map[string]int{}
+	for _, d := range diffs {
+		counts[d.Status]++
+	}
 	const maxDiffFiles = 100
 	total := len(diffs)
 	if len(diffs) > maxDiffFiles {
@@ -1269,10 +1416,12 @@ func (e *Engine) registerConflict(gameID string, peer Peer, localManifest delta.
 	e.mu.Lock()
 	e.activeConflicts[gameID] = &Conflict{
 		Peer: peer, LocalSnap: localSnap, RemoteSnap: remoteSnap,
-		LocalStats:  manifestStats(localManifest),
-		RemoteStats: manifestStats(remoteData.Manifest),
-		DiffFiles:   diffs,
-		DiffTotal:   total,
+		LocalStats:     manifestStats(localManifest),
+		RemoteStats:    manifestStats(remoteData.Manifest),
+		DiffFiles:      diffs,
+		DiffTotal:      total,
+		OnlyLocalTotal: counts["only-local"], OnlyRemoteTotal: counts["only-remote"], ChangedTotal: counts["changed"],
+		remoteVersion: remoteData.Version,
 	}
 	e.mu.Unlock()
 
@@ -1371,6 +1520,7 @@ func (e *Engine) applyLocalDeletions(gameID string, root syncRoot, d Decision) {
 		// spelling this disk actually holds.
 		full := delta.LocalNameFor(root.Path, relPath)
 		_ = os.Chmod(full, 0o666)
+		owntouch.MarkRemoved(full)
 		if err := os.Remove(full); err == nil {
 			e.Log("info", "deleted locally (peer deleted): "+relPath)
 		}
@@ -1385,6 +1535,7 @@ func (e *Engine) applyLocalDeletions(gameID string, root syncRoot, d Decision) {
 		}
 		full := delta.LocalNameFor(root.Path, relDir)
 		if info, err := os.Stat(full); err == nil && info.IsDir() {
+			owntouch.MarkRemoved(full)
 			if err := os.Remove(full); err == nil { // only removes empty dirs, matching rmdirSync
 				e.Log("info", "deleted directory locally (peer deleted): "+relDir)
 			}
@@ -1427,7 +1578,9 @@ func (e *Engine) createPulledDirsIn(gameID string, root syncRoot, dirsToPull []s
 		if !delta.IsSafePath(root.Path, relDir) {
 			continue
 		}
-		_ = os.MkdirAll(filepath.Join(root.Path, filepath.FromSlash(relDir)), 0o777)
+		full := filepath.Join(root.Path, filepath.FromSlash(relDir))
+		owntouch.Mark(full)
+		_ = os.MkdirAll(full, 0o777)
 	}
 }
 
@@ -1475,7 +1628,11 @@ func (e *Engine) pullFiles(ctx context.Context, peer Peer, gameID string, game s
 	// Make sure every remote directory exists before patching into it.
 	for _, dir := range remoteData.Manifest.Dirs {
 		if delta.IsSafePath(root.Path, dir) {
-			_ = os.MkdirAll(filepath.Join(root.Path, filepath.FromSlash(dir)), 0o777)
+			full := filepath.Join(root.Path, filepath.FromSlash(dir))
+			if _, err := os.Stat(full); err != nil {
+				owntouch.Mark(full)
+				_ = os.MkdirAll(full, 0o777)
+			}
 		}
 	}
 
@@ -1523,6 +1680,24 @@ func (e *Engine) pullFiles(ctx context.Context, peer Peer, gameID string, game s
 
 	tracker := newProgressTracker(totalBytes)
 	throttle := e.throttleFor(peer.Wan())
+	// What this transfer says about the connection (linkspeed.go): every
+	// measureWindow while it runs — a pull of a large save takes an hour over a
+	// slow link, and one measurement at its end left the link unmeasured all
+	// that time — and what is left when it ends, however it ends: a pull cut
+	// off half-way still moved what it moved.
+	const measureWindow = 30 * time.Second
+	var measureMu sync.Mutex
+	measuredAt, measuredBytes := time.Now(), int64(0)
+	measure := func(final bool) {
+		moved, _, _ := tracker.stats()
+		measureMu.Lock()
+		defer measureMu.Unlock()
+		if d := time.Since(measuredAt); final || d >= measureWindow {
+			e.noteLinkRate(peer, moved-measuredBytes, d)
+			measuredAt, measuredBytes = time.Now(), moved
+		}
+	}
+	defer measure(true)
 
 	// Progress reporter shared by the per-file loop and the block-group
 	// loop inside each file. Without in-file reporting, a single large
@@ -1539,6 +1714,7 @@ func (e *Engine) pullFiles(ctx context.Context, peer Peer, gameID string, game s
 		}
 		lastReport = time.Now()
 		reportMu.Unlock()
+		measure(false)
 
 		bytesPulled, speed, pct := tracker.stats()
 		ev := ProgressEvent{PeerName: peer.Name, BytesTransferred: bytesPulled, TotalBytes: totalBytes, SpeedBytesPerSec: speed, Percentage: pct}
@@ -1561,6 +1737,11 @@ func (e *Engine) pullFiles(ctx context.Context, peer Peer, gameID string, game s
 	// can record the lineage from confirmed fact rather than rediscovering it
 	// with a manifest round trip. See the sync-complete event below.
 	var pulled []string
+	// Small files are collected here and fetched many to a request once the
+	// per-file checks below have passed; see pullBatched.
+	batcher, canBatch := e.Transport.(BatchFetcher)
+	canBatch = canBatch && !peer.Wan() && remoteData.Proto >= ProtoBatchFiles
+	var batched []batchJob
 	for _, relPath := range filesToPull {
 		if !delta.IsSafePath(root.Path, relPath) {
 			return fmt.Errorf("path traversal attempt on pulled file %s", relPath)
@@ -1608,13 +1789,21 @@ func (e *Engine) pullFiles(ctx context.Context, peer Peer, gameID string, game s
 		if isFile, _ := delta.ResolveLocalSaveFilePath(root.Path); isFile {
 			localFilePath = root.Path // single-file save mode
 		}
+		if canBatch && len(indices) > 0 && changedBytes(remoteFile, indices) <= batchFileMaxBytes {
+			batched = append(batched, batchJob{relPath: relPath, localPath: localFilePath, remote: remoteFile, indices: indices})
+			continue
+		}
 		if err := e.pullFile(ctx, peer, FileRef{GameID: gameID, Root: root.Name, RelPath: relPath}, localFilePath,
 			remoteFile, indices, throttle, tracker, reportProgress); err != nil {
 			return err
 		}
 		if remoteFile.MtimeMs > 0 {
 			mtime := time.UnixMilli(int64(remoteFile.MtimeMs))
+			// Marked again first: until Settled below, the time the rename left
+			// is not the one to compare (owntouch).
+			owntouch.Mark(localFilePath)
 			_ = os.Chtimes(localFilePath, mtime, mtime)
+			owntouch.Settled(localFilePath)
 		}
 		pulled = append(pulled, relPath)
 		e.Log("info", "file updated: "+relPath)
@@ -1622,6 +1811,13 @@ func (e *Engine) pullFiles(ctx context.Context, peer Peer, gameID string, game s
 		// File-boundary progress reporting (always fires).
 		reportProgress(true)
 	}
+	if len(batched) > 0 {
+		done, err := e.pullBatched(ctx, batcher, peer, gameID, root.Name, batched, throttle, tracker, reportProgress)
+		pulled = append(pulled, done...)
+		if err != nil {
+			return err
+		}
+	}
 
 	// Said once, plainly, at the end. A sync that quietly leaves files behind
 	// is worse than one that fails: the save looks synced and is not, and the
@@ -1822,11 +2018,37 @@ func fetchWithRetry(ctx context.Context, t Transport, peer Peer, ref FileRef,
 	return nil, lastErr
 }
 
+// hasSnapshot reports whether the game has any snapshot on its active branch
+// here.
+//
+// A sync that finds both sides already identical records the peer's latest
+// snapshot only for a game with no history at all — saves identical from the
+// start (Steam Cloud, a copied folder) would otherwise never get one. It used
+// to record it for every peer it compared with: a full copy of the same save
+// per device, again with each new snapshot over there, which for a save of a
+// quarter-million files was gigabytes of identical archives.
+func (e *Engine) hasSnapshot(gameID string, game store.Game) bool {
+	if e.Snapshots == nil {
+		return false
+	}
+	_, err := e.Snapshots.LatestSnapshot(gameID, game.ActiveBranch)
+	return err == nil
+}
+
 // recordMirrorSnapshot zips the (just-updated) local save under the peer's
 // snapshot id so both devices show the same history entry.
 func (e *Engine) recordMirrorSnapshot(gameID string, game store.Game, peer Peer, remoteSnap SnapshotInfo, comment string) {
-	if _, err := e.Store.GetSnapshot(remoteSnap.ID); err == nil {
-		return // already mirrored
+	if _, known := e.Store.ResolveSnapshotID(remoteSnap.ID); known {
+		return // already mirrored, or known as the same files as one here
+	}
+	// The peer says what its snapshot holds: if a snapshot here holds exactly
+	// that, it is the same snapshot, and the save is not archived again.
+	if remoteSnap.ContentHash != "" {
+		if existing, ok, _ := e.Store.SnapshotByContent(gameID, game.ActiveBranch, remoteSnap.ContentHash); ok &&
+			snapshot.ArchiveExists(existing.ZipPath) {
+			_ = e.Store.AddSnapshotAlias(remoteSnap.ID, existing.ID)
+			return
+		}
 	}
 
 	settings, err := e.Store.GetSettings()
@@ -1851,16 +2073,40 @@ func (e *Engine) recordMirrorSnapshot(gameID string, game store.Game, peer Peer,
 	if rootsErr != nil {
 		mirrorRoots = nil
 	}
-	if _, err := snapshot.ZipRoots(game.SavePath, mirrorRoots, zipPath); err != nil {
+	var skipped []string
+	var captured []store.CapturedFile
+	if e.Snapshots != nil {
+		// Unchanged files copied from the newest snapshot, not compressed
+		// again: seconds rather than minutes for a large save.
+		skipped, captured, err = e.Snapshots.ArchiveSaveTo(gameID, game.ActiveBranch, zipPath)
+	} else {
+		skipped, captured, err = snapshot.ZipRootsCapturing(game.SavePath, mirrorRoots, zipPath)
+	}
+	if err != nil {
+		os.Remove(zipPath)
 		e.Log("warn", fmt.Sprintf("mirror snapshot zip failed: %v", err))
 		return
 	}
+	// Incomplete is not kept (snapshot.IncompleteError); the next sync with
+	// the peer records it again.
+	if len(skipped) > 0 {
+		os.Remove(zipPath)
+		e.Log("info", fmt.Sprintf("snapshot of %q from %s discarded: %d file(s) could not be read; taken again at the next sync",
+			game.Name, peer.Name, len(skipped)))
+		return
+	}
+	key := snapshot.ContentKey(captured)
+	if existing, ok, _ := e.Store.SnapshotByContent(gameID, game.ActiveBranch, key); ok && snapshot.ArchiveExists(existing.ZipPath) {
+		os.Remove(zipPath)
+		_ = e.Store.AddSnapshotAlias(remoteSnap.ID, existing.ID)
+		return
+	}
 	info, err := os.Stat(zipPath)
 	if err != nil {
 		return
 	}
 
-	_ = e.Store.CreateSnapshot(store.Snapshot{
+	if err := e.Store.CreateSnapshot(store.Snapshot{
 		ID:           remoteSnap.ID,
 		GameID:       gameID,
 		BranchName:   game.ActiveBranch,
@@ -1869,7 +2115,15 @@ func (e *Engine) recordMirrorSnapshot(gameID string, game store.Game, peer Peer,
 		IsSystemAuto: true,
 		ZipPath:      zipPath,
 		SizeBytes:    info.Size(),
-	})
+		ContentHash:  key,
+	}); err == nil {
+		_ = e.Store.RecordSnapshotFiles(remoteSnap.ID, captured)
+		if e.Snapshots != nil {
+			if snap, gerr := e.Store.GetSnapshot(remoteSnap.ID); gerr == nil {
+				e.Snapshots.PruneContainedBy(snap, captured)
+			}
+		}
+	}
 }
 
 // humanBytes formats a byte count as a short human-readable string.
diff --git a/internal/p2p/syncengine/engine_test.go b/internal/p2p/syncengine/engine_test.go
index 1ed654ff..22a2be33 100644
--- a/internal/p2p/syncengine/engine_test.go
+++ b/internal/p2p/syncengine/engine_test.go
@@ -224,6 +224,59 @@ func TestSync_PullNewRemoteFile(t *testing.T) {
 	}
 }
 
+// Saves identical on both sides from the start (Steam Cloud, a copied folder)
+// never pull anything — and used to never share the peer's history entry
+// either, so the game showed a snapshot on one device and none on the other.
+func TestSync_InSyncMirrorsPeerSnapshot(t *testing.T) {
+	env := setupEngine(t)
+	write(t, env.localDir, "save.dat", "same progress")
+	write(t, env.remoteDir, "save.dat", "same progress")
+	env.transport.latestSnap = &SnapshotInfo{ID: "snap_888", Timestamp: "2026-06-01T00:00:00.000Z", Comment: "Initial snapshot"}
+
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil {
+		t.Fatalf("SyncWithPeer error = %v", err)
+	}
+	if res.Status != "in_sync" {
+		t.Fatalf("result = %+v, want in_sync", res)
+	}
+	snap, err := env.store.GetSnapshot("snap_888")
+	if err != nil {
+		t.Fatalf("peer snapshot not mirrored on an in-sync pass: %v", err)
+	}
+	if !strings.Contains(snap.Comment, "Synced from peer: Remote Device") {
+		t.Errorf("mirror comment = %q", snap.Comment)
+	}
+
+	// A second in-sync pass must not take it again.
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatal(err)
+	}
+	game, err := env.store.GetGame("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	snaps, err := env.store.ListSnapshots("game1", game.ActiveBranch)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(snaps) != 1 {
+		t.Errorf("snapshots after two in-sync passes = %d, want 1", len(snaps))
+	}
+}
+
+// An empty save folder in sync with an empty peer has nothing to mirror.
+func TestSync_InSyncEmptyFolderTakesNoMirror(t *testing.T) {
+	env := setupEngine(t)
+	env.transport.latestSnap = &SnapshotInfo{ID: "snap_999", Timestamp: "2026-06-01T00:00:00.000Z", Comment: "Initial snapshot"}
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := env.store.GetSnapshot("snap_999"); err == nil {
+		t.Error("an empty folder was mirrored")
+	}
+}
+
 func TestSync_InsufficientDiskSpace(t *testing.T) {
 	env := setupEngine(t)
 	write(t, env.remoteDir, "save.dat", "remote progress that needs space")
diff --git a/internal/p2p/syncengine/hold.go b/internal/p2p/syncengine/hold.go
index 7060c241..b444156d 100644
--- a/internal/p2p/syncengine/hold.go
+++ b/internal/p2p/syncengine/hold.go
@@ -85,6 +85,11 @@ type emptiness struct {
 func (x emptiness) back(held map[string][]string) bool {
 	for name, files := range held {
 		for _, f := range files {
+			if delta.NeverSynced(f) {
+				// Recorded by a build that still counted it; never part of
+				// the save, so not something that has to come back.
+				continue
+			}
 			if _, ok := x.present[name][f]; !ok {
 				return false
 			}
diff --git a/internal/p2p/syncengine/ignore.go b/internal/p2p/syncengine/ignore.go
index 96901a0e..dad3e7f0 100644
--- a/internal/p2p/syncengine/ignore.go
+++ b/internal/p2p/syncengine/ignore.go
@@ -2,6 +2,7 @@ package syncengine
 
 import (
 	"context"
+	"strings"
 
 	"github.com/opensave/opensave/internal/delta"
 	"github.com/opensave/opensave/internal/ignore"
@@ -39,15 +40,85 @@ func (e *Engine) rulesFor(gameID string) ignore.Rules {
 	if err != nil {
 		return ignore.Rules{}
 	}
-	return ignore.Parse(game.SyncIgnore)
+	return ignore.Parse(e.IgnoreText(game))
+}
+
+// IgnoreText is the exclusion list a game is synced under: the rules written
+// for it, and the files the game database names as its settings
+// (DeviceSettingsFor) unless it is set to sync those too. Everything that
+// applies a game's rules takes them from here, so the two can never be
+// applied by one part of the app and not another.
+func (e *Engine) IgnoreText(game store.Game) string {
+	if game.SyncDeviceSettings || e.DeviceSettingsFor == nil {
+		return game.SyncIgnore
+	}
+	settings := e.DeviceSettingsFor(game)
+	if len(settings) == 0 {
+		return game.SyncIgnore
+	}
+	text := game.SyncIgnore
+	if text != "" && !strings.HasSuffix(text, "\n") {
+		text += "\n"
+	}
+	// The game's own rules come last, so a "!" line there can still bring a
+	// settings file back into syncing.
+	return "# device settings (game database)\n" + strings.Join(settings, "\n") + "\n" + text
+}
+
+// excluded says whether a path takes no part in syncing: a file no save is
+// made of on any device (delta.NeverSynced), or one the game's rules exclude.
+// The first is handled exactly like the second — the manifest served still
+// lists it, the decision never sees it — so a device on an older build, which
+// still counts it, is never told it was deleted.
+func excluded(rules ignore.Rules, p string) bool {
+	return delta.NeverSynced(p) || (!rules.Empty() && rules.Match(p))
+}
+
+func holdsNeverSynced(m delta.Manifest) bool {
+	for p := range m.Files {
+		if delta.NeverSynced(p) {
+			return true
+		}
+	}
+	for _, root := range m.Extra {
+		for p := range root.Files {
+			if delta.NeverSynced(p) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func anyNeverSynced(paths []string) bool {
+	for _, p := range paths {
+		if delta.NeverSynced(p) {
+			return true
+		}
+	}
+	return false
+}
+
+func anyNeverSyncedIn(paths map[string]struct{}) bool {
+	for p := range paths {
+		if delta.NeverSynced(p) {
+			return true
+		}
+	}
+	return false
 }
 
 // filterManifest returns a copy with every excluded path removed. The
 // original is untouched: it is still what gets served to peers.
 func filterManifest(m delta.Manifest, rules ignore.Rules) delta.Manifest {
-	if rules.Empty() {
+	if rules.Empty() && !holdsNeverSynced(m) {
 		return m
 	}
+	return filterManifestBy(m, func(p string) bool { return excluded(rules, p) })
+}
+
+// filterManifestBy is filterManifest with what to leave out given directly.
+func filterManifestBy(m delta.Manifest, leaveOut func(string) bool) delta.Manifest {
 	out := delta.Manifest{
 		Timestamp: m.Timestamp,
 		Files:     make(map[string]delta.FileEntry, len(m.Files)),
@@ -61,7 +132,7 @@ func filterManifest(m delta.Manifest, rules ignore.Rules) delta.Manifest {
 	// would be told their saves diverged, over a file neither of them is
 	// syncing.
 	for p, entry := range m.Files {
-		if rules.Match(p) {
+		if leaveOut(p) {
 			continue
 		}
 		out.Files[p] = entry
@@ -70,7 +141,7 @@ func filterManifest(m delta.Manifest, rules ignore.Rules) delta.Manifest {
 		}
 	}
 	for _, d := range m.Dirs {
-		if rules.Match(d) {
+		if leaveOut(d) {
 			continue
 		}
 		out.Dirs = append(out.Dirs, d)
@@ -83,13 +154,13 @@ func filterManifest(m delta.Manifest, rules ignore.Rules) delta.Manifest {
 		for name, root := range m.Extra {
 			sub := delta.RootManifest{Files: make(map[string]delta.FileEntry, len(root.Files))}
 			for p, entry := range root.Files {
-				if rules.Match(p) {
+				if leaveOut(p) {
 					continue
 				}
 				sub.Files[p] = entry
 			}
 			for _, d := range root.Dirs {
-				if rules.Match(d) {
+				if leaveOut(d) {
 					continue
 				}
 				sub.Dirs = append(sub.Dirs, d)
@@ -107,12 +178,12 @@ func filterManifest(m delta.Manifest, rules ignore.Rules) delta.Manifest {
 // decision reads "we both had this and now I don't" and propagates a deletion
 // to the peer.
 func filterLineage(paths map[string]struct{}, rules ignore.Rules) map[string]struct{} {
-	if rules.Empty() {
+	if rules.Empty() && !anyNeverSyncedIn(paths) {
 		return paths
 	}
 	out := make(map[string]struct{}, len(paths))
 	for p := range paths {
-		if rules.Match(p) {
+		if excluded(rules, p) {
 			continue
 		}
 		out[p] = struct{}{}
@@ -123,12 +194,12 @@ func filterLineage(paths map[string]struct{}, rules ignore.Rules) map[string]str
 // filterPathList drops excluded paths from a plain list, for the lineage that
 // is persisted back after a sync.
 func filterPathList(paths []string, rules ignore.Rules) []string {
-	if rules.Empty() {
+	if rules.Empty() && !anyNeverSynced(paths) {
 		return paths
 	}
 	out := make([]string, 0, len(paths))
 	for _, p := range paths {
-		if rules.Match(p) {
+		if excluded(rules, p) {
 			continue
 		}
 		out = append(out, p)
@@ -152,5 +223,5 @@ func (e *Engine) FilteredContentHash(gameID string, game store.Game) string {
 	if err != nil {
 		return ""
 	}
-	return filterManifest(m, ignore.Parse(game.SyncIgnore)).ManifestHash()
+	return filterManifest(m, ignore.Parse(e.IgnoreText(game))).ManifestHash()
 }
diff --git a/internal/p2p/syncengine/keeplocal_safety_test.go b/internal/p2p/syncengine/keeplocal_safety_test.go
new file mode 100644
index 00000000..064298f5
--- /dev/null
+++ b/internal/p2p/syncengine/keeplocal_safety_test.go
@@ -0,0 +1,34 @@
+package syncengine
+
+import (
+	"context"
+	"fmt"
+	"testing"
+)
+
+// "Keep mine" against a peer holding part of the save, where the peer never
+// gets to pull (a pull that times out, a device that goes away). The next
+// sync must not read the peer's missing files as deletions and remove them
+// from the device whose version the person chose to keep.
+func TestKeepLocal_PeerThatNeverPulled_DeletesNothingHere(t *testing.T) {
+	env := setupEngine(t)
+	for i := 0; i < 300; i++ {
+		write(t, env.localDir, fmt.Sprintf("world/map_%04d.bin", i), fmt.Sprintf("chunk %d", i))
+	}
+	for i := 0; i < 100; i++ {
+		write(t, env.remoteDir, fmt.Sprintf("world/map_%04d.bin", i), fmt.Sprintf("chunk %d", i))
+	}
+
+	// The person answers the conflict with "Keep mine".
+	if err := env.engine.markResolvedLocal(context.Background(), "game1", env.peer); err != nil {
+		t.Fatal(err)
+	}
+	// The peer's pull never happens; its folder stays as it was.
+
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatalf("SyncWithPeer: %v", err)
+	}
+	if n := countFiles(t, env.localDir); n != 300 {
+		t.Fatalf("after \"keep mine\" this device went from 300 to %d files — the peer's partial copy was read as deletions", n)
+	}
+}
diff --git a/internal/p2p/syncengine/linkspeed.go b/internal/p2p/syncengine/linkspeed.go
new file mode 100644
index 00000000..21af61f7
--- /dev/null
+++ b/internal/p2p/syncengine/linkspeed.go
@@ -0,0 +1,297 @@
+package syncengine
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"sort"
+	"time"
+
+	"github.com/opensave/opensave/internal/store"
+)
+
+// Network-aware syncing.
+//
+// Devices are not equally close. Two on the same home network move a save at
+// tens of MB a second; one reached over a VPN relay or the internet relay may
+// manage a hundred KB. A save that goes to every device at once spends hours
+// crawling over the slow links while the device next door, which could have
+// had it in seconds and then handed it on, waits in the same queue.
+//
+// So each device keeps how fast it has found its connection to every other —
+// measured from the transfers it makes, and from a short speed test when it
+// has nothing recent — and works fastest first: a sync goes to the close
+// devices first, and a device that is behind takes the newer save from the
+// fastest device that holds it. Before anything is measured, the kind of
+// address stands in: a home-network address is taken for fast, a VPN address
+// (Tailscale's range) for slower, the internet relay for slowest.
+
+// LinkStat is what this device knows about its connection to a peer.
+type LinkStat struct {
+	// BytesPerSec is the measured rate, 0 when nothing is measured yet.
+	BytesPerSec float64 `json:"bytesPerSec"`
+	MeasuredMs  int64   `json:"measuredMs"`
+	// Kind is what the address says: "lan", "vpn", "relay" or "internet".
+	Kind string `json:"kind"`
+}
+
+const (
+	// minMeasureBytes and minMeasureTime: smaller transfers say more about
+	// round trips than about the link, and are not counted.
+	minMeasureBytes = 512 << 10
+	minMeasureTime  = 500 * time.Millisecond
+	// probeEvery is how often a link is tested again when no transfer has
+	// measured it since; probeBytes how much a test moves.
+	probeEvery = 6 * time.Hour
+	probeBytes = 2 << 20
+	// probeRetry is how soon a speed test that failed is tried again.
+	probeRetry = 10 * time.Minute
+)
+
+// prior rates, by address kind, used until a link is measured.
+var priorRate = map[string]float64{
+	"lan":      40 << 20,
+	"vpn":      2 << 20,
+	"internet": 1 << 20,
+	"relay":    256 << 10,
+}
+
+var (
+	_, tailscaleNet, _ = net.ParseCIDR("100.64.0.0/10")
+	privateNets        = func() []*net.IPNet {
+		var out []*net.IPNet
+		for _, c := range []string{"10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "169.254.0.0/16", "fc00::/7", "fe80::/10"} {
+			_, n, _ := net.ParseCIDR(c)
+			out = append(out, n)
+		}
+		return out
+	}()
+)
+
+// linkKind says what a peer's address is.
+func linkKind(peer Peer) string {
+	if peer.Wan() {
+		return "relay"
+	}
+	ip := net.ParseIP(peer.Address)
+	if ip == nil {
+		return "internet"
+	}
+	if ip.IsLoopback() {
+		return "lan"
+	}
+	if tailscaleNet.Contains(ip) {
+		return "vpn"
+	}
+	for _, n := range privateNets {
+		if n.Contains(ip) {
+			return "lan"
+		}
+	}
+	return "internet"
+}
+
+func (e *Engine) loadLinksLocked() {
+	if e.links != nil {
+		return
+	}
+	e.links = map[string]store.PeerLink{}
+	if e.Store == nil {
+		return
+	}
+	if links, err := e.Store.PeerLinks(); err == nil {
+		e.links = links
+	}
+}
+
+// Link is what this device knows about its connection to a peer.
+func (e *Engine) Link(peer Peer) LinkStat {
+	e.linkMu.Lock()
+	defer e.linkMu.Unlock()
+	e.loadLinksLocked()
+	l := e.links[peer.ID]
+	return LinkStat{BytesPerSec: l.BytesPerSec, MeasuredMs: l.MeasuredMs, Kind: linkKind(peer)}
+}
+
+// expectedRate is how fast a transfer with a peer is expected to go: what was
+// measured, or the address kind's guess.
+func (e *Engine) expectedRate(peer Peer) float64 {
+	l := e.Link(peer)
+	if l.BytesPerSec > 0 {
+		return l.BytesPerSec
+	}
+	return priorRate[l.Kind]
+}
+
+// noteLinkRate records a transfer with a peer: bytes in d.
+func (e *Engine) noteLinkRate(peer Peer, bytes int64, d time.Duration) {
+	e.recordLinkRate(peer, bytes, d, minMeasureTime)
+}
+
+// recordLinkRate records bytes in d, when d is at least minTime.
+func (e *Engine) recordLinkRate(peer Peer, bytes int64, d, minTime time.Duration) {
+	if bytes < minMeasureBytes || d < minTime || d <= 0 {
+		return
+	}
+	rate := float64(bytes) / d.Seconds()
+	e.linkMu.Lock()
+	defer e.linkMu.Unlock()
+	e.loadLinksLocked()
+	l := e.links[peer.ID]
+	l.PeerID = peer.ID
+	if l.BytesPerSec > 0 {
+		// A running average, weighted to the latest: a link changes (a VPN
+		// that found a direct path, a laptop that moved networks), and the
+		// order should follow it within a transfer or two.
+		l.BytesPerSec = 0.4*l.BytesPerSec + 0.6*rate
+	} else {
+		l.BytesPerSec = rate
+	}
+	l.MeasuredMs = time.Now().UnixMilli()
+	e.links[peer.ID] = l
+	if e.Store != nil {
+		_ = e.Store.SavePeerLink(l)
+	}
+}
+
+// OrderBySpeed sorts peers fastest first, keeping the given order among
+// equals.
+func (e *Engine) OrderBySpeed(peers []Peer) []Peer {
+	out := append([]Peer(nil), peers...)
+	rates := make(map[string]float64, len(out))
+	for _, p := range out {
+		rates[p.ID] = e.expectedRate(p)
+	}
+	sort.SliceStable(out, func(i, j int) bool { return rates[out[i].ID] > rates[out[j].ID] })
+	return out
+}
+
+// SpeedProber is implemented by transports that can test a link: fetch n
+// bytes of nothing in particular from the peer and say how long it took.
+type SpeedProber interface {
+	ProbeSpeed(ctx context.Context, peer Peer, n int) (int64, time.Duration, error)
+}
+
+// ProbeLinkIfDue tests the link to a peer when nothing has measured it for a
+// while. Not over the internet relay, which is the slowest link whatever a
+// test says and would only be loaded further. Reports whether it ran.
+func (e *Engine) ProbeLinkIfDue(ctx context.Context, peer Peer) bool {
+	prober, ok := e.Transport.(SpeedProber)
+	if !ok || peer.Wan() {
+		return false
+	}
+	e.linkMu.Lock()
+	e.loadLinksLocked()
+	l := e.links[peer.ID]
+	now := time.Now()
+	// A link never measured is tried again soon after a test that failed;
+	// one measured before only when that has gone stale.
+	retryAfter := probeEvery
+	if l.BytesPerSec == 0 {
+		retryAfter = probeRetry
+	}
+	due := now.Sub(time.UnixMilli(l.MeasuredMs)) > probeEvery && now.Sub(time.UnixMilli(l.ProbedMs)) > retryAfter
+	if due {
+		l.PeerID = peer.ID
+		l.ProbedMs = now.UnixMilli()
+		e.links[peer.ID] = l
+		if e.Store != nil {
+			_ = e.Store.SavePeerLink(l)
+		}
+	}
+	e.linkMu.Unlock()
+	if !due {
+		return false
+	}
+	ctx, cancel := context.WithTimeout(ctx, 60*time.Second)
+	defer cancel()
+	n, d, err := prober.ProbeSpeed(ctx, peer, probeBytes)
+	if err == nil && d < minMeasureTime {
+		// Over a home network the test is over before it measures much:
+		// once more, larger, for a figure worth having.
+		n, d, err = prober.ProbeSpeed(ctx, peer, 4*probeBytes)
+	}
+	if err != nil {
+		// A link never measured is tried again after probeRetry (see due
+		// above): the peer may simply not have been ready — or not yet on a
+		// build that answers a speed test, which once left every link
+		// unmeasured for hours.
+		e.Log("info", fmt.Sprintf("speed test with %s did not finish: %v", peer.Name, err))
+		return true
+	}
+	// A test is a measurement however short: it moved nothing but the test,
+	// so no round trips of a sync's own are in it.
+	e.recordLinkRate(peer, n, d, time.Millisecond)
+	e.Log("info", fmt.Sprintf("connection to %s: %s/s (%s)", peer.Name, humanBytes(int64(e.Link(peer).BytesPerSec)), linkKind(peer)))
+	return true
+}
+
+// Waiting for close devices before slow ones.
+
+// fastLink is the rate from which a device counts as close, and slowerBy how
+// much slower the next device must be for this one to be waited for first.
+const (
+	fastLink   = 4 << 20
+	slowerBy   = 4.0
+	maxWaitFor = 20 * time.Minute
+)
+
+// NotePeerPulled is told that a peer finished (or gave up) pulling a game from
+// this device, and lets a sync waiting for it go on.
+func (e *Engine) NotePeerPulled(gameID, peerID string) {
+	e.linkMu.Lock()
+	ch := e.pullWaits[gameID+"|"+peerID]
+	delete(e.pullWaits, gameID+"|"+peerID)
+	e.linkMu.Unlock()
+	if ch != nil {
+		close(ch)
+	}
+}
+
+// waitForPull waits, after asking a close peer to take this device's save,
+// until it has — before the slow devices are asked, so the save reaches the
+// close one at full speed and the slow ones can then take it from whichever
+// device is closest to them. Bounded by how long the transfer should take,
+// and given up when the context ends.
+func (e *Engine) waitForPull(ctx context.Context, gameID string, peer Peer, bytes int64) {
+	ch := make(chan struct{})
+	e.linkMu.Lock()
+	if e.pullWaits == nil {
+		e.pullWaits = map[string]chan struct{}{}
+	}
+	e.pullWaits[gameID+"|"+peer.ID] = ch
+	e.linkMu.Unlock()
+	defer func() {
+		e.linkMu.Lock()
+		if e.pullWaits[gameID+"|"+peer.ID] == ch {
+			delete(e.pullWaits, gameID+"|"+peer.ID)
+		}
+		e.linkMu.Unlock()
+	}()
+	wait := 30*time.Second + time.Duration(3*float64(bytes)/e.expectedRate(peer)*float64(time.Second))
+	if wait > maxWaitFor {
+		wait = maxWaitFor
+	}
+	select {
+	case <-ch:
+	case <-time.After(wait):
+		e.Log("info", fmt.Sprintf("%s has not finished taking %s yet; going on with the other devices", peer.Name, gameID))
+	case <-ctx.Done():
+	}
+}
+
+// waitsForClose reports whether, having asked peers[i] to take this device's
+// save, the sync should wait for it before going on: it is a close device,
+// and a much slower one is still to come.
+func (e *Engine) waitsForClose(peers []Peer, i int) bool {
+	r := e.expectedRate(peers[i])
+	if r < fastLink {
+		return false
+	}
+	for _, p := range peers[i+1:] {
+		if e.expectedRate(p)*slowerBy <= r {
+			return true
+		}
+	}
+	return false
+}
diff --git a/internal/p2p/syncengine/linkspeed_test.go b/internal/p2p/syncengine/linkspeed_test.go
new file mode 100644
index 00000000..968dcc87
--- /dev/null
+++ b/internal/p2p/syncengine/linkspeed_test.go
@@ -0,0 +1,175 @@
+package syncengine
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestLinkKind(t *testing.T) {
+	cases := map[string]string{
+		"192.168.178.20": "lan",
+		"10.0.0.5":       "lan",
+		"127.0.0.1":      "lan",
+		"100.99.121.82":  "vpn",
+		"203.0.113.9":    "internet",
+	}
+	for addr, want := range cases {
+		if got := linkKind(Peer{Address: addr}); got != want {
+			t.Errorf("%s: %s, want %s", addr, got, want)
+		}
+	}
+	if got := linkKind(Peer{Address: "relay"}); got != "relay" {
+		t.Errorf("relay: %s", got)
+	}
+}
+
+// Before anything is measured the kind of address orders the devices; once a
+// link is measured, the measurement does.
+func TestOrderBySpeed(t *testing.T) {
+	env := setupEngine(t)
+	e := env.engine
+	home := Peer{ID: "home", Address: "192.168.178.20"}
+	vpn := Peer{ID: "vpn", Address: "100.99.121.82"}
+	relay := Peer{ID: "relay", Address: "relay"}
+
+	got := e.OrderBySpeed([]Peer{relay, vpn, home})
+	if got[0].ID != "home" || got[1].ID != "vpn" || got[2].ID != "relay" {
+		t.Errorf("by address: %v", ids(got))
+	}
+
+	// A VPN link that turns out fast, and a home one that turns out slow.
+	e.noteLinkRate(vpn, 200<<20, 2*time.Second)
+	e.noteLinkRate(home, 1<<20, 10*time.Second)
+	got = e.OrderBySpeed([]Peer{home, vpn, relay})
+	if got[0].ID != "vpn" {
+		t.Errorf("by measurement: %v", ids(got))
+	}
+	// Remembered across restarts.
+	e2 := New(env.store, nil, env.transport)
+	if e2.Link(vpn).BytesPerSec == 0 {
+		t.Error("the measured link was not kept")
+	}
+}
+
+func TestNoteLinkRate_IgnoresTinyTransfers(t *testing.T) {
+	env := setupEngine(t)
+	p := Peer{ID: "p", Address: "192.168.1.2"}
+	env.engine.noteLinkRate(p, 10<<10, 2*time.Second)
+	if env.engine.Link(p).BytesPerSec != 0 {
+		t.Error("a transfer of a few KB was taken as the link's speed")
+	}
+}
+
+// A close device is waited for before a much slower one, and the wait ends as
+// soon as it reports that it has finished.
+func TestWaitForClose(t *testing.T) {
+	env := setupEngine(t)
+	e := env.engine
+	home := Peer{ID: "home", Address: "192.168.178.20"}
+	vpn := Peer{ID: "vpn", Address: "100.99.121.82"}
+	peers := e.OrderBySpeed([]Peer{vpn, home})
+	if !e.waitsForClose(peers, 0) {
+		t.Fatal("a close device is not waited for before a slow one")
+	}
+	if e.waitsForClose(peers, 1) {
+		t.Error("the last device is waited for, with nothing after it")
+	}
+	if e.waitsForClose(e.OrderBySpeed([]Peer{home}), 0) {
+		t.Error("waited with no slower device to follow")
+	}
+
+	done := make(chan struct{})
+	start := time.Now()
+	go func() {
+		e.waitForPull(context.Background(), "game1", home, 1<<30)
+		close(done)
+	}()
+	time.Sleep(100 * time.Millisecond)
+	e.NotePeerPulled("game1", home.ID)
+	select {
+	case <-done:
+		if time.Since(start) > 5*time.Second {
+			t.Error("the wait did not end when the device reported")
+		}
+	case <-time.After(10 * time.Second):
+		t.Fatal("the wait did not end when the device reported")
+	}
+}
+
+type probeTransport struct {
+	*fakeTransport
+	fail  bool
+	calls int
+	// perMB is how long each MB takes; a second when unset.
+	perMB time.Duration
+}
+
+func (p *probeTransport) ProbeSpeed(ctx context.Context, peer Peer, n int) (int64, time.Duration, error) {
+	p.calls++
+	if p.fail {
+		return 0, 0, errors.New("peer returned 404")
+	}
+	if p.perMB > 0 {
+		return int64(n), time.Duration(n>>20) * p.perMB, nil
+	}
+	return int64(n), time.Second, nil
+}
+
+// A speed test that fails — a peer not yet updated to answer one — is tried
+// again after minutes, not hours; a measured link is not tested again soon.
+func TestProbeLink_RetriesAFailureSoon(t *testing.T) {
+	env := setupEngine(t)
+	pt := &probeTransport{fakeTransport: env.transport, fail: true}
+	env.engine.Transport = pt
+	p := Peer{ID: "p", Name: "P", Address: "192.168.1.9"}
+
+	if !env.engine.ProbeLinkIfDue(context.Background(), p) {
+		t.Fatal("an unmeasured link was not tested")
+	}
+	if env.engine.ProbeLinkIfDue(context.Background(), p) {
+		t.Fatal("tested again straight after a failure")
+	}
+	// Eleven minutes later.
+	env.engine.linkMu.Lock()
+	l := env.engine.links[p.ID]
+	l.ProbedMs = time.Now().Add(-11 * time.Minute).UnixMilli()
+	env.engine.links[p.ID] = l
+	env.engine.linkMu.Unlock()
+	pt.fail = false
+	if !env.engine.ProbeLinkIfDue(context.Background(), p) {
+		t.Fatal("a failed test was not tried again after ten minutes")
+	}
+	if env.engine.Link(p).BytesPerSec == 0 {
+		t.Fatal("the successful test was not recorded")
+	}
+	if env.engine.ProbeLinkIfDue(context.Background(), p) {
+		t.Error("a freshly measured link was tested again")
+	}
+}
+
+// A home network finishes the test before it has measured much: it is run
+// again larger, and the result is kept — not discarded as too short, which
+// left the fastest link unmeasured and sorted behind the slow ones.
+func TestProbeLink_AFastLinkIsMeasured(t *testing.T) {
+	env := setupEngine(t)
+	pt := &probeTransport{fakeTransport: env.transport, perMB: 20 * time.Millisecond}
+	env.engine.Transport = pt
+	p := Peer{ID: "lan", Name: "LAN", Address: "192.168.178.20"}
+	env.engine.ProbeLinkIfDue(context.Background(), p)
+	if got := env.engine.Link(p).BytesPerSec; got < 20<<20 {
+		t.Errorf("a 50 MB/s link measured as %.0f B/s", got)
+	}
+	if pt.calls != 2 {
+		t.Errorf("%d test(s), want the short one and a larger one", pt.calls)
+	}
+}
+
+func ids(ps []Peer) []string {
+	out := make([]string, len(ps))
+	for i, p := range ps {
+		out[i] = p.ID
+	}
+	return out
+}
diff --git a/internal/p2p/syncengine/multiroot.go b/internal/p2p/syncengine/multiroot.go
index c763a459..544e7e53 100644
--- a/internal/p2p/syncengine/multiroot.go
+++ b/internal/p2p/syncengine/multiroot.go
@@ -114,24 +114,20 @@ func (e *Engine) syncOneRoot(ctx context.Context, gameID string, game store.Game
 	rules := e.rulesFor(gameID)
 	remote := sr.remote
 	unfilteredLocal, unfilteredRemote := local, remote
-	if !rules.Empty() {
-		local = filterManifest(local, rules)
-		remote = filterManifest(remote, rules)
-	}
+	local = filterManifest(local, rules)
+	remote = filterManifest(remote, rules)
 
 	fileList, dirList, err := e.Store.GetSyncStateForRoot(gameID, peer.ID, sr.root.Name)
 	if err != nil {
 		return err
 	}
 	lineageFiles, lineageDirs := toSet(fileList), toSet(dirList)
-	if !rules.Empty() {
-		// Without this an exclusion added to a location that had already
-		// synced reads as "we both had this file and now I do not", and the
-		// deletion travels to the peer — destroying the copy the rule was
-		// written to protect.
-		lineageFiles = filterLineage(lineageFiles, rules)
-		lineageDirs = filterLineage(lineageDirs, rules)
-	}
+	// Without this an exclusion added to a location that had already
+	// synced reads as "we both had this file and now I do not", and the
+	// deletion travels to the peer — destroying the copy the rule was
+	// written to protect.
+	lineageFiles = filterLineage(lineageFiles, rules)
+	lineageDirs = filterLineage(lineageDirs, rules)
 	decision := Compute(local, remote, lineageFiles, lineageDirs)
 
 	if !decision.HasChanges() {
@@ -172,7 +168,7 @@ func (e *Engine) syncOneRoot(ctx context.Context, gameID string, game store.Game
 	// adds a rule. If a side's UNFILTERED state still hashes to the base then
 	// that side has not changed, and its filtered hash says the same thing in
 	// today's terms. Same translation the main folder does.
-	if !rules.Empty() && base != "" {
+	if base != "" {
 		switch base {
 		case unfilteredLocal.RootHash(delta.PrimaryRoot):
 			base = local.RootHash(delta.PrimaryRoot)
diff --git a/internal/p2p/syncengine/multiroot_conflict.go b/internal/p2p/syncengine/multiroot_conflict.go
index 1fea1ba6..401d69de 100644
--- a/internal/p2p/syncengine/multiroot_conflict.go
+++ b/internal/p2p/syncengine/multiroot_conflict.go
@@ -140,7 +140,17 @@ func (e *Engine) ResolveRootConflict(ctx context.Context, gameID, peerID, root,
 		if err := e.Store.SetAgreedHashForRoot(gameID, peerID, root, local.RootHash(delta.PrimaryRoot)); err != nil {
 			return err
 		}
-		files, dirs := IntersectLineage(local, local)
+		// Shared only what the peer verifiably holds too — see
+		// markResolvedLocal for what recording the whole local copy as shared
+		// did once the peer's pull did not finish.
+		var files, dirs []string
+		if remoteData, err := e.Transport.FetchManifest(ctx, peer, gameID, ManifestQuery{
+			Name: game.Name, SavePath: game.SavePath, AppID: game.AppID, CoverURL: game.CoverURL,
+		}); err == nil {
+			if remoteRoot, has := remoteData.Manifest.Extra[root]; has {
+				files, dirs = IntersectLineage(local, delta.Manifest{Files: remoteRoot.Files, Dirs: remoteRoot.Dirs})
+			}
+		}
 		_ = e.Store.SetSyncStateForRoot(gameID, peerID, root, files, dirs)
 		e.clearRootConflict(gameID, root)
 		e.Transport.TriggerPeerPull(peer, gameID)
diff --git a/internal/p2p/syncengine/neversynced_test.go b/internal/p2p/syncengine/neversynced_test.go
new file mode 100644
index 00000000..f104089b
--- /dev/null
+++ b/internal/p2p/syncengine/neversynced_test.go
@@ -0,0 +1,137 @@
+package syncengine
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+)
+
+// Steam's remotecache.vdf is each device's own bookkeeping: never taken from
+// a peer, never pushed, never deleted because the peer lacks it.
+func TestNeverSynced_LeftAloneOnBothSides(t *testing.T) {
+	env := setupEngine(t)
+	write(t, env.localDir, "save.dat", "progress")
+	write(t, env.localDir, "remotecache.vdf", "this device's steam")
+	write(t, env.remoteDir, "save.dat", "progress")
+	write(t, env.remoteDir, "remotecache.vdf", "the other device's steam")
+
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil {
+		t.Fatalf("SyncWithPeer error = %v", err)
+	}
+	if res.Status != "in_sync" {
+		t.Fatalf("status = %q, want in_sync: the saves are the same", res.Status)
+	}
+	got, _ := os.ReadFile(filepath.Join(env.localDir, "remotecache.vdf"))
+	if string(got) != "this device's steam" {
+		t.Errorf("this device's remotecache.vdf became %q", got)
+	}
+	if got, _ := os.ReadFile(filepath.Join(env.remoteDir, "remotecache.vdf")); string(got) != "the other device's steam" {
+		t.Errorf("the peer's remotecache.vdf became %q", got)
+	}
+}
+
+// The served manifest keeps listing it: a device on an older build would read
+// the gap as the file deleted, and delete its own.
+func TestNeverSynced_StillServed(t *testing.T) {
+	dir := t.TempDir()
+	write(t, dir, "remotecache.vdf", "steam")
+	m, err := delta.BuildManifest(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := m.Files["remotecache.vdf"]; !ok {
+		t.Fatal("the manifest a device serves left remotecache.vdf out")
+	}
+}
+
+// A build with a new list of such files re-takes the recorded hash quietly
+// when the save is the one it was recorded for, and the next change seen is
+// not called a new version over it. A save that did change is left to count.
+func TestNeverSynced_AdoptingTheViewIsNotAVersion(t *testing.T) {
+	env := setupEngine(t)
+	game, err := env.store.GetGame("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	game.AutoSync = true
+	if err := env.store.UpdateGame(game); err != nil {
+		t.Fatal(err)
+	}
+	write(t, env.localDir, "save.dat", "progress")
+	write(t, env.localDir, "remotecache.vdf", "steam")
+	env.engine.NoteLocalChange("game1") // the first version: named by content
+
+	m, err := delta.BuildManifest(env.localDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// As a build before the list recorded it.
+	rec, err := env.store.GetGameVersion("game1")
+	if err != nil || rec.Vector == "" {
+		t.Fatalf("no version recorded: %v", err)
+	}
+	vector := rec.Vector
+	rec.Hash = versionHashBeforeNeverSynced("", m)
+	if err := env.store.SaveGameVersion(rec); err != nil {
+		t.Fatal(err)
+	}
+
+	env.engine.AdoptExclusionView("game1", m, "", true)
+	env.engine.NoteLocalChange("game1")
+	rec, _ = env.store.GetGameVersion("game1")
+	if rec.Vector != vector {
+		t.Fatalf("version moved from %s to %s over a save nobody changed", vector, rec.Vector)
+	}
+	if rec.Hash != env.engine.versionHashOf("game1", m) {
+		t.Error("the recorded hash was not re-taken")
+	}
+
+	// Recorded for a save that has since changed: not adopted, so the change
+	// is a version.
+	write(t, env.localDir, "save.dat", "more progress")
+	delta.InvalidateRoot(env.localDir)
+	changed, _ := delta.BuildManifest(env.localDir)
+	rec.Hash = versionHashBeforeNeverSynced("", m)
+	_ = env.store.SaveGameVersion(rec)
+	env.engine.AdoptExclusionView("game1", changed, "", true)
+	env.engine.NoteLocalChange("game1")
+	rec, _ = env.store.GetGameVersion("game1")
+	if rec.Vector == vector {
+		t.Error("a real change after the upgrade did not become a version")
+	}
+}
+
+// Held only by the peer: not taken. Held only here: not pushed, and not
+// deleted because the peer lacks it.
+func TestNeverSynced_OneSidedIsNotADifference(t *testing.T) {
+	env := setupEngine(t)
+	write(t, env.localDir, "save.dat", "progress")
+	write(t, env.remoteDir, "save.dat", "progress")
+	write(t, env.remoteDir, "RemoteCache.VDF", "only there")
+
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil || res.Status != "in_sync" {
+		t.Fatalf("status = %q, %v; want in_sync", res.Status, err)
+	}
+	if _, err := os.Stat(filepath.Join(env.localDir, "RemoteCache.VDF")); err == nil {
+		t.Error("the peer's remotecache.vdf was pulled")
+	}
+
+	os.Remove(filepath.Join(env.remoteDir, "RemoteCache.VDF"))
+	write(t, env.localDir, "remotecache.vdf", "only here")
+	delta.InvalidateRoot(env.localDir)
+	res, err = env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil || res.Status != "in_sync" {
+		t.Fatalf("status = %q, %v; want in_sync", res.Status, err)
+	}
+	if _, err := os.Stat(filepath.Join(env.remoteDir, "remotecache.vdf")); err == nil {
+		t.Error("this device's remotecache.vdf was pushed")
+	}
+	if _, err := os.Stat(filepath.Join(env.localDir, "remotecache.vdf")); err != nil {
+		t.Error("this device's remotecache.vdf was deleted")
+	}
+}
diff --git a/internal/p2p/syncengine/partial_safety_test.go b/internal/p2p/syncengine/partial_safety_test.go
new file mode 100644
index 00000000..59588c39
--- /dev/null
+++ b/internal/p2p/syncengine/partial_safety_test.go
@@ -0,0 +1,113 @@
+package syncengine
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync/atomic"
+	"testing"
+)
+
+// A transfer that stops part-way must never turn into deletions.
+//
+// Seen in 2.3.1: a device that had received part of a 240k-file save offered
+// its partial copy back, the missing files were read as "deleted over there",
+// and 23k files were deleted on the device that still had them. 2.4 records
+// as shared only what both manifests showed (persistLineage), which should
+// rule that out; these hold it there, for the per-file and the batched pull.
+
+func countFiles(t *testing.T, dir string) int {
+	t.Helper()
+	n := 0
+	_ = filepath.Walk(dir, func(p string, info os.FileInfo, err error) error {
+		if err == nil && !info.IsDir() {
+			n++
+		}
+		return nil
+	})
+	return n
+}
+
+func writeSmallFiles(t *testing.T, dir string, n int) {
+	t.Helper()
+	for i := 0; i < n; i++ {
+		write(t, dir, fmt.Sprintf("world/map_%04d.bin", i), fmt.Sprintf("chunk %d", i))
+	}
+}
+
+func interruptedThenResumed(t *testing.T, env *engineEnv, total, stopAfter int) {
+	t.Helper()
+	writeSmallFiles(t, env.remoteDir, total)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	var fetched atomic.Int64
+	env.transport.onFetchBlocks = func() {
+		if fetched.Add(1) == int64(stopAfter) {
+			cancel()
+		}
+	}
+	if _, err := env.engine.SyncWithPeer(ctx, "game1", env.peer); err == nil {
+		t.Fatal("setup: the first sync was meant to be interrupted")
+	}
+	env.transport.onFetchBlocks = nil
+	cancel()
+
+	got := countFiles(t, env.localDir)
+	if got == 0 || got >= total {
+		t.Fatalf("setup: after the interruption this side has %d of %d files; want a partial copy", got, total)
+	}
+
+	// Resume. Nothing may be deleted anywhere, and everything must arrive.
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatalf("resumed sync: %v", err)
+	}
+	if len(env.transport.deletedOnPeer) > 0 {
+		t.Fatalf("the partial copy was propagated as %d deletions on the peer, e.g. %s",
+			len(env.transport.deletedOnPeer), env.transport.deletedOnPeer[0])
+	}
+	if n := countFiles(t, env.remoteDir); n != total {
+		t.Fatalf("peer has %d files, want all %d", n, total)
+	}
+	if n := countFiles(t, env.localDir); n != total {
+		t.Fatalf("this side has %d files after resuming, want %d", n, total)
+	}
+}
+
+func TestPartialPull_InterruptedThenResumed(t *testing.T) {
+	env := setupEngine(t)
+	interruptedThenResumed(t, env, 200, 60)
+}
+
+func TestPartialPull_InterruptedThenResumed_Batched(t *testing.T) {
+	// One batch in flight, so the first batch of 256 is written before the
+	// interruption lands in the second: a deterministic partial copy.
+	old := batchWorkers
+	batchWorkers = 1
+	t.Cleanup(func() { batchWorkers = old })
+	env, _ := setupBatchEngine(t, ProtoBatchFiles)
+	// Batches fetch through the same fake reader, one call per file; the
+	// 300th is inside the second batch.
+	interruptedThenResumed(t, env, 600, 300)
+}
+
+// The other side of the same situation: this device has everything, the peer
+// holds only part of it and never synced. The part it lacks is new to it, not
+// deleted by it.
+func TestPartialPeer_NeverReadAsDeletions(t *testing.T) {
+	env := setupEngine(t)
+	writeSmallFiles(t, env.localDir, 300)
+	for i := 0; i < 80; i++ {
+		write(t, env.remoteDir, fmt.Sprintf("world/map_%04d.bin", i), fmt.Sprintf("chunk %d", i))
+	}
+
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatalf("SyncWithPeer: %v", err)
+	}
+	if n := countFiles(t, env.localDir); n != 300 {
+		t.Fatalf("this side went from 300 to %d files — the peer's partial copy was read as deletions", n)
+	}
+	if len(env.transport.deletedOnPeer) > 0 {
+		t.Fatalf("deleted on peer: %v", env.transport.deletedOnPeer[:1])
+	}
+}
diff --git a/internal/p2p/syncengine/playing.go b/internal/p2p/syncengine/playing.go
new file mode 100644
index 00000000..c2abda1c
--- /dev/null
+++ b/internal/p2p/syncengine/playing.go
@@ -0,0 +1,47 @@
+package syncengine
+
+import "strings"
+
+// A game being played.
+//
+// A game autosaves every few minutes, and each autosave is a change of its
+// save. Synced as they come, every one of them reached every other device as
+// a version of its own, and each of those took a snapshot of it: eleven of
+// one game in 26 minutes on a device nobody was sitting at. And this device
+// took other devices' versions of a game running here, writing a save under
+// a game that was still using it.
+//
+// So while a game is played here (Playing, from the daemon's sessions), it
+// is not synced either way. Another device asking for it is told so, and
+// asks again later; nothing is taken for it. When the session ends, the save
+// as it was left goes to the other devices in one sync (daemon/sessions.go).
+
+// PlayingMessage is what a device answers when asked for a game being played
+// on it. It contains SettlingMessage, so a device on a build that predates
+// this takes it as "busy, ask again" too.
+const PlayingMessage = "This device is playing this game right now; its save follows when the session ends. " + SettlingMessage
+
+const playingMark = "is playing this game right now"
+
+// isPlaying reports whether an error from a peer is its "playing" answer.
+func isPlaying(err error) bool {
+	return err != nil && strings.Contains(err.Error(), playingMark)
+}
+
+// playingErr is SyncGame declining to sync a game being played here. It
+// counts as ErrHeld for callers that only want to know it was not an error.
+type playingErr struct{}
+
+func (playingErr) Error() string {
+	return "it is being played on this device; it syncs when the session ends"
+}
+
+func (playingErr) Is(target error) bool { return target == ErrHeld || target == ErrPlayingHere }
+
+// ErrPlayingHere is SyncGame declining to sync a game being played here.
+var ErrPlayingHere error = playingErr{}
+
+// PlayingHere reports whether a game is being played on this device.
+func (e *Engine) PlayingHere(gameID string) bool {
+	return e.Playing != nil && e.Playing(gameID)
+}
diff --git a/internal/p2p/syncengine/playing_test.go b/internal/p2p/syncengine/playing_test.go
new file mode 100644
index 00000000..90112fb3
--- /dev/null
+++ b/internal/p2p/syncengine/playing_test.go
@@ -0,0 +1,49 @@
+package syncengine
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// A game being played here is not synced either way until the session ends:
+// nothing is taken for it, and callers are not told it failed.
+func TestPlaying_NotSyncedWhilePlayedHere(t *testing.T) {
+	env := setupEngine(t)
+	write(t, env.localDir, "slot1.sav", "being played")
+	write(t, env.remoteDir, "slot1.sav", "another device's")
+	playing := true
+	env.engine.Playing = func(string) bool { return playing }
+
+	_, err := env.engine.SyncGame(context.Background(), "game1", []Peer{env.peer})
+	if !errors.Is(err, ErrPlayingHere) || !errors.Is(err, ErrHeld) {
+		t.Fatalf("err = %v, want ErrPlayingHere, quiet as ErrHeld", err)
+	}
+	if got, _ := os.ReadFile(filepath.Join(env.localDir, "slot1.sav")); string(got) != "being played" {
+		t.Fatalf("the save of a running game was replaced: %q", got)
+	}
+
+	playing = false
+	if _, err := env.engine.SyncGame(context.Background(), "game1", []Peer{env.peer}); err != nil {
+		t.Fatalf("after the session: %v", err)
+	}
+}
+
+// Another device playing the game answers "playing": nothing is compared or
+// stamped, and asking does not count as a failure.
+func TestPlaying_PeerPlayingIsWaitedFor(t *testing.T) {
+	env := setupEngine(t)
+	write(t, env.localDir, "slot1.sav", "here")
+	env.transport.manifestErr = errors.New(PlayingMessage)
+
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil || res.Status != "peer_playing" {
+		t.Fatalf("status = %q, %v; want peer_playing", res.Status, err)
+	}
+	// A device on an older build reads the same answer as "busy".
+	if !isSettling(errors.New(PlayingMessage)) {
+		t.Error("the playing answer is not recognisable as busy by older builds")
+	}
+}
diff --git a/internal/p2p/syncengine/quick.go b/internal/p2p/syncengine/quick.go
new file mode 100644
index 00000000..cf0c3906
--- /dev/null
+++ b/internal/p2p/syncengine/quick.go
@@ -0,0 +1,83 @@
+package syncengine
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/opensave/opensave/internal/store"
+)
+
+// quickInSync settles the common case of a sync in one small exchange: this
+// device's save still hashes to the base both devices last agreed on, and
+// the peer, asked whether it does too (ManifestQuery.IfHash), says
+// "unchanged". Then there is nothing to decide, and neither manifest needs
+// to cross the wire — for a save of a quarter-million files that is tens of
+// MB of JSON per peer, which the minute-long reconcile used to fetch every
+// minute to learn that nothing had changed.
+//
+// It only ever answers "in sync" when both sides provably hold the agreed
+// state. In every other case it reports not done and the full sync runs; a
+// full manifest that came back anyway (a peer that predates the question)
+// is handed over in prefetched so it is not fetched twice.
+//
+// Deliberately narrow:
+//   - no exclusion rules: the agreed base is taken over the filtered
+//     manifest, the peer hashes its unfiltered one;
+//   - no extra save locations: those carry bases of their own (RootHash);
+//   - no conflict waiting, and the same branch on both sides.
+func (e *Engine) quickInSync(ctx context.Context, gameID string, game store.Game, peer Peer, q ManifestQuery) (res Result, done bool, prefetched *ManifestResponse) {
+	agreed := e.Store.GetAgreedHash(gameID, peer.ID)
+	if agreed == "" || !e.rulesFor(gameID).Empty() {
+		return Result{}, false, nil
+	}
+	if roots, err := e.Store.GameRootPaths(gameID); err != nil || len(roots) > 0 {
+		return Result{}, false, nil
+	}
+	e.mu.Lock()
+	conflicted := e.activeConflicts[gameID] != nil
+	e.mu.Unlock()
+	if conflicted {
+		return Result{}, false, nil
+	}
+	local, err := e.ReadManifest(ctx, gameID, game.SavePath)
+	if err != nil || len(local.Extra) > 0 || local.ManifestHash() != agreed {
+		return Result{}, false, nil
+	}
+
+	q.IfHash = agreed
+	resp, err := e.Transport.FetchManifest(ctx, peer, gameID, q)
+	if err != nil {
+		// Let the full path fetch again and classify the error as it always has.
+		return Result{}, false, nil
+	}
+	if !resp.Unchanged {
+		return Result{}, false, &resp
+	}
+	if resp.ManifestHash != agreed || (resp.ActiveBranch != "" && resp.ActiveBranch != game.ActiveBranch) {
+		// Unchanged, but not in a way this shortcut may act on: the full
+		// exchange decides.
+		return Result{}, false, nil
+	}
+	// The same files; the versions still have to say the same (version.go).
+	if resp.Version != nil && game.AutoSync {
+		res, final := e.quickVersionCheck(game, peer, local, *resp.Version)
+		if !final {
+			return Result{}, false, nil
+		}
+		if res.Status == versionStatusWaiting {
+			return res, true, nil
+		}
+	}
+
+	// Same as the in-sync outcome of a full sync, minus what is already
+	// recorded: the base is unchanged and the lineage with it.
+	e.Transport.ReportSyncEvent(peer, gameID, "in-sync", map[string]any{
+		"peerName":     e.deviceName(),
+		"manifestHash": agreed,
+	})
+	if resp.LatestSnapshot != nil && len(local.Files) > 0 && !e.hasSnapshot(gameID, game) {
+		e.recordMirrorSnapshot(gameID, game, peer, *resp.LatestSnapshot,
+			fmt.Sprintf("Synced from peer: %s (%s)", peer.Name, resp.LatestSnapshot.Comment))
+	}
+	return Result{Status: "in_sync", Direction: "none"}, true, nil
+}
diff --git a/internal/p2p/syncengine/quick_test.go b/internal/p2p/syncengine/quick_test.go
new file mode 100644
index 00000000..3cfdf8d8
--- /dev/null
+++ b/internal/p2p/syncengine/quick_test.go
@@ -0,0 +1,117 @@
+package syncengine
+
+import (
+	"context"
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/snapshot"
+)
+
+// ifHashTransport answers IfHash the way a current peer does, and counts
+// how many full manifests it had to send.
+type ifHashTransport struct {
+	*fakeTransport
+	full, unchanged int
+}
+
+func (t *ifHashTransport) FetchManifest(ctx context.Context, peer Peer, gameID string, q ManifestQuery) (ManifestResponse, error) {
+	resp, err := t.fakeTransport.FetchManifest(ctx, peer, gameID, q)
+	if err != nil {
+		return resp, err
+	}
+	if q.IfHash != "" && resp.Manifest.ManifestHash() == q.IfHash {
+		t.unchanged++
+		return ManifestResponse{ActiveBranch: resp.ActiveBranch, LatestSnapshot: resp.LatestSnapshot,
+			Unchanged: true, ManifestHash: q.IfHash, Manifest: delta.Manifest{}}, nil
+	}
+	t.full++
+	return resp, nil
+}
+
+func setupIfHashEngine(t *testing.T) (*engineEnv, *ifHashTransport) {
+	t.Helper()
+	env := setupEngine(t)
+	tr := &ifHashTransport{fakeTransport: env.transport}
+	env.engine = New(env.store, snapshot.New(env.store), tr)
+	return env, tr
+}
+
+func TestQuickInSync_SkipsTheManifestWhenNothingChanged(t *testing.T) {
+	env, tr := setupIfHashEngine(t)
+	write(t, env.localDir, "save.dat", "same")
+	write(t, env.remoteDir, "save.dat", "same")
+
+	// First sync converges and records the agreed base.
+	if res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil || res.Status != "in_sync" {
+		t.Fatalf("first sync = %+v, %v", res, err)
+	}
+	if tr.full != 1 {
+		t.Fatalf("setup: full manifests = %d, want 1", tr.full)
+	}
+
+	// Nothing changed anywhere: settled without a full manifest.
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil || res.Status != "in_sync" {
+		t.Fatalf("second sync = %+v, %v", res, err)
+	}
+	if tr.full != 1 || tr.unchanged != 1 {
+		t.Errorf("second sync fetched %d full manifests and %d unchanged answers; want 0 more full, 1 unchanged", tr.full-1, tr.unchanged)
+	}
+}
+
+// A change on either side must go the full way — the shortcut may only ever
+// say "in sync" when both sides provably hold the agreed state.
+func TestQuickInSync_ChangesStillSync(t *testing.T) {
+	env, tr := setupIfHashEngine(t)
+	write(t, env.localDir, "save.dat", "same")
+	write(t, env.remoteDir, "save.dat", "same")
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatal(err)
+	}
+
+	// The peer changes.
+	write(t, env.remoteDir, "new.dat", "made on the other device")
+	res, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Status != "updated" || res.Direction != "pull" {
+		t.Fatalf("peer change: result = %+v, want updated/pull", res)
+	}
+
+	// This side changes.
+	write(t, env.localDir, "mine.dat", "made here")
+	before := env.transport.pullTriggers
+	res, err = env.engine.SyncWithPeer(context.Background(), "game1", env.peer)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Status == "in_sync" {
+		t.Fatalf("local change was reported in sync: %+v", res)
+	}
+	if env.transport.pullTriggers == before {
+		t.Error("the peer was not told to pull this side's change")
+	}
+	if tr.unchanged != 0 {
+		t.Errorf("unchanged answers = %d with a change on one side, want 0", tr.unchanged)
+	}
+}
+
+// A peer that predates the question sends the full manifest; it is used,
+// not fetched a second time.
+func TestQuickInSync_OldPeerManifestIsNotFetchedTwice(t *testing.T) {
+	env := setupEngine(t) // plain fake: ignores IfHash
+	write(t, env.localDir, "save.dat", "same")
+	write(t, env.remoteDir, "save.dat", "same")
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatal(err)
+	}
+	before := env.transport.manifestCalls
+	if _, err := env.engine.SyncWithPeer(context.Background(), "game1", env.peer); err != nil {
+		t.Fatal(err)
+	}
+	if got := env.transport.manifestCalls - before; got != 1 {
+		t.Errorf("manifest requests for one sync = %d, want 1", got)
+	}
+}
diff --git a/internal/p2p/syncengine/settle.go b/internal/p2p/syncengine/settle.go
index 34942e10..a66c1c8d 100644
--- a/internal/p2p/syncengine/settle.go
+++ b/internal/p2p/syncengine/settle.go
@@ -131,10 +131,44 @@ func (e *Engine) Writing(gameID string) (done func()) {
 		once = true
 		g := e.gateLocked(gameID)
 		g.writers--
+		if e.writtenAt == nil {
+			e.writtenAt = map[string]time.Time{}
+		}
+		e.writtenAt[gameID] = time.Now()
 		e.wakeLocked(gameID, g)
 	}
 }
 
+// writeEcho is how long after a sync stops writing a game its file events may
+// still be arriving.
+const writeEcho = 10 * time.Second
+
+// BeingWritten reports whether a sync on this device is writing a game's save
+// now, or was a moment ago — what the watcher asks before taking a burst it
+// could not attribute (an overflow of file events) for a change of the game's.
+// A pull of a save of many files overflows the event queue again and again,
+// and each one read as the game saving: an automatic snapshot of a half-pulled
+// save every few seconds, which pushed the real ones out of retention.
+func (e *Engine) BeingWritten(gameID string) bool {
+	if e.WritingNow(gameID) {
+		return true
+	}
+	e.settleMu.Lock()
+	defer e.settleMu.Unlock()
+	return time.Since(e.writtenAt[gameID]) < writeEcho
+}
+
+// WritingNow reports whether a sync on this device is writing a game's save at
+// this moment — what the watcher waits out before looking at a burst, so it
+// never judges a save half-way between two states. Not the moments after:
+// a change the game makes then is the game's, and its snapshot is not held up.
+func (e *Engine) WritingNow(gameID string) bool {
+	e.settleMu.Lock()
+	defer e.settleMu.Unlock()
+	g := e.gates[gameID]
+	return g != nil && (g.writers > 0 || g.waitingWriters > 0)
+}
+
 // Reading holds the game's save still for a sync to read it: it waits until
 // nothing is writing it, then keeps writers out until done is called. It
 // returns ErrSettling if ctx ends first. Hold it only for the reading itself —
diff --git a/internal/p2p/syncengine/transport.go b/internal/p2p/syncengine/transport.go
index c39e315c..c2afbf8d 100644
--- a/internal/p2p/syncengine/transport.go
+++ b/internal/p2p/syncengine/transport.go
@@ -2,6 +2,7 @@ package syncengine
 
 import (
 	"context"
+	"errors"
 
 	"github.com/opensave/opensave/internal/delta"
 )
@@ -25,12 +26,54 @@ type SnapshotInfo struct {
 	ID        string `json:"id"`
 	Timestamp string `json:"timestamp"`
 	Comment   string `json:"comment"`
+	// ContentHash names what the snapshot holds (snapshot.ContentKey): a
+	// device that already has a snapshot of exactly those files records this
+	// one's id as an alias of it rather than archiving the save again.
+	ContentHash string `json:"contentHash,omitempty"`
 }
 
 // ProtoMultiRoot is the protocol revision at which a peer understands games
 // with more than one save location.
 const ProtoMultiRoot = 1
 
+// ProtoBatchFiles is the revision at which a peer serves the blocks of many
+// files in one request (FetchFileBatch). Before it, every file cost its own
+// round trip — ~200ms each in practice, which for a save of a quarter-million
+// small files (Project Zomboid map chunks) meant half a day for 50MB.
+const ProtoBatchFiles = 2
+
+// ProtoVersions is the revision at which a peer keeps save versions
+// (version.go). A peer below it decides by the old file comparison, which
+// reads whatever its copy lacks as deleted; nothing is taken from such a peer
+// or deleted on its word (OldBuildRefused).
+const ProtoVersions = 3
+
+// FileBlocksRequest asks for some blocks of one file, as one entry of a batch.
+type FileBlocksRequest struct {
+	RelPath      string `json:"relPath"`
+	BlockIndices []int  `json:"blockIndices"`
+	BlockSize    int    `json:"blockSize"`
+}
+
+// FileBlocks is one file's answer inside a batch. Error is set, and Blocks
+// empty, when the responder could not read that file.
+type FileBlocks struct {
+	RelPath string      `json:"relPath"`
+	Blocks  []BlockData `json:"blocks"`
+	Error   string      `json:"error,omitempty"`
+}
+
+// BatchFetcher is implemented by transports that can fetch the blocks of
+// many files, all in one save location, in a single request. Optional: the
+// engine checks for it, and for a peer that advertised ProtoBatchFiles.
+type BatchFetcher interface {
+	FetchFileBatch(ctx context.Context, peer Peer, gameID, root string, files []FileBlocksRequest) ([]FileBlocks, error)
+}
+
+// ErrBatchUnsupported is what a transport returns when it cannot batch for
+// this peer (the relay path); the engine then pulls file by file.
+var ErrBatchUnsupported = errors.New("batched file fetch is not supported for this peer")
+
 // ManifestResponse is what a peer returns for a manifest request.
 type ManifestResponse struct {
 	Manifest       delta.Manifest `json:"manifest"`
@@ -56,6 +99,26 @@ type ManifestResponse struct {
 	// did (see hold.go). Without it an empty location is not taken as every
 	// file in it deleted — see emptiedUnconfirmed.
 	DeletionConfirmed bool `json:"deletionConfirmed,omitempty"`
+
+	// Unchanged answers a request that carried ManifestQuery.IfHash: the
+	// responder's save still hashes to exactly that, so Manifest is left
+	// empty. Only ever set when asked; a peer that predates it ignores the
+	// question and sends the full manifest, which is handled as before.
+	Unchanged    bool   `json:"unchanged,omitempty"`
+	ManifestHash string `json:"manifestHash,omitempty"`
+
+	// Version is the responder's version of the save (version.go), absent
+	// from builds that predate it and for games it does not version.
+	Version *VersionInfo `json:"version,omitempty"`
+	// Versions says the responder keeps save versions at all. The relay
+	// path sends it instead of a Proto, which there would also claim things
+	// that path does not serve.
+	Versions bool `json:"versions,omitempty"`
+}
+
+// KeepsVersions reports whether the responder keeps save versions.
+func (r ManifestResponse) KeepsVersions() bool {
+	return r.Versions || r.Proto >= ProtoVersions
 }
 
 // FileRef identifies one file inside one of a game's save locations.
@@ -81,6 +144,9 @@ type ManifestQuery struct {
 	// cover art, instead of a blank tile.
 	AppID    string
 	CoverURL string
+	// IfHash asks the peer to answer "unchanged" instead of its manifest when
+	// its save still hashes to this — the base both devices last agreed on.
+	IfHash string
 }
 
 // BlockData is one fetched block.
diff --git a/internal/p2p/syncengine/version.go b/internal/p2p/syncengine/version.go
new file mode 100644
index 00000000..41f880cd
--- /dev/null
+++ b/internal/p2p/syncengine/version.go
@@ -0,0 +1,1292 @@
+package syncengine
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"sort"
+	"strings"
+	"time"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/ignore"
+	"github.com/opensave/opensave/internal/owntouch"
+	"github.com/opensave/opensave/internal/store"
+)
+
+// Save versions.
+//
+// Every device knows which version of a game's save it holds, as a version
+// vector: one counter per device that has ever changed the save, moved only
+// when the save really changes there (the watcher saw the game or the user
+// write it — never a sync). Comparing two vectors answers, exactly, the
+// question the file-by-file comparison could only guess at from what each side
+// happens to hold:
+//
+//   - one is older than the other: that device is simply behind. It takes the
+//     newer save as a whole — every file, and every deletion — and only from a
+//     device that holds it completely. Nothing is ever taken from it, pushed
+//     by it, or deleted on its say-so, and it is a source for nobody.
+//   - equal: the same save.
+//   - neither: both changed it independently. Only then is anyone asked.
+//
+// A device also remembers the newest version it has heard of (the target)
+// until it holds it. That is what keeps two outdated devices from syncing with
+// each other: each knows something newer exists, so each waits for a device
+// that holds it, instead of reading the other's half-finished copy as the save
+// and passing on what it lacks as deletions.
+//
+// An empty folder holds no version at all, which is older than every version,
+// so a new or emptied device fills itself from a complete one and never raises
+// a conflict. A save that existed before this was introduced gets a version
+// named after its content (genesisKey), so devices that already held the same
+// save agree at once, and ones that did not are compared the way they always
+// were, until they agree.
+
+// VersionVector is writer key → counter.
+type VersionVector map[string]int64
+
+type versionOrder int
+
+const (
+	versionEqual versionOrder = iota
+	versionOlder
+	versionNewer
+	versionConcurrent
+)
+
+// genesisAll is the entry "Use this save everywhere" adds (UseSaveEverywhere):
+// a version holding it counts as having every save from before versions
+// existed behind it, whatever its content was named.
+const genesisAll = "g:*"
+
+// holds reports whether v includes entry k at counter n.
+func holds(v VersionVector, k string, n int64) bool {
+	if v[k] >= n {
+		return true
+	}
+	return k != genesisAll && strings.HasPrefix(k, "g:") && v[genesisAll] >= 1
+}
+
+// compareVersions says how a relates to b.
+func compareVersions(a, b VersionVector) versionOrder {
+	aBehind, bBehind := false, false
+	for k, bv := range b {
+		if !holds(a, k, bv) {
+			aBehind = true
+			break
+		}
+	}
+	for k, av := range a {
+		if !holds(b, k, av) {
+			bBehind = true
+			break
+		}
+	}
+	switch {
+	case !aBehind && !bBehind:
+		return versionEqual
+	case aBehind && !bBehind:
+		return versionOlder
+	case !aBehind && bBehind:
+		return versionNewer
+	default:
+		return versionConcurrent
+	}
+}
+
+// covers reports whether a is b or newer.
+func covers(a, b VersionVector) bool {
+	o := compareVersions(a, b)
+	return o == versionEqual || o == versionNewer
+}
+
+func mergeVersions(a, b VersionVector) VersionVector {
+	out := make(VersionVector, len(a)+len(b))
+	for k, v := range a {
+		out[k] = v
+	}
+	for k, v := range b {
+		if v > out[k] {
+			out[k] = v
+		}
+	}
+	return out
+}
+
+func (v VersionVector) clone() VersionVector { return mergeVersions(v, nil) }
+
+// genesisOnly reports whether every entry names a save that predates version
+// tracking (genesisKey), i.e. nobody has changed it since.
+func (v VersionVector) genesisOnly() bool {
+	if len(v) == 0 {
+		return false
+	}
+	for k := range v {
+		if !strings.HasPrefix(k, "g:") {
+			return false
+		}
+	}
+	return true
+}
+
+// String is a short, stable form for the log.
+func (v VersionVector) String() string {
+	if len(v) == 0 {
+		return "none"
+	}
+	keys := make([]string, 0, len(v))
+	for k := range v {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	parts := make([]string, len(keys))
+	for i, k := range keys {
+		short := k
+		if len(short) > 8 {
+			short = short[:8]
+		}
+		parts[i] = fmt.Sprintf("%s=%d", short, v[k])
+	}
+	return strings.Join(parts, ",")
+}
+
+// genesisKey names the version of a save that existed before versions did,
+// after its content: devices that hold the same files get the same name.
+// Directories are left out — they are created by any sync, and are not what
+// makes two saves the same.
+func genesisKey(m delta.Manifest) string {
+	return "g:" + filesKey(m, ignore.Rules{})[:16]
+}
+
+// filesKey hashes which files a save holds and what is in them: paths and
+// content hashes, nothing else — not folders, not times. Excluded paths are
+// left out.
+func filesKey(m delta.Manifest, rules ignore.Rules) string {
+	return filesKeyOf(m, func(p string) bool { return excluded(rules, p) })
+}
+
+func filesKeyOf(m delta.Manifest, leaveOut func(string) bool) string {
+	paths := make([]string, 0, len(m.Files))
+	for p := range m.Files {
+		if !leaveOut(p) {
+			paths = append(paths, p)
+		}
+	}
+	sort.Strings(paths)
+	h := sha256.New()
+	for _, p := range paths {
+		h.Write([]byte(p))
+		h.Write([]byte{0})
+		h.Write([]byte(m.Files[p].Hash))
+		h.Write([]byte{0})
+	}
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// versionHashOf is what is recorded with a version: the save's files as this
+// device syncs them, tagged with the exclusion rules it was taken under. A
+// save that no longer hashes to it was changed here since, whether or not the
+// watcher said so.
+//
+// Filtered, because a file nobody syncs — a device's own config — changing
+// is not a new version of the save. Tagged, because writing a rule is not one
+// either: it changes the filtered hash on every device at once, and each took
+// that as a change of its own, so the next sync asked about a divergence
+// nobody made. A hash taken under other rules is re-taken, not compared
+// (sameSaveAs). The files no save is made of (delta.NeverSyncedList) are in
+// the tag for the same reason.
+func (e *Engine) versionHashOf(gameID string, primary delta.Manifest) string {
+	text := ""
+	if game, err := e.Store.GetGame(gameID); err == nil {
+		text = e.IgnoreText(game)
+	}
+	return versionHashUnder(text, primary)
+}
+
+// versionHashUnder is versionHashOf under a given exclusion list.
+func versionHashUnder(text string, primary delta.Manifest) string {
+	tag := sha256.Sum256([]byte(text + "\x00" + delta.NeverSyncedList))
+	return hex.EncodeToString(tag[:4]) + ":" + filesKey(primary, ignore.Parse(text))
+}
+
+// versionHashBeforeNeverSynced is versionHashOf as builds before
+// delta.NeverSynced took it: the rules alone in the tag, those files counted.
+func versionHashBeforeNeverSynced(rulesText string, primary delta.Manifest) string {
+	tag := sha256.Sum256([]byte(rulesText))
+	rules := ignore.Parse(rulesText)
+	return hex.EncodeToString(tag[:4]) + ":" + filesKeyOf(primary, func(p string) bool {
+		return !rules.Empty() && rules.Match(p)
+	})
+}
+
+// AdoptExclusionView re-takes the hash recorded with this device's version of
+// a game under what it is excluded by now (IgnoreText), when the save is the
+// one it was recorded for under oldText — the files excluded then or now
+// aside. Run when the exclusions change without anyone writing a rule (a new
+// build's delta.NeverSyncedList, the game database naming settings files),
+// before anything compares: otherwise the only difference, which files are
+// counted, reads as the save changed here, and every device names the same
+// save a new version of its own at once.
+//
+// beforeNeverSynced says the old hash was taken by a build that predates
+// delta.NeverSynced, and counted those files.
+func (e *Engine) AdoptExclusionView(gameID string, primary delta.Manifest, oldText string, beforeNeverSynced bool) {
+	if _, err := e.Store.GetGame(gameID); err != nil {
+		return
+	}
+	// The base agreed with each device, where it is still this save: the
+	// two agreed on it, and the files now left out leave them agreeing on
+	// the rest. One that no longer matches is a real change, left to count.
+	oldRules := ignore.Parse(oldText)
+	var oldView delta.Manifest
+	if beforeNeverSynced && oldRules.Empty() {
+		oldView = primary
+	} else if beforeNeverSynced {
+		oldView = filterManifestBy(primary, func(p string) bool { return !oldRules.Empty() && oldRules.Match(p) })
+	} else {
+		oldView = filterManifest(primary, oldRules)
+	}
+	if was, now := oldView.ManifestHash(), filterManifest(primary, e.rulesFor(gameID)).ManifestHash(); was != now {
+		if peers, err := e.Store.ListPeers(); err == nil {
+			for _, p := range peers {
+				if e.Store.GetAgreedHash(gameID, p.ID) == was {
+					_ = e.Store.SetAgreedHash(gameID, p.ID, now)
+				}
+			}
+		}
+	}
+
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	gv, err := e.loadVersionLocked(gameID)
+	if err != nil || gv.rec.GameID == "" || gv.rec.Hash == "" {
+		return
+	}
+	was := versionHashUnder(oldText, primary)
+	if beforeNeverSynced {
+		was = versionHashBeforeNeverSynced(oldText, primary)
+	}
+	if now := e.versionHashOf(gameID, primary); gv.rec.Hash == was && was != now {
+		gv.rec.Hash = now
+		_ = e.saveVersionLocked(gv)
+	}
+}
+
+// sameSaveAs reports whether a hash recorded with a version still describes
+// the save that now hashes to current: the same, or taken under different
+// exclusion rules, which says nothing about the save.
+func sameSaveAs(recorded, current string) bool {
+	if recorded == current {
+		return true
+	}
+	r, _, okR := strings.Cut(recorded, ":")
+	c, _, okC := strings.Cut(current, ":")
+	return okR && okC && r != c
+}
+
+// VersionInfo is a device's version of one game, as it travels in a manifest
+// response. Absent from devices that predate it, which are synced the way they
+// always were.
+type VersionInfo struct {
+	Vector VersionVector `json:"vector"`
+	// Target is the newer version the device knows of and does not hold yet.
+	Target VersionVector `json:"target,omitempty"`
+	// HasFiles: the device holds at least one save file for the game.
+	HasFiles bool `json:"hasFiles"`
+	// Answered is the version someone on the device kept its own save over
+	// in a conflict (Unix ms in AnsweredAt). Resolution is mutual: the device
+	// holding that version is asked in turn, not overwritten.
+	Answered   VersionVector `json:"answered,omitempty"`
+	AnsweredAt int64         `json:"answeredAt,omitempty"`
+}
+
+// Outdated reports whether the device knows its save is behind another's.
+func (v VersionInfo) Outdated() bool {
+	return len(v.Target) > 0 && compareVersions(v.Vector, v.Target) == versionOlder
+}
+
+// gameVersion is the stored record with its vectors decoded.
+type gameVersion struct {
+	rec      store.GameVersion
+	vec      VersionVector
+	target   VersionVector
+	answered VersionVector
+}
+
+func (e *Engine) loadVersionLocked(gameID string) (gameVersion, error) {
+	rec, err := e.Store.GetGameVersion(gameID)
+	if err != nil {
+		return gameVersion{}, err
+	}
+	gv := gameVersion{rec: rec, vec: VersionVector{}}
+	_ = json.Unmarshal([]byte(rec.Vector), &gv.vec)
+	if gv.vec == nil {
+		gv.vec = VersionVector{}
+	}
+	if rec.Target != "" {
+		_ = json.Unmarshal([]byte(rec.Target), &gv.target)
+	}
+	if rec.Answered != "" {
+		_ = json.Unmarshal([]byte(rec.Answered), &gv.answered)
+	}
+	return gv, nil
+}
+
+func (e *Engine) saveVersionLocked(gv gameVersion) error {
+	if gv.rec.GameID == "" {
+		return nil
+	}
+	raw, _ := json.Marshal(gv.vec)
+	gv.rec.Vector = string(raw)
+	gv.rec.Target = ""
+	if len(gv.target) > 0 {
+		t, _ := json.Marshal(gv.target)
+		gv.rec.Target = string(t)
+	}
+	gv.rec.Answered = ""
+	if len(gv.answered) > 0 {
+		a, _ := json.Marshal(gv.answered)
+		gv.rec.Answered = string(a)
+	} else {
+		gv.rec.AnsweredAt = 0
+	}
+	return e.Store.SaveGameVersion(gv.rec)
+}
+
+// ensureGenesisLocked names a save that has files and no version yet. Not
+// while the device is taking a newer version: its folder is then a part-way
+// copy of someone else's, and naming that would make it a version of its own.
+func (gv *gameVersion) ensureGenesis(local delta.Manifest, hash string) bool {
+	if len(gv.vec) > 0 || len(local.Files) == 0 || len(gv.target) > 0 || gv.rec.Pulling {
+		return false
+	}
+	gv.vec = VersionVector{genesisKey(local): 1}
+	gv.rec.Hash = hash
+	return true
+}
+
+// refreshLocked brings this device's version up to date with its save before
+// anyone compares it: a save that predates versions is named, and one that no
+// longer hashes to what its version recorded was changed here — by the game,
+// whether or not the watcher has said so yet (it waits for the game to finish
+// writing, and while a game holds its files open that can be a long time). It
+// is a version of its own from now. Without this, a device whose change was
+// still unrecorded looked merely behind, and took the other's save over it.
+//
+// Not while a pull towards a newer version is unfinished: the folder is then a
+// part-way copy of someone else's save, not a change made here. Reports
+// whether the record changed.
+func (e *Engine) refreshLocked(gv *gameVersion, game store.Game, local delta.Manifest) bool {
+	hash := e.versionHashOf(game.ID, local)
+	if gv.ensureGenesis(local, hash) {
+		return true
+	}
+	if gv.rec.Pulling || len(gv.vec) == 0 || gv.rec.Hash == "" || gv.rec.Hash == hash {
+		return false
+	}
+	if sameSaveAs(gv.rec.Hash, hash) {
+		// Taken under other exclusion rules, so it cannot say whether the
+		// save changed: re-taken under the new ones. A change made at the
+		// same time still becomes a version — the watcher reports it
+		// (NoteLocalChange).
+		gv.rec.Hash = hash
+		return true
+	}
+	e.bumpLocked(gv, game.Name, hash)
+	return true
+}
+
+// learn takes in what a peer knows: a version newer than this device's
+// becomes its target, unless it already knows of one newer still.
+func (gv *gameVersion) learn(theirs VersionInfo) {
+	for _, c := range []VersionVector{theirs.Vector, theirs.Target} {
+		if len(c) == 0 || compareVersions(gv.vec, c) != versionOlder {
+			continue
+		}
+		if len(gv.target) == 0 || compareVersions(gv.target, c) == versionOlder {
+			gv.target = c.clone()
+		}
+	}
+	gv.dropReachedTarget()
+}
+
+// dropReachedTarget forgets a target this device's version covers — or has
+// gone past it sideways, by changing on its own: that is no longer "behind",
+// it is a conflict, and the version comparison says so by itself.
+func (gv *gameVersion) dropReachedTarget() {
+	if len(gv.target) > 0 && compareVersions(gv.vec, gv.target) != versionOlder {
+		gv.target = nil
+	}
+}
+
+func (gv gameVersion) info(hasFiles bool) VersionInfo {
+	v := VersionInfo{Vector: gv.vec.clone(), Target: gv.target.clone(), HasFiles: hasFiles}
+	if len(gv.answered) > 0 {
+		v.Answered, v.AnsweredAt = gv.answered.clone(), gv.rec.AnsweredAt
+	}
+	return v
+}
+
+// adopt makes this device hold theirs: their version, and their answer to a
+// conflict if they carry one — the device holding the version they kept
+// theirs over is still to be asked, whoever it meets.
+func (gv *gameVersion) adopt(vec VersionVector, theirs VersionInfo) {
+	gv.vec = vec.clone()
+	gv.answered, gv.rec.AnsweredAt = nil, 0
+	if len(theirs.Answered) > 0 {
+		gv.answered, gv.rec.AnsweredAt = theirs.Answered.clone(), theirs.AnsweredAt
+	}
+	gv.dropReachedTarget()
+}
+
+// LocalVersion is this device's version of a game, for a manifest response.
+// manifest is the save being served; a save that predates versions is named
+// here if it has not been yet. Nil when the game takes no part in versions:
+// one with automatic sync turned off is not watched, so nothing would ever
+// move its version — the peer then syncs it the way it always has.
+func (e *Engine) LocalVersion(game store.Game, manifest delta.Manifest) *VersionInfo {
+	if !game.AutoSync {
+		return nil
+	}
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	gv, err := e.loadVersionLocked(game.ID)
+	if err != nil {
+		return nil
+	}
+	if e.refreshLocked(&gv, game, manifest) {
+		_ = e.saveVersionLocked(gv)
+	}
+	v := gv.info(len(manifest.Files) > 0)
+	return &v
+}
+
+// NoteLocalChange moves this device's version on: the watcher saw the game
+// or the user change the save (never a sync — those are told apart by
+// internal/owntouch). Called before the change is offered to anyone.
+func (e *Engine) NoteLocalChange(gameID string) {
+	game, err := e.Store.GetGame(gameID)
+	if err != nil || !game.AutoSync {
+		return
+	}
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	gv, err := e.loadVersionLocked(gameID)
+	if err != nil || gv.rec.GameID == "" {
+		return
+	}
+	if gv.rec.Pulling {
+		// Files a pull towards a newer version wrote — this run's own, still
+		// arriving, or left by a run that stopped part-way (after a restart
+		// the watcher reports those as a change: it cannot know who wrote
+		// them). Not a version: the folder is a part-way copy of someone
+		// else's, and naming it would hand that copy to every other device.
+		// The pull finishing is what moves the version. Anything written here
+		// meanwhile is in the snapshot taken before the pull replaces it.
+		return
+	}
+	m, err := delta.BuildManifest(game.SavePath)
+	if err != nil {
+		return
+	}
+	hash := e.versionHashOf(gameID, m)
+	if len(gv.vec) == 0 {
+		// First change ever seen here, on a save that predates versions or a
+		// new one. Named after its content, so a device holding the same files
+		// agrees with it rather than conflicting.
+		if gv.ensureGenesis(m, hash) {
+			_ = e.saveVersionLocked(gv)
+		}
+		return
+	}
+	if hash == gv.rec.Hash {
+		// Already a version: a sync noticed it first (refreshLocked), or the
+		// change was only to files nobody syncs.
+		return
+	}
+	// The watcher saw the game or the user write the save: a version, even if
+	// the exclusion rules changed since the last one (writing a rule raises
+	// no file event, so that alone never arrives here).
+	e.bumpLocked(&gv, game.Name, hash)
+}
+
+// bumpLocked gives this device a new version of its own, after hash.
+func (e *Engine) bumpLocked(gv *gameVersion, name, hash string) {
+	gv.rec.Counter++
+	gv.vec = gv.vec.clone()
+	gv.vec[gv.rec.Key] = gv.rec.Counter
+	gv.rec.Hash = hash
+	gv.dropReachedTarget()
+	if err := e.saveVersionLocked(*gv); err == nil {
+		e.Log("info", fmt.Sprintf("%s changed on this device — version %s", name, gv.vec))
+	}
+}
+
+// versionAfterResolution records a conflict's answer as a version.
+//
+// Keeping the peer's save makes this device hold what both held: newer than
+// both, so the peer finds nothing to take and devices behind either take it.
+//
+// Keeping this device's own is a new version of its own — but deliberately
+// not one that includes the peer's: that would make the peer simply behind,
+// and its save would be replaced without anyone there being asked. Resolution
+// is mutual. The answer is recorded instead (Answered), so this device is not
+// asked again about the same version, and the peer, still independent of it,
+// asks in turn. Whoever answers last decides (syncByVersion).
+func (e *Engine) versionAfterResolution(gameID string, theirs *VersionInfo, keptLocal bool) {
+	if theirs == nil {
+		return
+	}
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	gv, err := e.loadVersionLocked(gameID)
+	if err != nil || gv.rec.GameID == "" {
+		return
+	}
+	if keptLocal {
+		gv.rec.Counter++
+		gv.vec = gv.vec.clone()
+		gv.vec[gv.rec.Key] = gv.rec.Counter
+		gv.answered, gv.rec.AnsweredAt = theirs.Vector.clone(), time.Now().UnixMilli()
+	} else {
+		gv.vec = mergeVersions(gv.vec, theirs.Vector)
+		gv.answered, gv.rec.AnsweredAt = nil, 0
+	}
+	gv.rec.Pulling = false // whatever was being taken, the answer replaces it
+	gv.rec.Hash = ""
+	if game, err := e.Store.GetGame(gameID); err == nil {
+		if m, err := delta.BuildManifest(game.SavePath); err == nil {
+			gv.rec.Hash = e.versionHashOf(gameID, m)
+		}
+	}
+	gv.dropReachedTarget()
+	_ = e.saveVersionLocked(gv)
+}
+
+// discardLocalVersion is for a "keep theirs" that did not finish: this
+// device's own version is given up, and it takes the peer's like any outdated
+// device would — whole, and only from a device that holds it.
+func (e *Engine) discardLocalVersion(gameID string, theirs *VersionInfo) {
+	if theirs == nil {
+		return
+	}
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	gv, err := e.loadVersionLocked(gameID)
+	if err != nil || gv.rec.GameID == "" {
+		return
+	}
+	gv.vec = VersionVector{}
+	gv.target = theirs.Vector.clone()
+	gv.rec.Hash = ""
+	_ = e.saveVersionLocked(gv)
+}
+
+// versionStatusWaiting: this device or the peer is behind, and the other
+// cannot give it the newer version either. Nothing happens; both wait for a
+// device that holds it.
+const versionStatusWaiting = "waiting"
+
+// syncByVersion decides a sync from the two devices' versions. handled false
+// leaves it to the file comparison: the two hold the same version and the
+// same files, or both saves predate versions and have not met since.
+//
+// local and remote are filtered by the exclusion rules; unfilteredLocal is
+// this device's save as it is on disk, which names a version.
+func (e *Engine) syncByVersion(ctx context.Context, game store.Game, peer Peer,
+	local, unfilteredLocal delta.Manifest, remote ManifestResponse) (Result, bool, error) {
+
+	gameID := game.ID
+	theirs := *remote.Version
+
+	e.versionMu.Lock()
+	gv, err := e.loadVersionLocked(gameID)
+	if err != nil || gv.rec.GameID == "" {
+		e.versionMu.Unlock()
+		return Result{}, false, nil
+	}
+	before := gv.target.clone()
+	changed := e.refreshLocked(&gv, game, unfilteredLocal)
+	gv.learn(theirs)
+	if changed || compareVersions(before, gv.target) != versionEqual || len(before) != len(gv.target) {
+		_ = e.saveVersionLocked(gv)
+	}
+	mine := gv.info(len(local.Files) > 0)
+	e.versionMu.Unlock()
+
+	waiting := func(why string) (Result, bool, error) {
+		e.logOnce(gameID+"|"+peer.ID, why)
+		return Result{Status: versionStatusWaiting, PeerID: peer.ID, PeerName: peer.Name}, true, nil
+	}
+	pushTo := func() (Result, bool, error) {
+		// Once per state: a peer still taking it is asked again on every
+		// pass until it has, and that is not news each time.
+		e.logOnce(gameID+"|"+peer.ID, fmt.Sprintf("%q: %s has an older version (%s, this device %s) — asking it to take this one",
+			game.Name, peer.Name, theirs.Vector, mine.Vector))
+		e.Transport.TriggerPeerPull(peer, gameID)
+		return Result{Status: "triggered_peer_pull", Direction: "push", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+	}
+
+	switch {
+	case mine.Outdated():
+		if theirs.Outdated() || !covers(theirs.Vector, mine.Target) {
+			return waiting(fmt.Sprintf("%q: this device is behind (holds %s, newest known %s); %s does not hold that version either — waiting for a device that does",
+				game.Name, mine.Vector, mine.Target, peer.Name))
+		}
+		return e.pullVersion(ctx, game, peer, local, remote, theirs.Vector)
+	case theirs.Outdated():
+		if covers(mine.Vector, theirs.Target) {
+			return pushTo()
+		}
+		return waiting(fmt.Sprintf("%q: %s is behind and this device does not hold the version it is waiting for either",
+			game.Name, peer.Name))
+	}
+
+	switch compareVersions(mine.Vector, theirs.Vector) {
+	case versionNewer:
+		return pushTo()
+	case versionOlder:
+		// learn made this device outdated above, so this is only reached if
+		// the record could not be read back; the peer holds the newer version
+		// completely, so take it.
+		return e.pullVersion(ctx, game, peer, local, remote, theirs.Vector)
+	case versionEqual:
+		e.forgetLogOnce(gameID + "|" + peer.ID)
+		if sameFiles(local, remote.Manifest) {
+			return Result{}, false, nil
+		}
+		// One version, different files. Each side checks its own save against
+		// its version before answering (refreshLocked), so this is a change
+		// that landed after the peer answered; its sync carries it.
+		return waiting(fmt.Sprintf("%q: same version as %s but the files differ — waiting for the change to be recorded",
+			game.Name, peer.Name))
+	}
+
+	// Concurrent: both changed independently.
+	e.forgetLogOnce(gameID + "|" + peer.ID)
+	if sameFiles(local, remote.Manifest) {
+		// …into the same save. Both record the union, which each computes the
+		// same way, and are equal from then on.
+		e.versionMu.Lock()
+		if gv, err := e.loadVersionLocked(gameID); err == nil {
+			gv.vec = mergeVersions(gv.vec, theirs.Vector)
+			gv.rec.Hash = e.versionHashOf(gameID, unfilteredLocal)
+			gv.dropReachedTarget()
+			_ = e.saveVersionLocked(gv)
+		}
+		e.versionMu.Unlock()
+		return Result{}, false, nil
+	}
+	if !mine.HasFiles {
+		// An empty folder is never one side of a conflict: it takes the save.
+		return e.pullVersion(ctx, game, peer, local, remote, mergeVersions(mine.Vector, theirs.Vector))
+	}
+	if !theirs.HasFiles {
+		return pushTo()
+	}
+	if mine.Vector.genesisOnly() || theirs.Vector.genesisOnly() {
+		// A save from before versions existed, never compared with this one
+		// since: nothing recorded says which is newer, so the files decide
+		// (transitionNewer) and the older device takes the newer save whole,
+		// as any device behind does — never file by file, which is how devices
+		// ended up holding mixtures of two saves that no game ever wrote. Only
+		// an exact tie is asked about.
+		switch transitionNewer(local, remote.Manifest, e.Store.GetAgreedHash(gameID, peer.ID)) {
+		case 1:
+			return pushTo()
+		case -1:
+			e.Log("info", fmt.Sprintf("%q: %s's save holds the more recent work — taking it whole (this device's is kept in a snapshot)",
+				game.Name, peer.Name))
+			return e.pullVersion(ctx, game, peer, local, remote, theirs.Vector)
+		}
+		e.Log("warn", fmt.Sprintf("%q differs from %s's and neither holds more recent work — asking", game.Name, peer.Name))
+		e.registerConflict(gameID, peer, local, remote)
+		return Result{Status: "conflict", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+	}
+
+	// Someone already answered: here, keeping this device's save over the
+	// peer's version (or one it has since moved on from), or there, keeping
+	// theirs over this one's.
+	iAnswered := len(mine.Answered) > 0 && covers(theirs.Vector, mine.Answered)
+	theyAnswered := len(theirs.Answered) > 0 && covers(mine.Vector, theirs.Answered)
+	switch {
+	case iAnswered && theyAnswered:
+		// Both were answered, each keeping its own. The later answer stands,
+		// as the person who gave it saw the other one too; the earlier one's
+		// save is in the snapshot taken before it is replaced.
+		// Answers in the same millisecond are settled the same way on both
+		// devices, or each would keep asking the other to take its own.
+		if mine.AnsweredAt > theirs.AnsweredAt ||
+			(mine.AnsweredAt == theirs.AnsweredAt && mine.Vector.String() > theirs.Vector.String()) {
+			return pushTo()
+		}
+		e.Log("info", fmt.Sprintf("%q: %s's answer to the conflict came later — taking its version", game.Name, peer.Name))
+		return e.pullVersion(ctx, game, peer, local, remote, mergeVersions(mine.Vector, theirs.Vector))
+	case iAnswered:
+		// Answered here; the peer is asked in turn when it syncs.
+		return pushTo()
+	}
+	e.Log("warn", fmt.Sprintf("%q was changed independently on this device (%s) and on %s (%s)",
+		game.Name, mine.Vector, peer.Name, theirs.Vector))
+	e.registerConflict(gameID, peer, local, remote)
+	return Result{Status: "conflict", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+}
+
+// pullVersion makes this device's save exactly the peer's — every file it
+// holds, and none it does not — and only then records the peer's version as
+// this device's. Until that, the device stays behind and keeps waiting.
+//
+// local and remote are filtered by the exclusion rules, so excluded files are
+// neither taken nor removed.
+func (e *Engine) pullVersion(ctx context.Context, game store.Game, peer Peer,
+	local delta.Manifest, remote ManifestResponse, adopt VersionVector) (Result, bool, error) {
+
+	gameID := game.ID
+	if until, blocked := e.unwritableUntil(gameID); blocked {
+		// Said once when it happened (below); waiting is the answer until then.
+		e.logOnce(gameID+"|unwritable", fmt.Sprintf("%q: waiting until %s before writing to its save folder again",
+			game.Name, until.Format("15:04")))
+		return Result{Status: "unwritable", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+	}
+	if len(remote.Manifest.Files) == 0 && len(local.Files) > 0 && !remote.DeletionConfirmed {
+		// The newer version is an empty folder nobody has confirmed emptying.
+		e.Log("info", fmt.Sprintf("%q holds none of %q's save files now, and has not confirmed deleting them — keeping this device's copies",
+			peer.Name, game.Name))
+		return Result{Status: "peer_holding", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+	}
+
+	var d Decision
+	for p, lf := range local.Files {
+		rf, ok := remote.Manifest.Files[p]
+		switch {
+		case !ok:
+			d.FilesToDeleteLocally = append(d.FilesToDeleteLocally, p)
+		case rf.Hash != lf.Hash:
+			d.FilesToPull = append(d.FilesToPull, p)
+		}
+	}
+	for p := range remote.Manifest.Files {
+		if _, ok := local.Files[p]; !ok {
+			d.FilesToPull = append(d.FilesToPull, p)
+		}
+	}
+	localDirs, remoteDirs := toSet(local.Dirs), toSet(remote.Manifest.Dirs)
+	for dir := range remoteDirs {
+		if _, ok := localDirs[dir]; !ok {
+			d.DirsToPull = append(d.DirsToPull, dir)
+		}
+	}
+	for dir := range localDirs {
+		if _, ok := remoteDirs[dir]; !ok {
+			d.DirsToDeleteLocally = append(d.DirsToDeleteLocally, dir)
+		}
+	}
+	sort.Strings(d.FilesToPull)
+
+	e.Log("info", fmt.Sprintf("%q: taking %s's newer version (%s): %d file(s) to fetch, %d to remove",
+		game.Name, peer.Name, adopt, len(d.FilesToPull), len(d.FilesToDeleteLocally)))
+
+	// Whatever this device holds is kept in a snapshot before any of it is
+	// replaced or removed.
+	replacing := len(d.FilesToDeleteLocally)
+	for _, p := range d.FilesToPull {
+		if _, ok := local.Files[p]; ok {
+			replacing++
+		}
+	}
+	if replacing > 0 {
+		if _, err := e.Snapshots.CreateBeforeReplacing(gameID, fmt.Sprintf("Before taking %s's newer version", peer.Name)); err != nil {
+			return Result{}, true, fmt.Errorf("refusing to replace %q's files: they could not be snapshotted first: %w", game.Name, err)
+		}
+	}
+
+	e.setPulling(gameID, true)
+	defer e.setPulling(gameID, false)
+
+	applied := e.Writing(gameID)
+	started := time.Now()
+	e.applyLocalDeletions(gameID, primaryRootOf(game), d)
+	if n := len(d.FilesToDeleteLocally); n > 0 {
+		e.RecordActivity(store.ActivityEvent{GameID: gameID, Kind: store.ActivityDeleted, Device: peer.Name, Files: n})
+		e.noteEmptiedByPeer(gameID, started)
+	}
+	e.createPulledDirs(gameID, game, d.DirsToPull)
+	if len(d.FilesToPull) > 0 {
+		if err := e.pullFiles(ctx, peer, gameID, game, primaryRootOf(game), local, remote, d.FilesToPull); err != nil {
+			applied()
+			if errors.Is(err, fs.ErrPermission) {
+				// Not something the next pass fixes: the folder needs other
+				// rights (one under Program Files does). Said once, plainly,
+				// and tried again later rather than on every pass.
+				e.markUnwritable(gameID)
+				e.Log("error", fmt.Sprintf("%q: the save folder %s cannot be written (%v). Give OpenSave write access to it, or track the game at another folder. Trying again in %s.",
+					game.Name, game.SavePath, err, unwritableRetry))
+			}
+			return Result{}, true, fmt.Errorf("taking %s's version of %q did not finish (it is resumed on the next sync): %w", peer.Name, game.Name, err)
+		}
+	}
+	applied()
+
+	// Only a save that is now exactly the peer's takes its version. Read
+	// straight from disk, not through the gate (settle.go): this sync is the
+	// one that wrote it, and after following a peer onto a branch it still
+	// holds the save — waiting for that would wait for itself until the
+	// sync's time ran out.
+	fresh, err := delta.BuildManifest(game.SavePath)
+	if err != nil {
+		return Result{}, true, err
+	}
+	freshHash := e.versionHashOf(gameID, fresh)
+	fresh = filterManifest(fresh, e.rulesFor(gameID))
+	changedSince := false
+	if !sameFiles(fresh, remote.Manifest) {
+		if !changedSincePulled(primaryRootOf(game).Path, fresh, remote.Manifest, d) {
+			return Result{}, true, fmt.Errorf("taking %s's version of %q did not finish: the save still differs; it is resumed on the next sync", peer.Name, game.Name)
+		}
+		changedSince = true
+	}
+
+	e.versionMu.Lock()
+	if gv, err := e.loadVersionLocked(gameID); err == nil {
+		theirs := VersionInfo{}
+		if remote.Version != nil {
+			theirs = *remote.Version
+		}
+		gv.adopt(adopt, theirs)
+		gv.rec.Hash = freshHash
+		gv.rec.Pulling = false
+		e.Log("success", fmt.Sprintf("%q now holds version %s from %s", game.Name, gv.vec, peer.Name))
+		if changedSince {
+			// The peer's version arrived whole, and was then changed here -
+			// the game saving straight after the pull. That is a version of
+			// this device's own, made from theirs, and goes back to them.
+			e.bumpLocked(&gv, game.Name, freshHash)
+		}
+		_ = e.saveVersionLocked(gv)
+	}
+	e.versionMu.Unlock()
+	if changedSince {
+		e.Transport.TriggerPeerPull(peer, gameID)
+	}
+
+	// The file comparison's own records, so it agrees if it is ever asked.
+	e.persistLineage(gameID, peer.ID, fresh, remote.Manifest)
+	_ = e.Store.SetAgreedHash(gameID, peer.ID, remote.Manifest.ManifestHash())
+
+	e.syncExtraRoots(ctx, gameID, game, peer, remote)
+	if !d.HasChanges() {
+		return Result{Status: "in_sync", Direction: "none", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+	}
+	return Result{Status: "updated", Direction: "pull", PeerID: peer.ID, PeerName: peer.Name}, true, nil
+}
+
+// unwritableRetry is how long a game whose save folder could not be written
+// waits before a pull into it is tried again.
+const unwritableRetry = 30 * time.Minute
+
+func (e *Engine) markUnwritable(gameID string) {
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	if e.unwritable == nil {
+		e.unwritable = map[string]time.Time{}
+	}
+	e.unwritable[gameID] = time.Now().Add(unwritableRetry)
+}
+
+func (e *Engine) unwritableUntil(gameID string) (time.Time, bool) {
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	until, ok := e.unwritable[gameID]
+	if !ok || time.Now().After(until) {
+		delete(e.unwritable, gameID)
+		return time.Time{}, false
+	}
+	return until, true
+}
+
+// setPulling marks a pull towards a newer version as running, so the files it
+// writes are never taken for a change made here. In memory for this run; in
+// the record from the start until the pull has completed, which only adopting
+// the version clears — a pull that stopped part-way leaves a folder that is
+// neither this device's version nor the peer's, and it must not be read as a
+// change made here (refreshLocked, NoteLocalChange) while it waits to resume.
+func (e *Engine) setPulling(gameID string, on bool) {
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	if e.pullingNow == nil {
+		e.pullingNow = map[string]bool{}
+	}
+	if !on {
+		delete(e.pullingNow, gameID)
+		return
+	}
+	e.pullingNow[gameID] = true
+	if gv, err := e.loadVersionLocked(gameID); err == nil && gv.rec.GameID != "" && !gv.rec.Pulling {
+		gv.rec.Pulling = true
+		_ = e.saveVersionLocked(gv)
+	}
+}
+
+// quickVersionCheck is the version half of quickInSync: both saves are the
+// agreed one, so the question is only whether the versions say the same.
+// done reports a final answer; otherwise the full sync decides.
+func (e *Engine) quickVersionCheck(game store.Game, peer Peer, local delta.Manifest, theirs VersionInfo) (res Result, done bool) {
+	e.versionMu.Lock()
+	gv, err := e.loadVersionLocked(game.ID)
+	if err != nil || gv.rec.GameID == "" {
+		e.versionMu.Unlock()
+		return Result{}, false
+	}
+	before := gv.target.clone()
+	changed := e.refreshLocked(&gv, game, local)
+	gv.learn(theirs)
+	if changed || compareVersions(before, gv.target) != versionEqual || len(before) != len(gv.target) {
+		_ = e.saveVersionLocked(gv)
+	}
+	mine := gv.info(len(local.Files) > 0)
+	e.versionMu.Unlock()
+
+	if compareVersions(mine.Vector, theirs.Vector) != versionEqual {
+		return Result{}, false
+	}
+	if mine.Outdated() || theirs.Outdated() {
+		// The same save, and at least one of the two knows a newer one exists
+		// that neither holds: nothing to do between these two.
+		return Result{Status: versionStatusWaiting, PeerID: peer.ID, PeerName: peer.Name}, true
+	}
+	return Result{}, true
+}
+
+// logOnce logs a line the first time it is the answer for key, and not again
+// while it stays the answer: waiting is re-decided every reconcile.
+func (e *Engine) logOnce(key, msg string) {
+	e.versionMu.Lock()
+	if e.loggedOnce == nil {
+		e.loggedOnce = map[string]string{}
+	}
+	same := e.loggedOnce[key] == msg
+	e.loggedOnce[key] = msg
+	e.versionMu.Unlock()
+	if !same {
+		e.Log("info", msg)
+	}
+}
+
+func (e *Engine) forgetLogOnce(key string) {
+	e.versionMu.Lock()
+	delete(e.loggedOnce, key)
+	e.versionMu.Unlock()
+}
+
+// transitionNewer says which of two saves from before versions is the newer,
+// when that is beyond doubt: 1 for local, -1 for remote, 0 when it is not.
+//
+// Beyond doubt means one of two things. The devices last agreed on a state
+// and one of them still holds exactly it: that one has not changed since, so
+// the other is newer. Or one save is newer in every file that differs — each
+// is newer there or missing on the other side — and the other holds no file
+// of its own written after the first's newest. That is what a save someone
+// played on looks like next to one nobody touched.
+//
+// It is deliberately not "the newer file wins, file by file": that makes a
+// mixture of two saves no game wrote. And not "the newest file wins", which a
+// mixture passes as easily as a real save. Here the device that gives way was
+// older in every file that differs, so it never loses anything newer than
+// what it takes — at worst it takes a copy that arrived only half-way, and
+// then the complete save, newer than both, replaces that too when it is
+// seen. States newer each in different files fail the test both ways and are
+// left to a person.
+func transitionNewer(local, remote delta.Manifest, agreed string) int {
+	if agreed != "" {
+		lh, rh := local.ManifestHash(), remote.ManifestHash()
+		switch {
+		case lh == agreed && rh != agreed:
+			return -1
+		case rh == agreed && lh != agreed:
+			return 1
+		}
+	}
+	// Two saves with no file in common are not two versions of the same
+	// files — a save kept as one file named differently on each device, say
+	// — and nothing about their times says one should replace the other.
+	if len(local.Files) > 0 && len(remote.Files) > 0 && !shareAPath(local, remote) {
+		return 0
+	}
+	l, r := newerInEveryFile(local, remote), newerInEveryFile(remote, local)
+	switch {
+	case l && !r:
+		return 1
+	case r && !l:
+		return -1
+	}
+	return newestWork(local, remote)
+}
+
+// newestWork settles two saves from before versions that neither rule above
+// could: the one holding the most recent work — the latest-written of the
+// files where they differ — is the save, and if that is a tie, the one newer
+// in more of those files. The other takes it whole, keeping a snapshot of its
+// own first.
+//
+// Before versions, devices that never ran a game held copies relayed from the
+// others at different times, and syncs file by file had left some holding
+// mixtures; those were asked about, about states nobody made. Taking the one
+// with the most recent work is what syncing is for — an older state is never
+// what anyone wants handed on — and whatever the other held stays in its
+// snapshot. Changes made since versions existed are not decided this way:
+// those are versions of their own, and a real conflict between them is asked.
+//
+// 0 only for an exact tie, which leaves it to a person.
+func newestWork(a, b delta.Manifest) int {
+	var aNewest, bNewest delta.Milli
+	aCount, bCount := 0, 0
+	consider := func(p string) {
+		af, inA := a.Files[p]
+		bf, inB := b.Files[p]
+		if inA && inB && af.Hash == bf.Hash {
+			return
+		}
+		if inA && af.MtimeMs > aNewest {
+			aNewest = af.MtimeMs
+		}
+		if inB && bf.MtimeMs > bNewest {
+			bNewest = bf.MtimeMs
+		}
+		if inA && inB {
+			switch {
+			case af.MtimeMs > bf.MtimeMs:
+				aCount++
+			case bf.MtimeMs > af.MtimeMs:
+				bCount++
+			}
+		}
+	}
+	for p := range a.Files {
+		consider(p)
+	}
+	for p := range b.Files {
+		if _, inA := a.Files[p]; !inA {
+			consider(p)
+		}
+	}
+	switch {
+	case aNewest > bNewest:
+		return 1
+	case bNewest > aNewest:
+		return -1
+	case aCount > bCount:
+		return 1
+	case bCount > aCount:
+		return -1
+	}
+	return 0
+}
+
+func shareAPath(a, b delta.Manifest) bool {
+	for p := range a.Files {
+		if _, ok := b.Files[p]; ok {
+			return true
+		}
+	}
+	return false
+}
+
+// newerInEveryFile reports whether a is newer than b wherever they differ,
+// and they do differ.
+func newerInEveryFile(a, b delta.Manifest) bool {
+	differ := false
+	var aNewest delta.Milli
+	for p, af := range a.Files {
+		if af.MtimeMs > aNewest {
+			aNewest = af.MtimeMs
+		}
+		bf, ok := b.Files[p]
+		if !ok {
+			differ = true
+			continue
+		}
+		if af.Hash != bf.Hash {
+			differ = true
+			if af.MtimeMs <= bf.MtimeMs {
+				return false
+			}
+		}
+	}
+	for p, bf := range b.Files {
+		if _, ok := a.Files[p]; ok {
+			continue
+		}
+		differ = true
+		if bf.MtimeMs >= aNewest {
+			return false
+		}
+	}
+	return differ
+}
+
+// StatusPeerOutdatedApp: the peer runs an OpenSave without save versions.
+// Nothing is taken from it until it is updated; it can still take from here.
+const StatusPeerOutdatedApp = "peer_outdated_app"
+
+// OldBuildDeleteMessage is the answer to a deletion asked for by a device
+// whose OpenSave does not keep save versions.
+const OldBuildDeleteMessage = "this device keeps save versions and takes no deletions from an OpenSave that does not — update OpenSave on the asking device"
+
+// notePeerApp records whether a peer's OpenSave keeps save versions, as its
+// last manifest answer said.
+func (e *Engine) notePeerApp(peerID string, versions bool) {
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	if e.peerVersions == nil {
+		e.peerVersions = map[string]bool{}
+	}
+	e.peerVersions[peerID] = versions
+}
+
+// PeerNeedsUpdate reports whether a peer is known to run an OpenSave without
+// save versions — one this device takes nothing from — so the person can be
+// told to update it.
+func (e *Engine) PeerNeedsUpdate(peerID string) bool {
+	e.versionMu.Lock()
+	defer e.versionMu.Unlock()
+	v, known := e.peerVersions[peerID]
+	return known && !v
+}
+
+// RefusedOldBuildDelete logs, once per game and device, that a deletion asked
+// for by an OpenSave without save versions was refused.
+func (e *Engine) RefusedOldBuildDelete(gameID, from string) {
+	e.logOnce("refused|"+gameID+"|"+from, fmt.Sprintf(
+		"%s asked to delete files of %q; refused — its OpenSave does not keep save versions and decides by the old file comparison. Update it there.",
+		from, gameID))
+}
+
+// supersedes reports whether theirs is newer than this device's version and
+// than the version the open conflict is with: both sides of the question are
+// behind it.
+func (e *Engine) supersedes(gameID string, theirs VersionInfo) bool {
+	e.mu.Lock()
+	c := e.activeConflicts[gameID]
+	e.mu.Unlock()
+	if c == nil || c.remoteVersion == nil {
+		return false
+	}
+	e.versionMu.Lock()
+	gv, err := e.loadVersionLocked(gameID)
+	e.versionMu.Unlock()
+	if err != nil || len(gv.vec) == 0 {
+		return false
+	}
+	return compareVersions(gv.vec, theirs.Vector) == versionOlder &&
+		compareVersions(c.remoteVersion.Vector, theirs.Vector) == versionOlder
+}
+
+// UseSaveEverywhere makes this device's save of a game the one every other
+// device takes: its version becomes newer than every save from before
+// versions existed, on any device, so each of those takes this one whole,
+// keeping a snapshot of its own first. For when several devices hold states
+// nobody can rank — copies that arrived half-way, mixtures of two saves —
+// and a person knows which device has the right one.
+//
+// A change made on another device since versions existed is not overruled:
+// that device and this one have each changed the save independently, and
+// the person there is asked, as for any conflict.
+func (e *Engine) UseSaveEverywhere(gameID string) (VersionVector, error) {
+	game, err := e.Store.GetGame(gameID)
+	if err != nil {
+		return nil, err
+	}
+	if !game.AutoSync {
+		return nil, errors.New("automatic sync is off for this game, so it does not take part in versions")
+	}
+	m, err := delta.BuildManifest(game.SavePath)
+	if err != nil {
+		return nil, err
+	}
+	if len(m.Files) == 0 {
+		return nil, errors.New("this device holds no save files for this game")
+	}
+	e.versionMu.Lock()
+	gv, err := e.loadVersionLocked(gameID)
+	if err != nil || gv.rec.GameID == "" {
+		e.versionMu.Unlock()
+		if err == nil {
+			err = errors.New("the game is not tracked")
+		}
+		return nil, err
+	}
+	gv.vec = mergeVersions(gv.vec, VersionVector{genesisAll: 1})
+	gv.rec.Counter++
+	gv.vec[gv.rec.Key] = gv.rec.Counter
+	gv.rec.Hash = e.versionHashOf(gameID, m)
+	gv.target, gv.answered, gv.rec.AnsweredAt = nil, nil, 0
+	gv.rec.Pulling = false
+	saveErr := e.saveVersionLocked(gv)
+	vec := gv.vec.clone()
+	e.versionMu.Unlock()
+	if saveErr != nil {
+		return nil, saveErr
+	}
+	e.mu.Lock()
+	_, conflicted := e.activeConflicts[gameID]
+	e.mu.Unlock()
+	if conflicted {
+		e.clearConflict(gameID)
+	}
+	e.Log("info", fmt.Sprintf("%q: this device's save is to be used everywhere — version %s", game.Name, vec))
+	return vec, nil
+}
+
+// changedSincePulled reports a pull that finished and whose save was then
+// changed here: every way the save now differs from the peer's is a change
+// someone other than OpenSave made since the pull was planned. A game saving,
+// or a person deleting a file, the moment a pull is done leaves exactly that.
+//
+//   - A file the pull did not touch was the same on both sides when the pull
+//     was planned (anything that differed was to be fetched or removed). If
+//     it differs now, it was changed here meanwhile.
+//   - A file the pull wrote or removed differs because of someone else only
+//     if someone other than OpenSave has written it, or put it back, since
+//     (owntouch). Still as OpenSave left it, it is the pull's own unfinished
+//     part: a pull that stopped part-way, resumed on the next sync as before.
+//
+// Without this, a save the game had just written, or a file just deleted,
+// was taken for the unfinished part of the pull and replaced with the peer's
+// on the next sync; only the snapshot taken before replacing it still held it.
+func changedSincePulled(root string, fresh, remote delta.Manifest, d Decision) bool {
+	touched := make(map[string]bool, len(d.FilesToPull)+len(d.FilesToDeleteLocally))
+	for _, p := range d.FilesToPull {
+		touched[p] = true
+	}
+	for _, p := range d.FilesToDeleteLocally {
+		touched[p] = true
+	}
+	changedByOthers := func(p string) bool {
+		return !touched[p] || !owntouch.Recent(delta.LocalNameFor(root, p))
+	}
+	differs := 0
+	for p, lf := range fresh.Files {
+		if rf, ok := remote.Files[p]; ok && rf.Hash == lf.Hash {
+			continue
+		}
+		differs++
+		if !changedByOthers(p) {
+			return false
+		}
+	}
+	for p := range remote.Files {
+		if _, ok := fresh.Files[p]; ok {
+			continue
+		}
+		differs++
+		if !changedByOthers(p) {
+			return false
+		}
+	}
+	return differs > 0
+}
diff --git a/internal/p2p/syncengine/version_test.go b/internal/p2p/syncengine/version_test.go
new file mode 100644
index 00000000..67e2d189
--- /dev/null
+++ b/internal/p2p/syncengine/version_test.go
@@ -0,0 +1,803 @@
+package syncengine
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/snapshot"
+	"github.com/opensave/opensave/internal/store"
+)
+
+func TestCompareVersions(t *testing.T) {
+	cases := []struct {
+		a, b VersionVector
+		want versionOrder
+	}{
+		{VersionVector{}, VersionVector{}, versionEqual},
+		{VersionVector{}, VersionVector{"x": 1}, versionOlder},
+		{VersionVector{"x": 2}, VersionVector{"x": 1}, versionNewer},
+		{VersionVector{"x": 1, "y": 1}, VersionVector{"x": 1, "y": 1}, versionEqual},
+		{VersionVector{"x": 2}, VersionVector{"x": 1, "y": 1}, versionConcurrent},
+		{VersionVector{"x": 1}, VersionVector{"x": 1, "y": 1}, versionOlder},
+	}
+	for _, c := range cases {
+		if got := compareVersions(c.a, c.b); got != c.want {
+			t.Errorf("compare(%v, %v) = %d, want %d", c.a, c.b, got, c.want)
+		}
+	}
+}
+
+// "Use this save everywhere" covers every save from before versions, not
+// changes made since.
+func TestCompareVersions_UseEverywhereCoversEveryOldSave(t *testing.T) {
+	everywhere := VersionVector{"g:aaaa": 1, genesisAll: 1, "x": 1}
+	if got := compareVersions(VersionVector{"g:bbbb": 1}, everywhere); got != versionOlder {
+		t.Errorf("an old save of other content vs everywhere = %d, want older", got)
+	}
+	if got := compareVersions(VersionVector{}, everywhere); got != versionOlder {
+		t.Errorf("an empty folder vs everywhere = %d, want older", got)
+	}
+	if got := compareVersions(VersionVector{"g:bbbb": 1, "y": 1}, everywhere); got != versionConcurrent {
+		t.Errorf("a save changed since versions vs everywhere = %d, want concurrent", got)
+	}
+}
+
+func setTimes(t *testing.T, dir, rel string, at time.Time) {
+	t.Helper()
+	if err := os.Chtimes(filepath.Join(dir, filepath.FromSlash(rel)), at, at); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// Two saves from before versions, one played on since they last matched:
+// that one is newer in every file that differs, and the other takes it whole
+// — no question, and no mixture.
+func TestVersions_TransitionTakesTheClearlyNewerSave(t *testing.T) {
+	m, n := newMesh(t, "played", "old")
+	played, old := n["played"], n["old"]
+	t0 := time.Now().Add(-48 * time.Hour)
+	t1 := time.Now().Add(-1 * time.Hour)
+	for _, d := range []*meshNode{played, old} {
+		write(t, d.dir, "slot1.sav", "day one")
+		write(t, d.dir, "meta.dat", "m1")
+		write(t, d.dir, "stale.tmp", "gone later")
+		setTimes(t, d.dir, "slot1.sav", t0)
+		setTimes(t, d.dir, "meta.dat", t0)
+		setTimes(t, d.dir, "stale.tmp", t0)
+	}
+	// Played on, before the update: changed, added, removed.
+	write(t, played.dir, "slot1.sav", "day five")
+	write(t, played.dir, "slot2.sav", "new slot")
+	_ = os.Remove(filepath.Join(played.dir, "stale.tmp"))
+	setTimes(t, played.dir, "slot1.sav", t1)
+	setTimes(t, played.dir, "slot2.sav", t1)
+
+	for _, pair := range [][2]*meshNode{{old, played}, {played, old}} {
+		res, err := syncPair(t, pair[0], pair[1])
+		if err != nil || res.Status == "conflict" {
+			t.Fatalf("%s↔%s: %+v, %v", pair[0].name, pair[1].name, res, err)
+		}
+	}
+	settleAll(t, played, old)
+	if !sameSave(old.files(t), played.files(t)) {
+		t.Errorf("the old device did not take the played save whole: %s", describe(old.files(t)))
+	}
+	if m.deletes() != 0 {
+		t.Errorf("%d deletion(s) were sent", m.deletes())
+	}
+}
+
+// Two copies relayed from other devices at different times, each newer in a
+// different file — the state syncing file by file used to leave. Nobody is
+// asked: the one holding the more recent work is the save, and the other
+// takes it whole, keeping its own in a snapshot. Never a mixture.
+func TestVersions_TransitionTakesTheMostRecentWorkWithoutAsking(t *testing.T) {
+	m, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	t0 := time.Now().Add(-48 * time.Hour)
+	t1 := time.Now().Add(-2 * time.Hour)
+	t2 := time.Now().Add(-1 * time.Hour)
+	write(t, a.dir, "one.sav", "a-new")
+	write(t, a.dir, "two.sav", "old")
+	write(t, b.dir, "one.sav", "old")
+	write(t, b.dir, "two.sav", "b-newest")
+	setTimes(t, a.dir, "one.sav", t1)
+	setTimes(t, a.dir, "two.sav", t0)
+	setTimes(t, b.dir, "one.sav", t0)
+	setTimes(t, b.dir, "two.sav", t2)
+	want := b.files(t)
+
+	for _, pair := range [][2]*meshNode{{a, b}, {b, a}} {
+		if res, err := syncPair(t, pair[0], pair[1]); err != nil || res.Status == "conflict" {
+			t.Fatalf("%s↔%s: %+v, %v — want no question", pair[0].name, pair[1].name, res, err)
+		}
+	}
+	settleAll(t, a, b)
+	if !sameSave(a.files(t), want) || !sameSave(b.files(t), want) {
+		t.Errorf("not both on the save with the most recent work: a %s, b %s", describe(a.files(t)), describe(b.files(t)))
+	}
+	if m.deletes() != 0 {
+		t.Errorf("%d deletion(s) were sent", m.deletes())
+	}
+}
+
+// Two saves with no file in common — one file, named differently on each
+// device — are not two versions of the same files: asked about, never one
+// replacing the other because it is newer.
+func TestVersions_TransitionAsksWhenNoFileIsShared(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	write(t, a.dir, "alice.sav", "A's world")
+	write(t, b.dir, "bob.sav", "B's world")
+	setTimes(t, a.dir, "alice.sav", time.Now().Add(-time.Hour))
+	setTimes(t, b.dir, "bob.sav", time.Now().Add(-48*time.Hour))
+	bBefore := b.files(t)
+	for _, pair := range [][2]*meshNode{{a, b}, {b, a}} {
+		if res, err := syncPair(t, pair[0], pair[1]); err != nil || res.Status != "conflict" {
+			t.Fatalf("%s↔%s: %+v, %v — want a question", pair[0].name, pair[1].name, res, err)
+		}
+	}
+	if !sameSave(b.files(t), bBefore) {
+		t.Error("b's save was replaced by one with no file in common")
+	}
+}
+
+// Only an exact tie — the same latest time, newer in as many files — is asked
+// about: there is nothing to tell the two apart.
+func TestVersions_TransitionAsksOnlyOnAnExactTie(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	t1 := time.Now().Add(-1 * time.Hour)
+	write(t, a.dir, "slot.sav", "a-content")
+	write(t, b.dir, "slot.sav", "b-content")
+	setTimes(t, a.dir, "slot.sav", t1)
+	setTimes(t, b.dir, "slot.sav", t1)
+	aBefore, bBefore := a.files(t), b.files(t)
+
+	res, err := syncPair(t, a, b)
+	if err != nil || res.Status != "conflict" {
+		t.Fatalf("an exact tie: %+v, %v — want a question", res, err)
+	}
+	if !sameSave(a.files(t), aBefore) || !sameSave(b.files(t), bBefore) {
+		t.Error("a save changed before anyone answered")
+	}
+}
+
+// "Use this save everywhere" on the device with the right save: every other
+// device takes it whole — even one already waiting on a question about two
+// mixed states — and nobody is asked.
+func TestVersions_UseThisSaveEverywhere(t *testing.T) {
+	m, n := newMesh(t, "right", "mixed1", "mixed2")
+	right, mixed1, mixed2 := n["right"], n["mixed1"], n["mixed2"]
+	t0 := time.Now().Add(-48 * time.Hour)
+	t1 := time.Now().Add(-1 * time.Hour)
+	writeMany(t, right.dir, "map", 12)
+	write(t, right.dir, "players.db", "current")
+	// Two states nothing tells apart: one file each, the same time.
+	write(t, mixed1.dir, "players.db", "old-1")
+	write(t, mixed2.dir, "players.db", "old-2")
+	setTimes(t, mixed1.dir, "players.db", t1)
+	setTimes(t, mixed2.dir, "players.db", t1)
+	_ = t0
+
+	if res, _ := syncPair(t, mixed1, mixed2); res.Status != "conflict" {
+		t.Fatalf("setup: an exact tie should be asked about, got %+v", res)
+	}
+	if _, err := right.eng.UseSaveEverywhere("game1"); err != nil {
+		t.Fatal(err)
+	}
+	settleAll(t, right, mixed1, mixed2)
+	want := right.files(t)
+	for _, d := range []*meshNode{mixed1, mixed2} {
+		if !sameSave(d.files(t), want) {
+			t.Errorf("%s did not take the save used everywhere: %s", d.name, describe(d.files(t)))
+		}
+		if len(d.eng.ActiveConflicts()) != 0 {
+			t.Errorf("%s still has a conflict open", d.name)
+		}
+	}
+	if m.deletes() != 0 {
+		t.Errorf("%d deletion(s) were sent", m.deletes())
+	}
+}
+
+// A change made on another device after versions existed is not overruled by
+// "Use this save everywhere": that is a conflict like any other.
+func TestVersions_UseEverywhereDoesNotOverruleALaterChange(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	write(t, a.dir, "slot.sav", "start")
+	settleAll(t, a, b)
+	b.change(t, func(dir string) { write(t, dir, "slot.sav", "b-played") })
+	if _, err := a.eng.UseSaveEverywhere("game1"); err != nil {
+		t.Fatal(err)
+	}
+	bBefore := b.files(t)
+	settleAll(t, a, b)
+	if !sameSave(b.files(t), bBefore) {
+		t.Error("b's change since the update was replaced without anyone being asked")
+	}
+	if len(a.eng.ActiveConflicts())+len(b.eng.ActiveConflicts()) == 0 {
+		t.Error("nobody was asked")
+	}
+}
+
+// --- a network of real engines -------------------------------------------
+
+// meshNode is one device: its own store, save folder and engine.
+type meshNode struct {
+	name string
+	dir  string
+	st   *store.Store
+	eng  *Engine
+	// blockBudget, when >= 0, is how many more files this device serves
+	// before every further block request fails: a pull that stops part-way.
+	blockBudget int
+	// proto is the protocol revision it answers with; below ProtoVersions it
+	// stands for an OpenSave that keeps no save versions.
+	proto int
+}
+
+func (n *meshNode) peer() Peer {
+	return Peer{ID: "node_" + n.name, Name: n.name, Address: "127.0.0.1", Port: 1}
+}
+
+type mesh struct {
+	mu      sync.Mutex
+	nodes   map[string]*meshNode // by peer id
+	offline map[string]bool
+	// deletesSent counts DeleteRemote calls: the version path never makes one.
+	deletesSent int
+}
+
+type meshTransport struct {
+	m    *mesh
+	self *meshNode
+}
+
+var errUnreachable = errors.New("peer unreachable")
+
+func (t *meshTransport) target(peer Peer) (*meshNode, error) {
+	t.m.mu.Lock()
+	defer t.m.mu.Unlock()
+	n := t.m.nodes[peer.ID]
+	if n == nil || t.m.offline[peer.ID] {
+		return nil, errUnreachable
+	}
+	return n, nil
+}
+
+// FetchManifest answers the way the real route does: the save, and the
+// device's version of it.
+func (t *meshTransport) FetchManifest(ctx context.Context, peer Peer, gameID string, q ManifestQuery) (ManifestResponse, error) {
+	n, err := t.target(peer)
+	if err != nil {
+		return ManifestResponse{}, err
+	}
+	game, err := n.st.GetGame(gameID)
+	if err != nil {
+		return ManifestResponse{}, errors.New("Game not found.")
+	}
+	m, err := delta.BuildManifest(n.dir)
+	if err != nil {
+		return ManifestResponse{}, err
+	}
+	return ManifestResponse{
+		Manifest:          m,
+		ActiveBranch:      game.ActiveBranch,
+		DeletionConfirmed: n.eng.DeletionConfirmed(gameID),
+		Version:           n.eng.LocalVersion(game, m),
+		Proto:             n.proto,
+	}, nil
+}
+
+func (t *meshTransport) FetchBlocks(ctx context.Context, peer Peer, ref FileRef, blockIndices []int, blockSize int) ([]BlockData, error) {
+	n, err := t.target(peer)
+	if err != nil {
+		return nil, err
+	}
+	t.m.mu.Lock()
+	if n.blockBudget == 0 {
+		t.m.mu.Unlock()
+		return nil, errors.New("connection reset")
+	}
+	if n.blockBudget > 0 {
+		n.blockBudget--
+	}
+	t.m.mu.Unlock()
+	full := filepath.Join(n.dir, filepath.FromSlash(ref.RelPath))
+	entry, err := delta.HashFile(full)
+	if err != nil {
+		return nil, err
+	}
+	raw, err := os.ReadFile(full)
+	if err != nil {
+		return nil, err
+	}
+	var out []BlockData
+	for _, idx := range blockIndices {
+		if idx >= len(entry.Blocks) {
+			continue
+		}
+		start := idx * entry.BlockSize
+		end := start + entry.Blocks[idx].Length
+		if end > len(raw) {
+			end = len(raw)
+		}
+		out = append(out, BlockData{Index: idx, Data: raw[start:end], Length: end - start})
+	}
+	return out, nil
+}
+
+func (t *meshTransport) DeleteRemote(ctx context.Context, peer Peer, ref FileRef) error {
+	n, err := t.target(peer)
+	if err != nil {
+		return err
+	}
+	t.m.mu.Lock()
+	t.m.deletesSent++
+	t.m.mu.Unlock()
+	return os.RemoveAll(filepath.Join(n.dir, filepath.FromSlash(ref.RelPath)))
+}
+
+func (t *meshTransport) TriggerPeerPull(peer Peer, gameID string)                                 {}
+func (t *meshTransport) ReportSyncEvent(peer Peer, gameID, eventType string, data map[string]any) {}
+
+func newMesh(t *testing.T, names ...string) (*mesh, map[string]*meshNode) {
+	t.Helper()
+	m := &mesh{nodes: map[string]*meshNode{}, offline: map[string]bool{}}
+	byName := map[string]*meshNode{}
+	for _, name := range names {
+		root := t.TempDir()
+		dir := filepath.Join(root, "saves")
+		if err := os.MkdirAll(dir, 0o777); err != nil {
+			t.Fatal(err)
+		}
+		st, err := store.Open(filepath.Join(root, "opensave.db"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Cleanup(func() { st.Close() })
+		if err := st.EnsureDefaultSettings(root, filepath.Join(root, "backups")); err != nil {
+			t.Fatal(err)
+		}
+		if err := st.CreateGame(store.Game{ID: "game1", Name: "Game One", SavePath: dir, AutoSync: true, MaxSnapshots: 50}); err != nil {
+			t.Fatal(err)
+		}
+		n := &meshNode{name: name, dir: dir, st: st, blockBudget: -1, proto: ProtoVersions}
+		n.eng = New(st, snapshot.New(st), &meshTransport{m: m, self: n})
+		m.nodes[n.peer().ID] = n
+		byName[name] = n
+	}
+	for _, a := range byName {
+		for _, b := range byName {
+			if a != b {
+				p := b.peer()
+				if err := a.st.UpsertPeer(store.Peer{ID: p.ID, Name: p.Name, Address: p.Address, Port: p.Port, Status: "online"}); err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+	}
+	return m, byName
+}
+
+func (m *mesh) setOffline(n *meshNode, off bool) {
+	m.mu.Lock()
+	m.offline[n.peer().ID] = off
+	m.mu.Unlock()
+}
+
+func (m *mesh) deletes() int {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return m.deletesSent
+}
+
+// syncPair runs one sync of a with b, as a's engine would.
+func syncPair(t *testing.T, a, b *meshNode) (Result, error) {
+	t.Helper()
+	return a.eng.SyncWithPeer(context.Background(), "game1", b.peer())
+}
+
+// change edits the save the way a game does, then tells the engine — the
+// watcher's part.
+func (n *meshNode) change(t *testing.T, edit func(dir string)) {
+	t.Helper()
+	edit(n.dir)
+	n.eng.NoteLocalChange("game1")
+}
+
+func (n *meshNode) files(t *testing.T) map[string]string {
+	t.Helper()
+	m, err := delta.BuildManifest(n.dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := map[string]string{}
+	for p, f := range m.Files {
+		out[p] = f.Hash
+	}
+	return out
+}
+
+func (n *meshNode) version(t *testing.T) VersionInfo {
+	t.Helper()
+	game, err := n.st.GetGame("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	m, err := delta.BuildManifest(n.dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return *n.eng.LocalVersion(game, m)
+}
+
+func sameSave(a, b map[string]string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for p, h := range a {
+		if b[p] != h {
+			return false
+		}
+	}
+	return true
+}
+
+func describe(files map[string]string) string {
+	paths := make([]string, 0, len(files))
+	for p := range files {
+		paths = append(paths, p)
+	}
+	sort.Strings(paths)
+	if len(paths) > 6 {
+		return fmt.Sprintf("%d files (%v …)", len(paths), paths[:6])
+	}
+	return fmt.Sprintf("%v", paths)
+}
+
+func writeMany(t *testing.T, dir, prefix string, n int) {
+	t.Helper()
+	for i := 0; i < n; i++ {
+		write(t, dir, fmt.Sprintf("%s/chunk_%03d.bin", prefix, i), fmt.Sprintf("%s-%d", prefix, i))
+	}
+}
+
+// settleAll syncs every reachable node with every other, a few rounds, the
+// way devices that are all online settle.
+func settleAll(t *testing.T, nodes ...*meshNode) {
+	t.Helper()
+	for round := 0; round < 4; round++ {
+		for _, a := range nodes {
+			for _, b := range nodes {
+				if a != b {
+					_, _ = syncPair(t, a, b)
+				}
+			}
+		}
+	}
+}
+
+// --- scenarios ---------------------------------------------------------------
+
+// The case this exists for. Three devices share a save; the server changes it
+// a lot. One laptop starts taking the new version and is cut off part-way; the
+// server then goes offline. The two laptops must not sync with each other in
+// the meantime — not the half-finished copy, and not its missing files read as
+// deletions — and when the server is back, both end up with its save exactly.
+func TestVersions_IncompleteDevicesWaitForTheNewestOne(t *testing.T) {
+	m, n := newMesh(t, "server", "laptop", "desk")
+	server, laptop, desk := n["server"], n["laptop"], n["desk"]
+
+	writeMany(t, server.dir, "map", 40)
+	write(t, server.dir, "players.db", "v1")
+	settleAll(t, server, laptop, desk)
+	original := server.files(t)
+	for _, d := range []*meshNode{laptop, desk} {
+		if !sameSave(d.files(t), original) {
+			t.Fatalf("%s did not take the initial save: %s", d.name, describe(d.files(t)))
+		}
+	}
+
+	// The desk is switched off for a while; the server plays and the laptop
+	// takes that. The desk is now two versions behind, the laptop one —
+	// which makes the laptop newer than the desk, the comparison a device
+	// part-way through a pull must never win.
+	m.setOffline(desk, true)
+	server.change(t, func(dir string) { write(t, dir, "players.db", "v1.5") })
+	settleAll(t, server, laptop)
+	m.setOffline(desk, false)
+
+	// The server plays on: new chunks, some removed, one changed.
+	server.change(t, func(dir string) {
+		writeMany(t, dir, "map2", 30)
+		for i := 0; i < 5; i++ {
+			_ = os.Remove(filepath.Join(dir, "map", fmt.Sprintf("chunk_%03d.bin", i)))
+		}
+		write(t, dir, "players.db", "v2")
+	})
+	newest := server.files(t)
+
+	// The laptop starts taking it and is cut off after 10 files.
+	server.blockBudget = 10
+	if _, err := syncPair(t, laptop, server); err == nil {
+		t.Fatal("a pull cut off part-way reported success")
+	}
+	server.blockBudget = -1
+	if !laptop.version(t).Outdated() {
+		t.Fatalf("the laptop does not know it is behind: %+v", laptop.version(t))
+	}
+	m.setOffline(server, true)
+
+	// Server gone. The laptop and desk meet, repeatedly, both ways.
+	deskBefore := desk.files(t)
+	laptopBefore := laptop.files(t)
+	for i := 0; i < 3; i++ {
+		for _, pair := range [][2]*meshNode{{desk, laptop}, {laptop, desk}} {
+			res, err := syncPair(t, pair[0], pair[1])
+			if err != nil {
+				t.Fatalf("%s↔%s: %v", pair[0].name, pair[1].name, err)
+			}
+			if res.Status == "conflict" {
+				t.Fatalf("%s↔%s raised a conflict between two copies of the same history", pair[0].name, pair[1].name)
+			}
+		}
+	}
+	if !sameSave(desk.files(t), deskBefore) {
+		t.Errorf("the desk changed while the newest device was away: was %s, now %s",
+			describe(deskBefore), describe(desk.files(t)))
+	}
+	if !sameSave(laptop.files(t), laptopBefore) {
+		t.Errorf("the laptop's partial copy changed while the newest device was away: was %s, now %s",
+			describe(laptopBefore), describe(laptop.files(t)))
+	}
+	if !desk.version(t).Outdated() {
+		t.Error("the desk never learned from the laptop that a newer version exists")
+	}
+	if m.deletes() != 0 {
+		t.Errorf("%d deletion(s) were sent to another device", m.deletes())
+	}
+
+	// The server returns; everyone ends with exactly its save.
+	m.setOffline(server, false)
+	settleAll(t, server, laptop, desk)
+	for _, d := range []*meshNode{server, laptop, desk} {
+		if !sameSave(d.files(t), newest) {
+			t.Errorf("%s does not hold the newest save: %s", d.name, describe(d.files(t)))
+		}
+		if v := d.version(t); v.Outdated() {
+			t.Errorf("%s still thinks it is behind: %+v", d.name, v)
+		}
+	}
+	if m.deletes() != 0 {
+		t.Errorf("%d deletion(s) were sent to another device", m.deletes())
+	}
+}
+
+// An empty folder — a new device, or one whose copy never arrived — is never
+// one side of a conflict, and never makes the device with the save lose files.
+func TestVersions_AnEmptyDeviceTakesTheSaveWithoutAsking(t *testing.T) {
+	m, n := newMesh(t, "full", "empty")
+	full, empty := n["full"], n["empty"]
+	writeMany(t, full.dir, "map", 100)
+	want := full.files(t)
+
+	// The device with the save syncs first: nothing of its own may go.
+	res, err := syncPair(t, full, empty)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Status == "conflict" {
+		t.Fatal("a save against an empty folder raised a conflict")
+	}
+	if !sameSave(full.files(t), want) {
+		t.Fatalf("the device with the save lost files to an empty one: %s", describe(full.files(t)))
+	}
+	if res, err := syncPair(t, empty, full); err != nil || res.Status == "conflict" {
+		t.Fatalf("the empty device's sync: %+v, %v", res, err)
+	}
+	if !sameSave(empty.files(t), want) {
+		t.Errorf("the empty device did not take the save: %s", describe(empty.files(t)))
+	}
+	if m.deletes() != 0 {
+		t.Errorf("%d deletion(s) were sent", m.deletes())
+	}
+}
+
+// A save that existed before versions did, identical on two devices, is one
+// version at once: no conflict, nothing moved.
+func TestVersions_IdenticalSavesFromBeforeVersionsAgree(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	writeMany(t, a.dir, "s", 5)
+	writeMany(t, b.dir, "s", 5)
+	res, err := syncPair(t, a, b)
+	if err != nil || res.Status != "in_sync" {
+		t.Fatalf("identical saves: %+v, %v", res, err)
+	}
+	if compareVersions(a.version(t).Vector, b.version(t).Vector) != versionEqual {
+		t.Errorf("identical saves got different versions: %v vs %v", a.version(t).Vector, b.version(t).Vector)
+	}
+}
+
+// A deletion made in the game is part of the new version, and reaches the
+// other device as that — whole — however many files it is.
+func TestVersions_DeletionsTravelWithTheVersion(t *testing.T) {
+	m, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	writeMany(t, a.dir, "map", 20)
+	settleAll(t, a, b)
+	a.change(t, func(dir string) {
+		_ = os.RemoveAll(filepath.Join(dir, "map"))
+		write(t, dir, "fresh.sav", "new world")
+	})
+	settleAll(t, a, b)
+	if !sameSave(b.files(t), a.files(t)) {
+		t.Errorf("b = %s, want %s", describe(b.files(t)), describe(a.files(t)))
+	}
+	if m.deletes() != 0 {
+		t.Errorf("the newer device sent %d deletion(s) itself; the older one takes them", m.deletes())
+	}
+}
+
+// Both devices changed the save while apart: that, and only that, is asked
+// about. Resolution is mutual — keeping A's on A asks B in turn rather than
+// overwriting it, and A is not asked again. Once B agrees, the answer spreads,
+// and a third device that had changed nothing takes it without being asked.
+func TestVersions_IndependentChangesAskEachSideOnceAndTheAnswerSpreads(t *testing.T) {
+	_, n := newMesh(t, "a", "b", "c")
+	a, b, c := n["a"], n["b"], n["c"]
+	write(t, a.dir, "slot.sav", "start")
+	settleAll(t, a, b, c)
+
+	a.change(t, func(dir string) { write(t, dir, "slot.sav", "a-played") })
+	b.change(t, func(dir string) { write(t, dir, "slot.sav", "b-played") })
+
+	res, err := syncPair(t, a, b)
+	if err != nil || res.Status != "conflict" {
+		t.Fatalf("independent changes: %+v, %v — want a conflict", res, err)
+	}
+	if _, err := a.eng.ResolveConflict(context.Background(), "game1", b.peer().ID, "keep-local"); err != nil {
+		t.Fatal(err)
+	}
+	bBefore := b.files(t)
+	settleAll(t, a, b, c)
+	if len(a.eng.ActiveConflicts()) != 0 {
+		t.Error("A was asked again after answering")
+	}
+	if len(b.eng.ActiveConflicts()) == 0 {
+		t.Fatal("B was never asked: A's answer must not decide B's save on its own")
+	}
+	if !sameSave(b.files(t), bBefore) {
+		t.Error("B's save changed before anyone there answered")
+	}
+	if _, err := b.eng.ResolveConflict(context.Background(), "game1", a.peer().ID, "keep-remote"); err != nil {
+		t.Fatal(err)
+	}
+	settleAll(t, a, b, c)
+	for _, d := range []*meshNode{b, c} {
+		if got := d.files(t)["slot.sav"]; got != a.files(t)["slot.sav"] {
+			t.Errorf("%s did not take the kept version", d.name)
+		}
+		if len(d.eng.ActiveConflicts()) != 0 {
+			t.Errorf("%s still has a conflict after both answered", d.name)
+		}
+	}
+}
+
+// Both sides answer "keep mine" (or "keep both"): the later answer stands and
+// both end on one save, instead of asking each other forever.
+func TestVersions_TwoAnswersSettleOnTheLater(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	write(t, a.dir, "slot.sav", "start")
+	settleAll(t, a, b)
+	a.change(t, func(dir string) { write(t, dir, "slot.sav", "a-played") })
+	b.change(t, func(dir string) { write(t, dir, "slot.sav", "b-played") })
+	if res, _ := syncPair(t, a, b); res.Status != "conflict" {
+		t.Fatalf("A: %+v, want a conflict", res)
+	}
+	if res, _ := syncPair(t, b, a); res.Status != "conflict" {
+		t.Fatalf("B: %+v, want a conflict", res)
+	}
+	if _, err := a.eng.ResolveConflict(context.Background(), "game1", b.peer().ID, "keep-local"); err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(5 * time.Millisecond)
+	if _, err := b.eng.ResolveConflict(context.Background(), "game1", a.peer().ID, "keep-local"); err != nil {
+		t.Fatal(err)
+	}
+	settleAll(t, a, b)
+	if got := a.files(t)["slot.sav"]; got != b.files(t)["slot.sav"] {
+		t.Fatal("the two never settled on one save")
+	}
+	if len(a.eng.ActiveConflicts())+len(b.eng.ActiveConflicts()) != 0 {
+		t.Error("a conflict is still open after both answered")
+	}
+	if got, _ := os.ReadFile(filepath.Join(a.dir, "slot.sav")); string(got) != "b-played" {
+		t.Errorf("A holds %q; B answered last, so its save should stand", got)
+	}
+}
+
+// A change the watcher never reported — made while the app was closed — still
+// becomes a version once the two devices meet, instead of leaving them on one
+// version with different files, each waiting for the other.
+func TestVersions_AnUnnoticedChangeIsFoundWhenTheDevicesMeet(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	write(t, a.dir, "slot.sav", "start")
+	settleAll(t, a, b)
+	write(t, a.dir, "slot.sav", "played offline") // no NoteLocalChange
+	settleAll(t, b, a)
+	if got := b.files(t)["slot.sav"]; got != a.files(t)["slot.sav"] {
+		t.Errorf("the unnoticed change never reached b")
+	}
+}
+
+// A device whose OpenSave keeps no save versions is never a source: its half
+// of a save is not taken, and nothing here changes on its account. It is
+// reported as needing an update.
+func TestVersions_NothingIsTakenFromAnOldBuild(t *testing.T) {
+	_, n := newMesh(t, "current", "old")
+	current, old := n["current"], n["old"]
+	writeMany(t, current.dir, "map", 10)
+	writeMany(t, old.dir, "map", 3)
+	write(t, old.dir, "only-old.bin", "x")
+	old.proto = ProtoBatchFiles
+	before := current.files(t)
+
+	res, err := syncPair(t, current, old)
+	if err != nil || res.Status != StatusPeerOutdatedApp {
+		t.Fatalf("sync with an old build: %+v, %v — want %s", res, err, StatusPeerOutdatedApp)
+	}
+	if !sameSave(current.files(t), before) {
+		t.Error("the save changed on account of a device without save versions")
+	}
+	if !current.eng.PeerNeedsUpdate(old.peer().ID) {
+		t.Error("the old build is not reported as needing an update")
+	}
+	old.proto = ProtoVersions
+	_, _ = syncPair(t, current, old)
+	if current.eng.PeerNeedsUpdate(old.peer().ID) {
+		t.Error("still reported as needing an update after it was updated")
+	}
+}
+
+// Files a sync writes are not a change made here: the pull-in-progress record
+// keeps them from becoming a version, even across a restart.
+func TestVersions_PulledFilesAreNotANewVersion(t *testing.T) {
+	_, n := newMesh(t, "a", "b")
+	a, b := n["a"], n["b"]
+	writeMany(t, a.dir, "map", 10)
+	settleAll(t, a, b)
+	before := b.version(t).Vector
+
+	a.change(t, func(dir string) { writeMany(t, dir, "map3", 10) })
+	a.blockBudget = 3
+	_, _ = syncPair(t, b, a)
+	a.blockBudget = -1
+
+	// What a restart leaves: the record says a pull was running, and the
+	// watcher reports the half-written folder as a change.
+	rec, _ := b.st.GetGameVersion("game1")
+	rec.Pulling = true
+	_ = b.st.SaveGameVersion(rec)
+	b.eng.NoteLocalChange("game1")
+	if got := b.version(t).Vector; compareVersions(got, before) != versionEqual {
+		t.Fatalf("a half-finished pull became a version of its own: %v (was %v)", got, before)
+	}
+	settleAll(t, a, b)
+	if !sameSave(b.files(t), a.files(t)) {
+		t.Errorf("b did not finish taking a's version: %s", describe(b.files(t)))
+	}
+}
diff --git a/internal/p2p/transport_lan.go b/internal/p2p/transport_lan.go
index 1955b640..44dcf3d6 100644
--- a/internal/p2p/transport_lan.go
+++ b/internal/p2p/transport_lan.go
@@ -35,6 +35,23 @@ type lanTransport struct {
 
 var lanClient = &http.Client{Timeout: 30 * time.Second}
 
+// lanBulkClient is for the two requests whose size grows with the save: a
+// manifest, and a batch of files. Thirty seconds was fine for a few hundred
+// files and not for a few hundred thousand over a VPN; the manifest then never
+// arrived, every sync of that game failed, and the devices never converged.
+// The sync's own context still bounds it (perPeerSyncTimeout), and the
+// transport still gives up on a peer that does not answer at all.
+var lanBulkClient = &http.Client{
+	Timeout: 10 * time.Minute,
+	Transport: func() http.RoundTripper {
+		t := http.DefaultTransport.(*http.Transport).Clone()
+		// Headers arrive once the peer has built its manifest, which for a big
+		// save not hashed recently takes a while — but not this long.
+		t.ResponseHeaderTimeout = 3 * time.Minute
+		return t
+	}(),
+}
+
 func peerURL(peer syncengine.Peer, route string) string {
 	return fmt.Sprintf("http://%s:%d/api/p2p%s", peer.Address, peer.Port, route)
 }
@@ -54,9 +71,17 @@ func (t *lanTransport) FetchManifest(ctx context.Context, peer syncengine.Peer,
 	if q.CoverURL != "" {
 		params.Set("coverUrl", q.CoverURL)
 	}
+	if q.IfHash != "" {
+		params.Set("ifHash", q.IfHash)
+	}
 
 	var resp syncengine.ManifestResponse
-	err := t.getJSON(ctx, peer, peerURL(peer, "/manifest/"+gameID)+"?"+params.Encode(), &resp)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, peerURL(peer, "/manifest/"+gameID)+"?"+params.Encode(), nil)
+	if err != nil {
+		return resp, err
+	}
+	t.sign(req, peer, nil)
+	err = doJSONWith(lanBulkClient, req, &resp)
 	return resp, err
 }
 
@@ -67,17 +92,95 @@ func (t *lanTransport) FetchBlocks(ctx context.Context, peer syncengine.Peer, re
 	// No encodings advertised: on a LAN the wire is typically faster than the
 	// compressor, so the bytes saved cost more than they're worth. Responses
 	// are still decoded, so a peer that compresses anyway is handled.
-	err := t.postJSON(ctx, peer, peerURL(peer, "/blocks/"+ref.GameID), map[string]any{
+	//
+	// Given time for what it asks for. "LAN" here is any direct address,
+	// a VPN one (Tailscale) included, and a few MB of blocks at a VPN relay's
+	// hundred-odd KB a second outlasted the fixed 30 seconds every request
+	// used to get: the request failed, and with it the whole pull, over and
+	// over. Sized like the relay's (slowLinkBytesPerSec), within the bulk
+	// client's ceiling.
+	expected := int64(len(blockIndices)) * int64(max(blockSize, 1))
+	ctx, cancel := context.WithTimeout(ctx, 30*time.Second+time.Duration(expected/(slowLinkBytesPerSec/2))*time.Second)
+	defer cancel()
+	raw, err := json.Marshal(map[string]any{
 		"relPath": ref.RelPath, "root": ref.Root, "blockIndices": blockIndices, "blockSize": blockSize,
-	}, &resp)
+	})
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, peerURL(peer, "/blocks/"+ref.GameID), bytes.NewReader(raw))
 	if err != nil {
 		return nil, err
 	}
+	req.Header.Set("Content-Type", "application/json")
+	t.sign(req, peer, raw)
+	if err := doJSONWith(lanBulkClient, req, &resp); err != nil {
+		return nil, err
+	}
 	return decodeBlocks(resp.Blocks)
 }
 
+// ProbeSpeed times fetching n bytes from the peer's speed test
+// (syncengine.SpeedProber). The clock starts once the answer begins to
+// arrive, so the figure is the link's rate rather than its round trip.
+func (t *lanTransport) ProbeSpeed(ctx context.Context, peer syncengine.Peer, n int) (int64, time.Duration, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, peerURL(peer, fmt.Sprintf("/speedtest?n=%d", n)), nil)
+	if err != nil {
+		return 0, 0, err
+	}
+	t.sign(req, peer, nil)
+	resp, err := lanBulkClient.Do(req)
+	if err != nil {
+		return 0, 0, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return 0, 0, fmt.Errorf("peer returned %d", resp.StatusCode)
+	}
+	start := time.Now()
+	got, err := io.Copy(io.Discard, resp.Body)
+	if err != nil {
+		return got, time.Since(start), err
+	}
+	return got, time.Since(start), nil
+}
+
+// FetchFileBatch fetches the blocks of many files in one request. Only asked
+// of a peer that advertised syncengine.ProtoBatchFiles.
+func (t *lanTransport) FetchFileBatch(ctx context.Context, peer syncengine.Peer, gameID, root string, files []syncengine.FileBlocksRequest) ([]syncengine.FileBlocks, error) {
+	var resp struct {
+		Files []syncengine.FileBlocks `json:"files"`
+	}
+	raw, err := json.Marshal(map[string]any{"root": root, "files": files})
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, peerURL(peer, "/files/"+gameID), bytes.NewReader(raw))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	t.sign(req, peer, raw)
+	if err := doJSONWith(lanBulkClient, req, &resp); err != nil {
+		return nil, err
+	}
+	for i := range resp.Files {
+		if resp.Files[i].Error != "" {
+			continue
+		}
+		blocks, err := decodeBlocks(resp.Files[i].Blocks)
+		if err != nil {
+			return nil, fmt.Errorf("%s: %w", resp.Files[i].RelPath, err)
+		}
+		resp.Files[i].Blocks = blocks
+	}
+	return resp.Files, nil
+}
+
 func (t *lanTransport) DeleteRemote(ctx context.Context, peer syncengine.Peer, ref syncengine.FileRef) error {
-	return t.postJSON(ctx, peer, peerURL(peer, "/delete-file/"+ref.GameID), map[string]any{"relPath": ref.RelPath, "root": ref.Root}, nil)
+	// "versioned": this device keeps save versions; a peer that does too
+	// refuses deletions from one that does not (handleDeleteFile).
+	return t.postJSON(ctx, peer, peerURL(peer, "/delete-file/"+ref.GameID), map[string]any{"relPath": ref.RelPath, "root": ref.Root, "versioned": true}, nil)
 }
 
 // TriggerPeerPull tells a peer that this device holds newer content.
@@ -224,7 +327,11 @@ func (t *lanTransport) sign(req *http.Request, peer syncengine.Peer, body []byte
 }
 
 func doJSON(req *http.Request, out any) error {
-	resp, err := lanClient.Do(req)
+	return doJSONWith(lanClient, req, out)
+}
+
+func doJSONWith(client *http.Client, req *http.Request, out any) error {
+	resp, err := client.Do(req)
 	if err != nil {
 		return err
 	}
diff --git a/internal/p2p/transport_wan.go b/internal/p2p/transport_wan.go
index 859e91ef..b2b5d90d 100644
--- a/internal/p2p/transport_wan.go
+++ b/internal/p2p/transport_wan.go
@@ -3,6 +3,7 @@ package p2p
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"time"
 
@@ -90,7 +91,7 @@ func (t *wanTransport) FetchBlocks(ctx context.Context, peer syncengine.Peer, re
 }
 
 func (t *wanTransport) DeleteRemote(ctx context.Context, peer syncengine.Peer, ref syncengine.FileRef) error {
-	_, err := t.wan.Request(ctx, peer.ID, "/delete-file/"+ref.GameID, "POST", map[string]any{"relPath": ref.RelPath, "root": ref.Root})
+	_, err := t.wan.Request(ctx, peer.ID, "/delete-file/"+ref.GameID, "POST", map[string]any{"relPath": ref.RelPath, "root": ref.Root, "versioned": true})
 	return err
 }
 
@@ -154,6 +155,30 @@ func (t *routingTransport) FetchBlocks(ctx context.Context, peer syncengine.Peer
 	return t.pick(peer).FetchBlocks(ctx, peer, ref, blockIndices, blockSize)
 }
 
+// FetchFileBatch batches over the LAN only. Relay messages are sealed and
+// size-limited per client (see BatchIndices), so the relay keeps pulling file
+// by file.
+func (t *routingTransport) FetchFileBatch(ctx context.Context, peer syncengine.Peer, gameID, root string, files []syncengine.FileBlocksRequest) ([]syncengine.FileBlocks, error) {
+	if peer.Wan() {
+		return nil, syncengine.ErrBatchUnsupported
+	}
+	b, ok := t.lan.(syncengine.BatchFetcher)
+	if !ok {
+		return nil, syncengine.ErrBatchUnsupported
+	}
+	return b.FetchFileBatch(ctx, peer, gameID, root, files)
+}
+
+// ProbeSpeed tests LAN links only (syncengine.SpeedProber); the relay is the
+// slowest link whatever a test would say.
+func (t *routingTransport) ProbeSpeed(ctx context.Context, peer syncengine.Peer, n int) (int64, time.Duration, error) {
+	p, ok := t.lan.(syncengine.SpeedProber)
+	if peer.Wan() || !ok {
+		return 0, 0, errors.New("no speed test over the relay")
+	}
+	return p.ProbeSpeed(ctx, peer, n)
+}
+
 func (t *routingTransport) DeleteRemote(ctx context.Context, peer syncengine.Peer, ref syncengine.FileRef) error {
 	return t.pick(peer).DeleteRemote(ctx, peer, ref)
 }
diff --git a/internal/p2p/wanclient_handlers.go b/internal/p2p/wanclient_handlers.go
index 95203dd3..39668b2d 100644
--- a/internal/p2p/wanclient_handlers.go
+++ b/internal/p2p/wanclient_handlers.go
@@ -172,6 +172,7 @@ func (w *WanClient) handleMessage(ctx context.Context, msg RelayMessage) {
 			if w.engine.Sync.Progress.OnSyncError != nil {
 				w.engine.Sync.Progress.OnSyncError(msg.GameID, ev)
 			}
+			w.engine.Sync.NotePeerPulled(msg.GameID, msg.From)
 		}
 
 	case "request":
@@ -603,6 +604,9 @@ func (w *WanClient) serveManifest(route string, body json.RawMessage, peerID str
 	if w.engine.holdingBack(game) {
 		return 404, map[string]string{"error": syncengine.HeldMessage}
 	}
+	if w.engine.Sync.PlayingHere(game.ID) {
+		return 503, map[string]string{"error": syncengine.PlayingMessage}
+	}
 	// As on the LAN: a save a sync here is writing is not described
 	// (syncengine/settle.go). Each request runs on its own goroutine, so the
 	// wait does not hold up the relay traffic that finishes the write.
@@ -617,12 +621,20 @@ func (w *WanClient) serveManifest(route string, body json.RawMessage, peerID str
 		return 500, map[string]string{"error": err.Error()}
 	}
 	w.engine.Sync.NoteServed(game.ID, peerID, manifest) // syncengine/served.go
+	// "versions" says this device keeps save versions, and "version" is its
+	// version of this save (syncengine/version.go). Not "proto": over the
+	// relay that would also claim extra save locations and batched blocks,
+	// which this path does not serve.
 	resp := map[string]any{
 		"gameId": gameID, "activeBranch": game.ActiveBranch, "manifest": manifest,
 		"deletionConfirmed": w.engine.Sync.DeletionConfirmed(game.ID),
+		"versions":          true,
+	}
+	if v := w.engine.Sync.LocalVersion(game, manifest); v != nil {
+		resp["version"] = v
 	}
 	if latest, err := w.engine.Snapshots.LatestSnapshot(gameID, ""); err == nil {
-		resp["latestSnapshot"] = syncengine.SnapshotInfo{ID: latest.ID, Timestamp: latest.Timestamp, Comment: latest.Comment}
+		resp["latestSnapshot"] = syncengine.SnapshotInfo{ID: latest.ID, Timestamp: latest.Timestamp, Comment: latest.Comment, ContentHash: latest.ContentHash}
 	} else {
 		resp["latestSnapshot"] = nil
 	}
@@ -678,6 +690,9 @@ func (w *WanClient) serveDeleteFile(route string, rawBody json.RawMessage, fromP
 	gameID := route[strings.LastIndex(route, "/")+1:]
 	var body struct {
 		RelPath string `json:"relPath"`
+		// Versioned: the asking device keeps save versions (see the LAN
+		// route, handleDeleteFile).
+		Versioned bool `json:"versioned"`
 	}
 	if err := json.Unmarshal(rawBody, &body); err != nil || body.RelPath == "" {
 		return 400, map[string]string{"error": "relPath is required."}
@@ -686,6 +701,17 @@ func (w *WanClient) serveDeleteFile(route string, rawBody json.RawMessage, fromP
 	if err != nil {
 		return 404, map[string]string{"error": "Game not found."}
 	}
+	if w.engine.Sync.PlayingHere(game.ID) {
+		return 503, map[string]string{"error": syncengine.PlayingMessage}
+	}
+	if game.AutoSync && !body.Versioned {
+		from := fromPeerID
+		if p, err := w.engine.Store.GetPeer(fromPeerID); err == nil && p.Name != "" {
+			from = p.Name
+		}
+		w.engine.Sync.RefusedOldBuildDelete(game.Name, from)
+		return 409, map[string]string{"error": syncengine.OldBuildDeleteMessage}
+	}
 	if !delta.IsSafePath(game.SavePath, body.RelPath) {
 		return 403, map[string]string{"error": "invalid path"}
 	}
diff --git a/internal/presets/detectsettings.go b/internal/presets/detectsettings.go
new file mode 100644
index 00000000..5fe85f2b
--- /dev/null
+++ b/internal/presets/detectsettings.go
@@ -0,0 +1,361 @@
+package presets
+
+import (
+	"bytes"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+	"unicode/utf16"
+)
+
+// Detecting a game's settings files by what they hold, for games the game
+// database does not cover (DeviceSettings covers the ones it does).
+//
+// What tells a device's settings from a save is its contents: resolution,
+// vsync, shadow quality, upscaler — the vocabulary of one machine's display,
+// which no save has a use for. Names were measured against the Ludusavi
+// manifest's own labels (16,700 settings entries, 9,000 save entries) and do
+// not tell them apart on their own: "settings" names 1,521 settings files and
+// 70 saves, ".ini" 1,831 and 137, ".json" 359 and 216. So a name only adds
+// weight to what the contents say, by how reliably the manifest shows it to
+// mean settings; a word that names saves rules a file out.
+//
+// What this misses stays syncing, which is how every file synced before; what
+// it wrongly takes is caught by how a save behaves — it changes with every
+// session — and given back (daemon/detectsettings.go).
+
+// DetectedSettings is one file found to be a game's settings.
+type DetectedSettings struct {
+	Path   string // relative to the save folder, slash-separated
+	Reason string
+}
+
+const (
+	detectMaxSize  = 256 << 10
+	detectMaxFiles = 4000 // text-like files read per folder, at most
+)
+
+var detectExts = map[string]bool{
+	"ini": true, "cfg": true, "conf": true, "config": true, "xml": true, "json": true, "lua": true,
+	"txt": true, "yaml": true, "yml": true, "toml": true, "vdf": true, "set": true, "settings": true,
+	"opt": true, "prefs": true, "local": true, "properties": true,
+}
+
+// Terms of a machine's display and graphics, as sequences of words. Matched
+// in identifiers split at case changes and separators, so
+// "_enableFrameGeneration", "r_materialAniso" and "Shadow Quality" all count.
+var strongTerms = [][]string{
+	{"resolution"}, {"fullscreen"}, {"windowed"}, {"borderless"}, {"vsync"}, {"v", "sync"},
+	{"antialiasing"}, {"anti", "aliasing"}, {"aniso"}, {"anisotropic"}, {"anisotropy"},
+	{"msaa"}, {"fxaa"}, {"taa"}, {"smaa"}, {"ssao"}, {"hbao"}, {"ssr"}, {"bloom"}, {"gamma"},
+	{"brightness"}, {"hdr"}, {"dlss"}, {"fsr"}, {"xess"}, {"upscaling"}, {"upscaler"},
+	{"monitor"}, {"adapter"}, {"gpu"}, {"vram"}, {"tessellation"}, {"raytracing"},
+	{"ray", "tracing"}, {"fov"}, {"field", "view"}, {"lod"}, {"framerate"}, {"frame", "rate"},
+	{"refreshrate"}, {"refresh", "rate"}, {"frame", "generation"}, {"frame", "limit"},
+	{"fps", "limit"}, {"max", "fps"}, {"texture", "quality"}, {"texture", "filtering"},
+	{"shadow", "quality"}, {"shadow", "resolution"}, {"motion", "blur"}, {"depth", "field"},
+	{"view", "distance"}, {"draw", "distance"}, {"render", "scale"}, {"resolution", "scale"},
+	{"display", "mode"}, {"window", "mode"}, {"screen", "mode"}, {"ambient", "occlusion"},
+	{"screen", "width"}, {"screen", "height"}, {"quality", "preset"}, {"graphics", "quality"},
+	{"film", "grain"}, {"lens", "flare"}, {"volumetric"}, {"scalability"}, {"supersampling"},
+	{"sharpening"}, {"chromatic", "aberration"}, {"vignette"}, {"directx"}, {"vulkan"},
+	{"dx11"}, {"dx12"}, {"d3d11"}, {"d3d12"},
+}
+
+var weakTerms = []string{
+	"width", "height", "quality", "preset", "driver", "spec", "shadows", "shadow", "texture",
+	"textures", "video", "display", "graphics", "render", "screen", "fps", "sync", "filtering",
+}
+
+// Words in a file name, weighted by how reliably the Ludusavi manifest shows
+// them to mean settings rather than a save.
+var nameWeights = map[string]float64{
+	"options": 1.5, "option": 1.5, "graphics": 1.5, "graphic": 1.5, "video": 1.5, "display": 1.5,
+	"render": 1.5, "engine": 1.5, "scalability": 1.5, "gameusersettings": 1.5, "screen": 1,
+	"settings": 0.5, "setting": 0.5, "config": 0.5, "configuration": 0.5, "cfg": 0.5,
+	"prefs": 0.5, "preferences": 0.5,
+}
+
+// Words that name saves somewhere in the manifest often enough that a file
+// carrying one is never taken, whatever it holds. Logs are not settings
+// either, though leaving them out would harm nothing.
+var saveWords = map[string]bool{
+	"save": true, "saves": true, "savegame": true, "savedata": true, "slot": true, "progress": true,
+	"autosave": true, "quicksave": true, "world": true, "player": true, "character": true,
+	"campaign": true, "level": true, "levels": true, "achievement": true, "achievements": true,
+	"achievments": true, "stats": true, "unlock": true, "unlocks": true, "inventory": true,
+	"profile": true, "career": true, "highscore": true, "highscores": true, "score": true,
+	"scores": true, "records": true, "besttimes": true, "gamedata": true, "log": true,
+	"logs": true, "crash": true, "output": true, "history": true,
+}
+
+var identRe = regexp.MustCompile(`[A-Za-z][A-Za-z0-9]*`)
+var camelRe = regexp.MustCompile(`[A-Z]?[a-z]+|[A-Z]+(?:[^a-z]|$)|[0-9]+`)
+
+// words splits text into lowercase words: identifiers, then at case changes
+// ("FrameGeneration" → frame generation), keeping a whole short identifier
+// too ("dx11", "vsync").
+func words(text string) []string {
+	var out []string
+	for _, id := range identRe.FindAllString(text, -1) {
+		parts := camelRe.FindAllString(id, -1)
+		low := strings.ToLower(id)
+		if len(parts) > 1 && len(low) <= 6 {
+			out = append(out, low)
+		}
+		for _, p := range parts {
+			p = strings.ToLower(p)
+			if p == "of" || p == "the" || p == "b" || p == "r" || p == "e" {
+				continue // filler, and engine prefixes like r_ and bEnable
+			}
+			out = append(out, p)
+		}
+	}
+	return out
+}
+
+// textOf returns a file's text, decoding UTF-16, or false for binary.
+func textOf(b []byte) (string, bool) {
+	switch {
+	case bytes.HasPrefix(b, []byte{0xFF, 0xFE}), bytes.HasPrefix(b, []byte{0xFE, 0xFF}):
+		big := b[0] == 0xFE
+		b = b[2:]
+		u := make([]uint16, len(b)/2)
+		for i := range u {
+			if big {
+				u[i] = uint16(b[2*i])<<8 | uint16(b[2*i+1])
+			} else {
+				u[i] = uint16(b[2*i+1])<<8 | uint16(b[2*i])
+			}
+		}
+		return string(utf16.Decode(u)), true
+	case len(b) >= 4 && b[1] == 0 && b[3] == 0 && b[0] != 0 && b[2] != 0:
+		// UTF-16LE without a mark, as some engines write their XML.
+		u := make([]uint16, len(b)/2)
+		for i := range u {
+			u[i] = uint16(b[2*i+1])<<8 | uint16(b[2*i])
+		}
+		return string(utf16.Decode(u)), true
+	case bytes.IndexByte(b, 0) >= 0:
+		return "", false
+	}
+	return string(b), true
+}
+
+// contentScore counts the distinct display terms a text holds.
+func contentScore(text string) (strong int, score float64) {
+	ws := words(text)
+	present := make(map[string]bool, len(ws))
+	pairs := make(map[[2]string]bool, len(ws))
+	for i, w := range ws {
+		present[w] = true
+		if i+1 < len(ws) {
+			pairs[[2]string{w, ws[i+1]}] = true
+		}
+	}
+	for _, t := range strongTerms {
+		if (len(t) == 1 && present[t[0]]) || (len(t) == 2 && pairs[[2]string{t[0], t[1]}]) {
+			strong++
+		}
+	}
+	weak := 0.0
+	for _, w := range weakTerms {
+		if present[w] {
+			weak += 0.5
+		}
+	}
+	if weak > 2 {
+		weak = 2
+	}
+	return strong, float64(strong) + weak
+}
+
+// nameSignal weighs a file's name: the support it lends, and whether a word
+// in it names saves.
+func nameSignal(rel string) (weight float64, saveWord, strongName bool) {
+	base := path.Base(rel)
+	ext := strings.ToLower(strings.TrimPrefix(path.Ext(base), "."))
+	stem := strings.TrimSuffix(base, path.Ext(base))
+	ws := words(stem)
+	low := strings.ToLower(stem)
+	for w := range nameWeights {
+		if len(w) >= 6 && strings.Contains(low, w) {
+			ws = append(ws, w)
+		}
+	}
+	seen := map[string]bool{}
+	for _, w := range ws {
+		if seen[w] {
+			continue
+		}
+		seen[w] = true
+		if saveWords[w] {
+			saveWord = true
+		}
+		weight += nameWeights[w]
+		if nameWeights[w] >= 1.5 {
+			strongName = true
+		}
+	}
+	if strings.Contains(low, "save") || strings.Contains(low, "slot") || ext == "log" {
+		saveWord = true
+	}
+	if ext == "ini" || ext == "cfg" || ext == "conf" {
+		weight += 0.5
+	}
+	if weight > 2 {
+		weight = 2
+	}
+	return weight, saveWord, strongName
+}
+
+// unrealSettings reports a file in an Unreal Engine game's settings folder,
+// Saved/Config/<platform>: the engine writes a machine's settings there and
+// nothing else, often as files too short to judge by content.
+func unrealSettings(rel string) bool {
+	segs := strings.Split(strings.ToLower(rel), "/")
+	if len(segs) < 2 || !strings.HasSuffix(segs[len(segs)-1], ".ini") {
+		return false
+	}
+	for i := 0; i+1 < len(segs)-1; i++ {
+		if segs[i] == "config" && (strings.HasPrefix(segs[i+1], "windows") || strings.HasPrefix(segs[i+1], "linux") ||
+			strings.HasPrefix(segs[i+1], "mac")) {
+			return true
+		}
+	}
+	return false
+}
+
+// judgeSettings decides one file from its path and bytes.
+func judgeSettings(rel string, b []byte) (bool, string) {
+	weight, saveWord, strongName := nameSignal(rel)
+	if saveWord && !strongName {
+		return false, ""
+	}
+	if unrealSettings(rel) && !saveWord {
+		return true, "Unreal Engine settings folder"
+	}
+	text, ok := textOf(b)
+	if !ok {
+		return false, ""
+	}
+	strong, score := contentScore(text)
+	switch {
+	case saveWord:
+		// "user_engine_option_save.xml": a strong settings word and a save
+		// word together. Taken only on overwhelming contents.
+		if strong >= 6 && score+weight >= 8 {
+			return true, "display settings in its contents"
+		}
+	case len(b) > 32<<10:
+		// A long file holds a lot besides; a save with a settings section is
+		// long. Only a file that is mostly settings is taken.
+		if strong >= 6 {
+			return true, "display settings in its contents"
+		}
+	case strong >= 3 && score+weight >= 4:
+		return true, "display settings in its contents"
+	case strong >= 2 && weight >= 1.5 && score+weight >= 4:
+		return true, "display settings in its contents, and its name"
+	}
+	return false, ""
+}
+
+// DetectSettings finds the settings files in root by what they hold. The
+// game's save locations in the game database (by name or App ID) are never
+// taken, whatever they hold.
+func (ds *DeviceSettings) DetectSettings(name, appID, root string) []DetectedSettings {
+	var savePats []string
+	if ds != nil {
+		g, ok := ds.byID[appID]
+		if !ok || appID == "" {
+			g, ok = ds.byName[normalizeGameName(stripNameSuffixes(name))]
+		}
+		if ok {
+			for _, vars := range ds.sc.ludusaviVarSets(g, ds.protonIx) {
+				for _, tpl := range g.Paths {
+					for _, expanded := range expandTemplate(tpl, vars, ds.bases(g.Installs), g.SteamID) {
+						if rel, ok := relativeUnder(root, expanded); ok {
+							savePats = append(savePats, rel)
+						}
+					}
+				}
+			}
+		}
+	}
+	return detectIn(root, savePats)
+}
+
+// isSaveLocation reports whether rel is, or is inside, one of the game
+// database's save locations (relative patterns). A location naming a folder
+// covers what is in it; one with a file extension names that file.
+func isSaveLocation(rel string, savePats []string) bool {
+	segs := strings.Split(strings.ToLower(rel), "/")
+	for _, p := range savePats {
+		ps := strings.Split(p, "/")
+		if len(ps) > len(segs) {
+			continue
+		}
+		match := true
+		for i, pc := range ps {
+			if ok, err := path.Match(pc, segs[i]); err != nil || !ok {
+				match = false
+				break
+			}
+		}
+		if !match {
+			continue
+		}
+		if len(ps) < len(segs) || path.Ext(ps[len(ps)-1]) != "" {
+			return true
+		}
+	}
+	return false
+}
+
+func detectIn(root string, savePats []string) []DetectedSettings {
+	var out []DetectedSettings
+	read := 0
+	_ = filepath.WalkDir(root, func(p string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return nil
+		}
+		if d.IsDir() {
+			if p != root && strings.HasPrefix(d.Name(), ".") {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		ext := strings.ToLower(strings.TrimPrefix(filepath.Ext(p), "."))
+		if !detectExts[ext] || read >= detectMaxFiles {
+			return nil
+		}
+		rel, err := filepath.Rel(root, p)
+		if err != nil {
+			return nil
+		}
+		rel = filepath.ToSlash(rel)
+		if strings.Contains(strings.ToLower(rel), ".sync-conflict-") || isSaveLocation(rel, savePats) {
+			return nil
+		}
+		info, err := d.Info()
+		if err != nil || info.Size() > detectMaxSize {
+			return nil
+		}
+		b, err := os.ReadFile(p)
+		if err != nil {
+			return nil
+		}
+		read++
+		if ok, reason := judgeSettings(rel, b); ok {
+			out = append(out, DetectedSettings{Path: rel, Reason: reason})
+		}
+		return nil
+	})
+	sort.Slice(out, func(i, j int) bool { return out[i].Path < out[j].Path })
+	return out
+}
diff --git a/internal/presets/detectsettings_test.go b/internal/presets/detectsettings_test.go
new file mode 100644
index 00000000..f499e375
--- /dev/null
+++ b/internal/presets/detectsettings_test.go
@@ -0,0 +1,65 @@
+package presets
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"unicode/utf16"
+)
+
+func utf16le(s string) []byte {
+	u := utf16.Encode([]rune(s))
+	b := []byte{0xFF, 0xFE}
+	for _, r := range u {
+		b = append(b, byte(r), byte(r>>8))
+	}
+	return b
+}
+
+func TestJudgeSettings(t *testing.T) {
+	crimson := `<EngineOptionSave _selectPreset="Custom"><EngineOptionVideo Name="_videoOption">
+<OptionBool Name="_enableFrameGeneration"/><OptionInt Name="_shadowQuality"/><OptionInt Name="_textureQuality"/>
+<OptionBool Name="_vSync"/><OptionInt Name="_displayMode"/><OptionInt Name="_resolution"/><OptionInt Name="_upscaler" Value="DLSS"/>
+<OptionFloat Name="_brightness"/><OptionInt Name="_antiAliasing"/></EngineOptionVideo></EngineOptionSave>`
+	doom := "configVersion 9\nr_materialAniso \"16\"\nr_mode \"18\"\nr_fullscreen \"1\"\nr_swapInterval \"1\"\nr_lodScale \"1\"\nr_gamma \"1.2\"\n"
+	cases := []struct {
+		name string
+		path string
+		data []byte
+		want bool
+	}{
+		{"display settings", "videoconfig.txt", []byte("setting.fullscreen 1\nsetting.mat_vsync 0\nsetting.gamma 1\nsetting.defaultres 1920\nsetting.ssao 1\n"), true},
+		{"camelCase identifiers, save word outweighed", "user_engine_option_save.xml", []byte(crimson), true},
+		{"engine abbreviations", "DOOMEternalConfig.local", []byte(doom), true},
+		{"UTF-16", "GraphicsConfig.xml", utf16le(`<config><ScreenMode>1</ScreenMode><Resolution w="1920"/><Vsync>1</Vsync><AntiAliasing>2</AntiAliasing><ShadowQuality>3</ShadowQuality><TextureQuality>3</TextureQuality></config>`), true},
+		{"Unreal settings folder", "Saved/Config/WindowsNoEditor/Input.ini", []byte("[/Script/Engine.InputSettings]\n"), true},
+		{"save word, weak settings", "savegame.json", []byte(`{"fullscreen":true,"vsync":true,"resolution":"1080p","gold":12}`), false},
+		{"progress in a settings-like file", "config.cfg", []byte("level 4\ngold 120\nhealth 80\n"), false},
+		{"a log mentioning the GPU", "output_log.txt", []byte("GPU: RTX; resolution 1920; vsync on; fullscreen; monitor 1; hdr off"), false},
+		{"binary", "graphics.cfg", []byte{0x01, 0x00, 0x02, 0x00, 0x03}, false},
+		{"long save with a settings section", "state.json", []byte(`{"fullscreen":1,"vsync":1,"resolution":1,"gamma":1,"hdr":0,` + strings.Repeat(`"item":1,`, 5000) + `"x":0}`), false},
+	}
+	for _, c := range cases {
+		if got, _ := judgeSettings(c.path, c.data); got != c.want {
+			t.Errorf("%s: %s judged settings=%v, want %v", c.name, c.path, got, c.want)
+		}
+	}
+}
+
+// The game database's save locations are never taken, whatever they hold.
+func TestDetectIn_SaveLocationsAreNeverTaken(t *testing.T) {
+	root := t.TempDir()
+	settings := "fullscreen=1\nvsync=1\nresolution=1920x1080\nshadow_quality=3\nanti_aliasing=2\n"
+	for _, p := range []string{"video.ini", "slots/1/video.ini", "keep/video.ini"} {
+		full := filepath.Join(root, filepath.FromSlash(p))
+		_ = os.MkdirAll(filepath.Dir(full), 0o777)
+		if err := os.WriteFile(full, []byte(settings), 0o666); err != nil {
+			t.Fatal(err)
+		}
+	}
+	got := detectIn(root, []string{"slots", "keep/video.ini"})
+	if len(got) != 1 || got[0].Path != "video.ini" {
+		t.Errorf("detected %+v, want only video.ini", got)
+	}
+}
diff --git a/internal/presets/devicesettings.go b/internal/presets/devicesettings.go
new file mode 100644
index 00000000..1270ee97
--- /dev/null
+++ b/internal/presets/devicesettings.go
@@ -0,0 +1,142 @@
+package presets
+
+import (
+	"path"
+	"sort"
+	"strings"
+)
+
+// DeviceSettings answers, for a tracked game, which of the files in its save
+// folders are its settings rather than its save: what the Ludusavi manifest
+// tags "config" and not "save" (indexedGame.Config). A device's graphics
+// settings are the usual case — a game that keeps them beside its saves had
+// one PC's resolution and quality handed to every other.
+//
+// The answer is a list of exclusion patterns, in the syntax of a game's own
+// exclusion rules and relative to the save folder they fall in, so it is
+// applied by exactly the machinery that applies those: never synced in either
+// direction, never deleted on another device, still in every snapshot.
+type DeviceSettings struct {
+	sc       *Scanner
+	byID     map[string]indexedGame
+	byName   map[string]indexedGame
+	protonIx map[string][]string
+	bases    func([]string) []string
+}
+
+// DeviceSettings loads the manifest index once for any number of lookups.
+// Nil when there is no index.
+func (sc *Scanner) DeviceSettings() *DeviceSettings {
+	if sc == nil || sc.CacheFile == "" {
+		return nil
+	}
+	idx := sc.loadManifestIndex()
+	if len(idx) == 0 {
+		return nil
+	}
+	ds := &DeviceSettings{sc: sc, byID: map[string]indexedGame{}, byName: map[string]indexedGame{}}
+	for _, g := range idx {
+		if len(g.Config) == 0 && len(g.Paths) == 0 {
+			continue
+		}
+		if g.SteamID != "" {
+			ds.byID[g.SteamID] = g
+		}
+		if key := normalizeGameName(g.Name); key != "" {
+			ds.byName[key] = g
+		}
+	}
+	if sc.goos() == "linux" {
+		ds.protonIx = sc.protonPrefixIndex()
+	}
+	ds.bases = sc.installBaseCandidates()
+	return ds
+}
+
+// Patterns returns the exclusion patterns for a game's settings that lie
+// inside one of roots (its save folders; the first is the main one), each
+// relative to the root it is in. Patterns for any other root than the first
+// are not returned: a game's rules are written against its main folder.
+func (ds *DeviceSettings) Patterns(name, appID string, roots []string) []string {
+	if ds == nil || len(roots) == 0 {
+		return nil
+	}
+	g, ok := ds.byID[appID]
+	if !ok || appID == "" {
+		g, ok = ds.byName[normalizeGameName(stripNameSuffixes(name))]
+	}
+	if !ok {
+		return nil
+	}
+	installBases := ds.bases(g.Installs)
+	seen := map[string]bool{}
+	var out []string
+	for _, vars := range ds.sc.ludusaviVarSets(g, ds.protonIx) {
+		for _, tpl := range g.Config {
+			for _, expanded := range expandTemplate(tpl, vars, installBases, g.SteamID) {
+				rel, ok := relativeUnder(roots[0], expanded)
+				if !ok || coversEverything(rel) || seen[rel] {
+					continue
+				}
+				seen[rel] = true
+				out = append(out, "/"+rel)
+			}
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
+// relativeUnder returns pattern relative to root when it names something
+// strictly inside it, comparing component by component so that a wildcard the
+// template had (a store user id) matches the folder actually tracked.
+// Case-insensitive, as exclusion rules are.
+func relativeUnder(root, pattern string) (string, bool) {
+	split := func(p string) []string {
+		// Either separator on every system: a pattern written for Windows
+		// (Ludusavi's data, a Proton prefix) is matched on Linux too, and
+		// path.Match would read a backslash as an escape.
+		p = strings.ToLower(path.Clean(strings.ReplaceAll(p, `\`, "/")))
+		var parts []string
+		for _, c := range strings.Split(p, "/") {
+			if c != "" {
+				parts = append(parts, c)
+			}
+		}
+		return parts
+	}
+	r, p := split(root), split(pattern)
+	if len(r) == 0 || len(p) <= len(r) {
+		return "", false
+	}
+	for i, rc := range r {
+		if ok, err := path.Match(p[i], rc); err != nil || !ok {
+			return "", false
+		}
+	}
+	return strings.Join(p[len(r):], "/"), true
+}
+
+// coversEverything reports a relative pattern that would exclude the whole
+// folder — a settings entry naming the folder the save is tracked at. Never
+// applied: a wrong exclusion loses saves, a missed one only syncs a setting.
+func coversEverything(rel string) bool {
+	first := strings.SplitN(rel, "/", 2)[0]
+	return strings.Trim(first, "*") == "" || first == "*.*"
+}
+
+// InstallNames returns the names the game database knows a game's install
+// folder by ("Crimson Desert"), found by App ID or name.
+func (ds *DeviceSettings) InstallNames(name, appID string) []string {
+	if ds == nil {
+		return nil
+	}
+	g, ok := ds.byID[appID]
+	if !ok || appID == "" {
+		g, ok = ds.byName[normalizeGameName(stripNameSuffixes(name))]
+	}
+	if !ok {
+		return nil
+	}
+	return append([]string(nil), g.Installs...)
+}
diff --git a/internal/presets/devicesettings_test.go b/internal/presets/devicesettings_test.go
new file mode 100644
index 00000000..019641c4
--- /dev/null
+++ b/internal/presets/devicesettings_test.go
@@ -0,0 +1,82 @@
+package presets
+
+import (
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+// Crimson Desert's shape: the save is a per-account folder inside the tracked
+// one, and the graphics settings sit beside it. The settings are named; the
+// save is not; an entry tagged both counts as save.
+func TestDeviceSettings_SettingsInsideTheSaveFolder(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("templates below use Windows placeholders")
+	}
+	home := t.TempDir()
+	t.Setenv("USERPROFILE", home)
+	t.Setenv("HOME", home)
+	local := filepath.Join(home, "AppData", "Local")
+	t.Setenv("LOCALAPPDATA", local)
+
+	sc := manifestScanner(t, `
+Pearl Game:
+  files:
+    "<winLocalAppData>/Pearl/CD/save/<storeUserId>":
+      tags: [save]
+    "<winLocalAppData>/Pearl/CD/save/user_engine_option_save.xml":
+      tags: [config]
+    "<winLocalAppData>/Pearl/CD/save/both.cfg":
+      tags: [config, save]
+    "<winLocalAppData>/Pearl/CD/save/*":
+      tags: [config]
+    "<winLocalAppData>/Pearl/Elsewhere/settings.ini":
+      tags: [config]
+  steam:
+    id: 123
+`)
+	ds := sc.DeviceSettings()
+	if ds == nil {
+		t.Fatal("no index")
+	}
+	root := filepath.Join(local, "Pearl", "CD", "save")
+	want := []string{"/user_engine_option_save.xml"}
+	if got := ds.Patterns("Pearl Game", "123", []string{root}); !reflect.DeepEqual(got, want) {
+		t.Errorf("by App ID: %v, want %v", got, want)
+	}
+	if got := ds.Patterns("Pearl Game", "", []string{root}); !reflect.DeepEqual(got, want) {
+		t.Errorf("by name: %v, want %v", got, want)
+	}
+	// Tracked at the per-account folder: the settings are not inside it.
+	if got := ds.Patterns("Pearl Game", "123", []string{filepath.Join(root, "22202")}); len(got) != 0 {
+		t.Errorf("at the account folder: %v, want none", got)
+	}
+	if got := ds.Patterns("Another Game", "999", []string{root}); len(got) != 0 {
+		t.Errorf("unknown game: %v", got)
+	}
+}
+
+func TestRelativeUnder(t *testing.T) {
+	for _, c := range []struct {
+		root, pattern, want string
+		ok                  bool
+	}{
+		{`C:\Users\A\Saved Games\Game`, `C:\Users\A\Saved Games\Game\game.cfg`, "game.cfg", true},
+		{`C:\Users\A/Saved Games/Game`, `c:\users\a\saved games\game\Config\x.ini`, "config/x.ini", true},
+		{`C:\Users\A\Game\save\22202`, `C:\Users\A\Game\save\*\opt.xml`, "opt.xml", true},
+		{`C:\Users\A\Game\save`, `C:\Users\A\*\save\opt.xml`, "opt.xml", true},
+		{`C:\Users\A\Game`, `C:\Users\A\Game`, "", false},
+		{`C:\Users\A\Game`, `C:\Users\A\Other\x.ini`, "", false},
+	} {
+		got, ok := relativeUnder(c.root, c.pattern)
+		if got != c.want || ok != c.ok {
+			t.Errorf("relativeUnder(%q, %q) = %q, %v; want %q, %v", c.root, c.pattern, got, ok, c.want, c.ok)
+		}
+	}
+	for rel, want := range map[string]bool{"*": true, "*.*": true, "**/x": true, "*.cfg": false, "config": false} {
+		if coversEverything(rel) != want {
+			t.Errorf("coversEverything(%q) = %v", rel, !want)
+		}
+	}
+}
diff --git a/internal/presets/ludusavi.go b/internal/presets/ludusavi.go
index 7a6ea160..31027066 100644
--- a/internal/presets/ludusavi.go
+++ b/internal/presets/ludusavi.go
@@ -68,6 +68,11 @@ type indexedGame struct {
 	// index built by an older release, which reads as "none" — the field is
 	// additive, so an index on disk from a previous version still loads.
 	Registry []string `json:"r,omitempty"`
+	// Config is where the game keeps its settings and nothing else: entries
+	// the manifest tags "config" and not "save". A device's own graphics
+	// settings among them; inside a tracked save folder they are left out of
+	// syncing (DeviceSettingsPatterns).
+	Config []string `json:"c,omitempty"`
 }
 
 // manifestPaths derives the manifest + index locations from the scanner's
@@ -89,7 +94,7 @@ func (sc *Scanner) manifestPaths() (yamlPath, indexPath string) {
 // reading the file it knows and a newer one builds its own, so a user moving
 // between the two is never handed an index whose shape their code predates.
 // Bump this whenever indexedGame gains or loses a field that a scan depends on.
-const indexFileName = "ludusavi-index-v2.json"
+const indexFileName = "ludusavi-index-v3.json"
 
 // scanLudusavi expands the manifest's save-path templates and returns the
 // locations that actually exist on this machine.
@@ -728,13 +733,17 @@ func buildManifestIndex(yamlPath string) []indexedGame {
 
 	games := make([]indexedGame, 0, len(manifest))
 	for name, mg := range manifest {
-		var paths []string
+		var paths, config []string
 		for tpl, entry := range mg.Files {
+			if entryIsConfigEntry(tpl, entry) {
+				config = append(config, tpl)
+			}
 			if !entryIsSaveEntry(tpl, entry) {
 				continue
 			}
 			paths = append(paths, tpl)
 		}
+		sort.Strings(config)
 		var regKeys []string
 		for key, entry := range mg.Registry {
 			if !registryEntryIsSave(key, entry) {
@@ -763,7 +772,7 @@ func buildManifestIndex(yamlPath string) []indexedGame {
 			continue
 		}
 		sort.Strings(regKeys) // stable index across rebuilds
-		g := indexedGame{Name: name, Paths: paths, Registry: regKeys, SteamID: steamID}
+		g := indexedGame{Name: name, Paths: paths, Registry: regKeys, SteamID: steamID, Config: config}
 		for dir := range mg.InstallDir {
 			g.Installs = append(g.Installs, dir)
 		}
@@ -799,6 +808,34 @@ func registryEntryIsSave(key string, entry manifestFileEntry) bool {
 }
 
 func entryIsSaveEntry(tpl string, entry manifestFileEntry) bool {
+	if len(entry.Tags) > 0 && !hasTag(entry, "save") {
+		return false
+	}
+	return entryResolvable(tpl, entry)
+}
+
+// entryIsConfigEntry keeps manifest file entries that are a game's settings
+// and nothing else: tagged "config" and not "save". An entry carrying both
+// is part of the save, whatever else it holds.
+func entryIsConfigEntry(tpl string, entry manifestFileEntry) bool {
+	if !hasTag(entry, "config") || hasTag(entry, "save") {
+		return false
+	}
+	return entryResolvable(tpl, entry)
+}
+
+func hasTag(entry manifestFileEntry, tag string) bool {
+	for _, t := range entry.Tags {
+		if t == tag {
+			return true
+		}
+	}
+	return false
+}
+
+// entryResolvable reports whether a file template can plausibly resolve on
+// a platform this app runs on.
+func entryResolvable(tpl string, entry manifestFileEntry) bool {
 	// Placeholders no supported platform can resolve.
 	if strings.Contains(tpl, "<winDir>") || strings.Contains(tpl, "<dataDrive>") {
 		return false
@@ -812,19 +849,6 @@ func entryIsSaveEntry(tpl string, entry manifestFileEntry) bool {
 		return false
 	}
 
-	if len(entry.Tags) > 0 {
-		hasSave := false
-		for _, t := range entry.Tags {
-			if t == "save" {
-				hasSave = true
-				break
-			}
-		}
-		if !hasSave {
-			return false
-		}
-	}
-
 	if len(entry.When) == 0 {
 		return true
 	}
diff --git a/internal/sessions/procs_windows.go b/internal/sessions/procs_windows.go
index 1fb4ceb8..73450d37 100644
--- a/internal/sessions/procs_windows.go
+++ b/internal/sessions/procs_windows.go
@@ -3,14 +3,18 @@
 package sessions
 
 import (
+	"time"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
 )
 
 // listProcesses walks the process table and asks each process for its full
-// program path. A process this user may not look at (a service, another
-// user's) is skipped: no game of theirs is one of ours.
+// program path, how much memory it holds and how much processor time it has
+// used. A process whose path cannot be read is listed by its file name alone
+// (Proc.Name): a game protected against tampering keeps its path from
+// everyone, and matching by name within its install folder's programs is
+// still exact enough.
 func listProcesses() ([]Proc, error) {
 	snap, err := windows.CreateToolhelp32Snapshot(windows.TH32CS_SNAPPROCESS, 0)
 	if err != nil {
@@ -27,8 +31,12 @@ func listProcesses() ([]Proc, error) {
 	buf := make([]uint16, windows.MAX_LONG_PATH)
 	for {
 		if pid := entry.ProcessID; pid > 4 {
-			if exe := imagePath(pid, buf); exe != "" {
-				out = append(out, Proc{PID: int(pid), Exe: exe})
+			if p, ok := inspect(pid, buf); ok {
+				out = append(out, p)
+			} else if name := windows.UTF16ToString(entry.ExeFile[:]); name != "" {
+				// Its path is not ours to read: a game protected against
+				// tampering, run elevated, or another user's. Known by name.
+				out = append(out, Proc{PID: int(pid), Name: name})
 			}
 		}
 		if err := windows.Process32Next(snap, &entry); err != nil {
@@ -38,15 +46,46 @@ func listProcesses() ([]Proc, error) {
 	return out, nil
 }
 
-func imagePath(pid uint32, buf []uint16) string {
+var procGetProcessMemoryInfo = windows.NewLazySystemDLL("kernel32.dll").NewProc("K32GetProcessMemoryInfo")
+
+// processMemoryCounters is PROCESS_MEMORY_COUNTERS.
+type processMemoryCounters struct {
+	cb                         uint32
+	PageFaultCount             uint32
+	PeakWorkingSetSize         uintptr
+	WorkingSetSize             uintptr
+	QuotaPeakPagedPoolUsage    uintptr
+	QuotaPagedPoolUsage        uintptr
+	QuotaPeakNonPagedPoolUsage uintptr
+	QuotaNonPagedPoolUsage     uintptr
+	PagefileUsage              uintptr
+	PeakPagefileUsage          uintptr
+}
+
+func inspect(pid uint32, buf []uint16) (Proc, bool) {
 	h, err := windows.OpenProcess(windows.PROCESS_QUERY_LIMITED_INFORMATION, false, pid)
 	if err != nil {
-		return ""
+		return Proc{}, false
 	}
 	defer windows.CloseHandle(h)
 	n := uint32(len(buf))
 	if err := windows.QueryFullProcessImageName(h, 0, &buf[0], &n); err != nil {
-		return ""
+		return Proc{}, false
+	}
+	p := Proc{PID: int(pid), Exe: windows.UTF16ToString(buf[:n])}
+
+	var creation, exit, kernel, user windows.Filetime
+	timesOK := windows.GetProcessTimes(h, &creation, &exit, &kernel, &user) == nil
+	var mem processMemoryCounters
+	mem.cb = uint32(unsafe.Sizeof(mem))
+	r, _, _ := procGetProcessMemoryInfo.Call(uintptr(h), uintptr(unsafe.Pointer(&mem)), uintptr(mem.cb))
+	if timesOK && r != 0 {
+		p.Measured = true
+		p.Memory = uint64(mem.WorkingSetSize)
+		// FILETIME counts 100-nanosecond intervals.
+		ticks := (uint64(kernel.HighDateTime)<<32 | uint64(kernel.LowDateTime)) +
+			(uint64(user.HighDateTime)<<32 | uint64(user.LowDateTime))
+		p.CPU = time.Duration(ticks * 100)
 	}
-	return windows.UTF16ToString(buf[:n])
+	return p, true
 }
diff --git a/internal/sessions/sessions.go b/internal/sessions/sessions.go
index 1f47e5c3..c3c3ceac 100644
--- a/internal/sessions/sessions.go
+++ b/internal/sessions/sessions.go
@@ -25,6 +25,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode"
 )
 
 // Proc is a running process, as much of it as matching needs.
@@ -32,8 +33,20 @@ type Proc struct {
 	PID int
 	// Exe is the program's full path, where the system says.
 	Exe string
+	// Name is the program's file name, where the system gives no path: a
+	// game protected against tampering lets nobody read where it runs from,
+	// but the process list still says what it is called.
+	Name string
 	// Args is the command line, where the system says (Linux).
 	Args []string
+	// Measured says Memory and CPU were read: the program's working set in
+	// bytes, and the processor time it has used so far. A game being played
+	// holds hundreds of megabytes and keeps the processor busy; what a game
+	// leaves behind when it closes — a crash handler, a stub of the same name
+	// — holds next to nothing and does nothing (Idle).
+	Measured bool
+	Memory   uint64
+	CPU      time.Duration
 	// SteamAppID is the Steam app the process was started for, where Steam
 	// tagged it (Linux: SteamAppId in the environment, or AppId=… on the
 	// command line of Steam's launcher).
@@ -49,6 +62,14 @@ type Target struct {
 	// Dirs are the game's install folders: anything running from inside one
 	// is the game.
 	Dirs []string
+	// FolderNames are names the game's install folder goes by, for a game
+	// installed where no library lists it ("G:GamesCrimson Desert"):
+	// a program running from inside a folder of one of these names is the
+	// game. Normalised with FolderName.
+	FolderNames []string
+	// Programs are the file names of the programs in the install folders, for
+	// a process known only by name (Proc.Name). Lower-case.
+	Programs []string
 }
 
 // Running reports which targets have a process running, from a list of
@@ -74,6 +95,20 @@ func matches(p Proc, t Target) bool {
 	if p.Exe != "" && t.Exe != "" && samePath(p.Exe, t.Exe) {
 		return true
 	}
+	if p.Exe == "" && p.Name != "" {
+		name := strings.ToLower(p.Name)
+		if t.Exe != "" && strings.EqualFold(filepath.Base(t.Exe), name) {
+			return true
+		}
+		for _, prog := range t.Programs {
+			if prog == name {
+				return true
+			}
+		}
+	}
+	if p.Exe != "" && FolderOf(p, t) != "" {
+		return true
+	}
 	for _, dir := range t.Dirs {
 		if dir == "" {
 			continue
@@ -252,3 +287,53 @@ func (t *Tracker) Finish(gameID string, at time.Time) {
 // List returns the processes running now. Where the system cannot say, it
 // returns none and an error, and nothing is ever seen playing.
 func List() ([]Proc, error) { return listProcesses() }
+
+// FolderName normalises a folder or game name for comparing the two: lower
+// case, letters, digits and single spaces.
+func FolderName(name string) string {
+	var b strings.Builder
+	for _, r := range strings.ToLower(name) {
+		switch {
+		case unicode.IsLetter(r) || unicode.IsDigit(r):
+			b.WriteRune(r)
+		default:
+			b.WriteRune(' ')
+		}
+	}
+	return strings.Join(strings.Fields(b.String()), " ")
+}
+
+// FolderOf returns the folder, among those above a process's program, that
+// is named as one of a target's install folders (Target.FolderNames), or "".
+func FolderOf(p Proc, t Target) string {
+	if p.Exe == "" || len(t.FolderNames) == 0 {
+		return ""
+	}
+	dir := filepath.Dir(p.Exe)
+	for dir != "" && dir != filepath.Dir(dir) {
+		name := FolderName(filepath.Base(dir))
+		for _, want := range t.FolderNames {
+			if name == want {
+				return dir
+			}
+		}
+		dir = filepath.Dir(dir)
+	}
+	return ""
+}
+
+// Idle reports a process that holds next to no memory and has used no
+// processor time since it was last seen (prevCPU, when it was): not a game
+// being played, whatever it is called. A process that could not be measured
+// is never called idle.
+func Idle(p Proc, prevCPU time.Duration, seenBefore bool) bool {
+	if !p.Measured || p.Memory >= idleMemory {
+		return false
+	}
+	return !seenBefore || p.CPU-prevCPU < idleCPU
+}
+
+const (
+	idleMemory = 32 << 20               // a game holds far more
+	idleCPU    = 100 * time.Millisecond // per poll: 1% of ten seconds
+)
diff --git a/internal/sessions/sessions_test.go b/internal/sessions/sessions_test.go
index c4311de4..87131882 100644
--- a/internal/sessions/sessions_test.go
+++ b/internal/sessions/sessions_test.go
@@ -105,3 +105,67 @@ func TestListFindsThisProcess(t *testing.T) {
 		t.Errorf("this process (%d, %s) was not found among %d: %v", os.Getpid(), self, len(procs), running)
 	}
 }
+
+// A game protected against tampering keeps its path from everyone; its
+// process is still recognised by name, among its install folder's programs.
+func TestRunning_ProtectedProcessByName(t *testing.T) {
+	target := Target{GameID: "cd", Dirs: []string{`C:\Games\Crimson Desert`}, Programs: []string{"crimsondesert.exe"}}
+	procs := []Proc{{PID: 11, Name: "explorer.exe"}, {PID: 12, Name: "CrimsonDesert.exe"}}
+	if got := Running(procs, []Target{target}); got["cd"] != 12 {
+		t.Errorf("Running = %v, want the protected process recognised by name", got)
+	}
+	if got := Running([]Proc{{PID: 13, Name: "game.exe"}}, []Target{target}); len(got) != 0 {
+		t.Errorf("a program not in the game's folder was taken for it: %v", got)
+	}
+}
+
+// A game installed in a folder no library lists is found by the folder's
+// name, wherever that folder is.
+func TestRunning_ByInstallFolderName(t *testing.T) {
+	// Paths as this system writes them: a process's program is a native path.
+	root := string(filepath.Separator)
+	if runtime.GOOS == "windows" {
+		root = `G:\`
+	}
+	games := filepath.Join(root, "Games")
+	target := Target{GameID: "cd", FolderNames: []string{FolderName("Crimson Desert")}}
+	procs := []Proc{
+		{PID: 5, Exe: filepath.Join(root, "Windows", "explorer.exe")},
+		{PID: 6, Exe: filepath.Join(games, "Crimson Desert", "bin64", "CrimsonDesert.exe")},
+	}
+	if got := Running(procs, []Target{target}); got["cd"] != 6 {
+		t.Fatalf("Running = %v, want the game found by its folder", got)
+	}
+	if dir := FolderOf(procs[1], target); dir != filepath.Join(games, "Crimson Desert") {
+		t.Errorf("FolderOf = %q", dir)
+	}
+	other := []Proc{{PID: 7, Exe: filepath.Join(games, "Crimson Desert Tools", "editor.exe")}}
+	if got := Running(other, []Target{target}); len(got) != 0 {
+		t.Errorf("a folder whose name only begins the same was taken for the game: %v", got)
+	}
+}
+
+// What a game leaves behind when it closes is not the game being played: next
+// to no memory, no processor time since it was last seen.
+func TestIdle(t *testing.T) {
+	stub := Proc{PID: 1, Exe: `G:\Games\Crimson Desert\bin64\CrimsonDesert.exe`, Measured: true, Memory: 900 << 10, CPU: 2 * time.Second}
+	if !Idle(stub, 2*time.Second, true) {
+		t.Error("a lingering stub counts as the game")
+	}
+	if !Idle(stub, 0, false) {
+		t.Error("a stub seen for the first time counts as the game")
+	}
+	busy := stub
+	busy.CPU = 3 * time.Second
+	if Idle(busy, 2*time.Second, true) {
+		t.Error("a small process using the processor is called idle")
+	}
+	game := stub
+	game.Memory = 3 << 30
+	if Idle(game, 2*time.Second, true) {
+		t.Error("a game holding gigabytes is called idle while paused")
+	}
+	if Idle(Proc{PID: 2, Name: "CrimsonDesert.exe"}, 0, false) {
+		t.Error("a process that could not be measured is called idle")
+	}
+}
diff --git a/internal/snapshot/content.go b/internal/snapshot/content.go
new file mode 100644
index 00000000..ccb7fb73
--- /dev/null
+++ b/internal/snapshot/content.go
@@ -0,0 +1,397 @@
+package snapshot
+
+import (
+	"archive/zip"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+
+	"github.com/opensave/opensave/internal/delta"
+	"github.com/opensave/opensave/internal/store"
+)
+
+// Snapshot content.
+//
+// A snapshot is named by what it holds: every file's name in the archive and
+// the SHA-256 of its content, hashed together. The file hashes are the ones the
+// sync compares (the hash cache both use), so the same save gives the same
+// value on every device — which is how a device knows the snapshot another
+// device took is one it already has, and keeps the save archived once.
+//
+// Before this, a save of a quarter-million files was archived again for every
+// device it was compared with, and again on every new snapshot over there:
+// gigabytes of identical archives for one 780 MB save.
+
+// contentEntry is one file of a snapshot: its name in the archive and its hash.
+type contentEntry struct{ name, hash string }
+
+func contentKeyOf(entries []contentEntry) string {
+	sort.Slice(entries, func(i, j int) bool { return entries[i].name < entries[j].name })
+	h := sha256.New()
+	for _, e := range entries {
+		h.Write([]byte(e.name))
+		h.Write([]byte{0})
+		h.Write([]byte(e.hash))
+		h.Write([]byte{0})
+	}
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// archiveName is the name a captured file has in the archive.
+func archiveName(c store.CapturedFile) string {
+	if c.Root == "" {
+		return c.Path
+	}
+	return RootPrefix + c.Root + "/" + c.Path
+}
+
+// ContentKey names what a snapshot holds, from its recorded files.
+func ContentKey(files []store.CapturedFile) string {
+	entries := make([]contentEntry, 0, len(files))
+	for _, f := range files {
+		entries = append(entries, contentEntry{archiveName(f), f.Hash})
+	}
+	return contentKeyOf(entries)
+}
+
+// ContentKeyOfArchive names what a snapshot holds by reading its archive: for
+// a snapshot recorded without its file list (one taken for a sync with a peer).
+func ContentKeyOfArchive(zipPath string) (string, error) {
+	files, err := ArchiveFiles(zipPath)
+	if err != nil {
+		return "", err
+	}
+	return ContentKey(files), nil
+}
+
+// ArchiveFiles reads a snapshot's archive and lists its files with their
+// content hashes, as the snapshot would have recorded them.
+func ArchiveFiles(zipPath string) ([]store.CapturedFile, error) {
+	path, done, err := OpenArchive(zipPath)
+	if err != nil {
+		return nil, err
+	}
+	defer done()
+	r, err := zip.OpenReader(path)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	out := make([]store.CapturedFile, 0, len(r.File))
+	for _, f := range r.File {
+		if f.FileInfo().IsDir() || strings.HasSuffix(f.Name, "/") || strings.HasSuffix(f.Name, delta.TmpSuffix) {
+			continue
+		}
+		rc, err := f.Open()
+		if err != nil {
+			return nil, fmt.Errorf("%s: %w", f.Name, err)
+		}
+		h := sha256.New()
+		_, err = io.Copy(h, rc)
+		rc.Close()
+		if err != nil {
+			return nil, fmt.Errorf("%s: %w", f.Name, err)
+		}
+		c := store.CapturedFile{Path: f.Name, Hash: hex.EncodeToString(h.Sum(nil))}
+		if rest, ok := strings.CutPrefix(f.Name, RootPrefix); ok {
+			if root, p, ok := strings.Cut(rest, "/"); ok {
+				c.Root, c.Path = root, p
+			}
+		}
+		out = append(out, c)
+	}
+	return out, nil
+}
+
+// filesOf is what a snapshot holds, from its recorded file list — read from
+// its archive once, and recorded, when it has none.
+func (m *Manager) filesOf(snap store.Snapshot) ([]store.CapturedFile, error) {
+	if files, err := m.Store.SnapshotFiles(snap.ID); err == nil && len(files) > 0 {
+		return files, nil
+	}
+	files, err := ArchiveFiles(snap.ZipPath)
+	if err != nil {
+		return nil, err
+	}
+	if len(files) > 0 {
+		_ = m.Store.RecordSnapshotFiles(snap.ID, files)
+	}
+	return files, nil
+}
+
+// contentHashOf works out what a snapshot holds (filesOf).
+func (m *Manager) contentHashOf(snap store.Snapshot) (string, error) {
+	files, err := m.filesOf(snap)
+	if err != nil {
+		return "", err
+	}
+	return ContentKey(files), nil
+}
+
+// PruneContained removes automatic snapshots that a newer snapshot on the same
+// branch holds entirely: every file of the older one is in the newer one, with
+// the same content. Such a snapshot is only a step on the way to the newer
+// one — a pull that stopped part-way, a save that only gained files — and
+// restoring it would give nothing the newer one lacks but the absence of what
+// was added since. The other way round is kept: a newer snapshot contained in
+// an older one is a save something was deleted from, on purpose. Pinned
+// snapshots and ones taken by hand are never removed.
+//
+// The largest snapshots are looked at first as the ones that may hold others —
+// they hold the most — and passes repeat until one removes nothing. After
+// that, each new snapshot is checked against the older ones as it is taken
+// (PruneContainedBy), which is all it takes to stay that way.
+func (m *Manager) PruneContained(gameID string) (removed int, freed int64, err error) {
+	for {
+		n, f, err := m.pruneContainedPass(gameID)
+		removed += n
+		freed += f
+		if err != nil || n == 0 {
+			return removed, freed, err
+		}
+	}
+}
+
+func (m *Manager) pruneContainedPass(gameID string) (removed int, freed int64, err error) {
+	snaps, err := m.Store.AllSnapshotsOfGame(gameID) // oldest first
+	if err != nil {
+		return 0, 0, err
+	}
+	byBranch := map[string][]store.Snapshot{}
+	counts := map[string]int{}
+	for _, s := range snaps {
+		if !ArchiveExists(s.ZipPath) {
+			continue
+		}
+		files, ferr := m.filesOf(s)
+		if ferr != nil {
+			continue
+		}
+		counts[s.ID] = len(files)
+		byBranch[s.BranchName] = append(byBranch[s.BranchName], s)
+	}
+	for _, list := range byBranch {
+		order := make(map[string]int, len(list)) // position in time
+		for i, s := range list {
+			order[s.ID] = i
+		}
+		holders := append([]store.Snapshot(nil), list...)
+		sort.SliceStable(holders, func(i, j int) bool { return counts[holders[i].ID] > counts[holders[j].ID] })
+		gone := map[string]bool{}
+		for _, holder := range holders {
+			if gone[holder.ID] || counts[holder.ID] == 0 {
+				continue
+			}
+			holderFiles, ferr := m.filesOf(holder)
+			if ferr != nil {
+				continue
+			}
+			have := make(map[string]string, len(holderFiles))
+			for _, f := range holderFiles {
+				have[archiveName(f)] = f.Hash
+			}
+			for _, older := range list[:order[holder.ID]] {
+				if gone[older.ID] || older.Pinned || !older.IsSystemAuto || counts[older.ID] == 0 || counts[older.ID] > len(holderFiles) {
+					continue
+				}
+				olderFiles, ferr := m.filesOf(older)
+				if ferr != nil || !containedIn(olderFiles, have) {
+					continue
+				}
+				identical := len(olderFiles) == len(holderFiles)
+				if identical {
+					// The same files: its id still leads to them. Before the
+					// delete, so no moment exists where the id leads nowhere;
+					// while the row is there it still resolves to itself.
+					_ = m.Store.RepointSnapshotAliases(older.ID, holder.ID)
+					_ = m.Store.AddSnapshotAlias(older.ID, holder.ID)
+					m.carryName(older, &holder)
+				}
+				f, derr := m.DeleteSnapshot(gameID, older.ID)
+				if derr != nil {
+					continue
+				}
+				gone[older.ID] = true
+				removed++
+				freed += f
+				if m.Log != nil {
+					m.Log("info", fmt.Sprintf("snapshot %s removed: every file of it is in the newer %s", older.ID, holder.ID))
+				}
+			}
+		}
+	}
+	return removed, freed, nil
+}
+
+func containedIn(files []store.CapturedFile, have map[string]string) bool {
+	for _, f := range files {
+		if have[archiveName(f)] != f.Hash {
+			return false
+		}
+	}
+	return true
+}
+
+// PruneContainedBy checks the older snapshots of snap's branch against snap,
+// just taken, and removes those it holds entirely (PruneContained). Only when
+// PruneOnCreate is set; in the background, counted for shutdown.
+func (m *Manager) PruneContainedBy(snap store.Snapshot, files []store.CapturedFile) {
+	if !m.PruneOnCreate || len(files) == 0 {
+		return
+	}
+	m.inFlight.Add()
+	go func() {
+		defer m.inFlight.Done()
+		have := make(map[string]string, len(files))
+		for _, f := range files {
+			have[archiveName(f)] = f.Hash
+		}
+		snaps, err := m.Store.ListSnapshots(snap.GameID, snap.BranchName)
+		if err != nil {
+			return
+		}
+		for _, older := range snaps {
+			if older.ID == snap.ID || older.Timestamp >= snap.Timestamp || older.Pinned || !older.IsSystemAuto ||
+				!ArchiveExists(older.ZipPath) {
+				continue
+			}
+			olderFiles, ferr := m.filesOf(older)
+			if ferr != nil || len(olderFiles) == 0 || len(olderFiles) > len(files) || !containedIn(olderFiles, have) {
+				continue
+			}
+			identical := len(olderFiles) == len(files)
+			if identical {
+				// Aliased before the delete (see PruneContained).
+				_ = m.Store.RepointSnapshotAliases(older.ID, snap.ID)
+				_ = m.Store.AddSnapshotAlias(older.ID, snap.ID)
+				m.carryName(older, &snap)
+			}
+			_, derr := m.DeleteSnapshot(snap.GameID, older.ID)
+			if derr == nil && m.Log != nil {
+				m.Log("info", fmt.Sprintf("snapshot %s removed: every file of it is in the newer %s", older.ID, snap.ID))
+			}
+		}
+	}()
+}
+
+// ArchiveSaveTo archives a game's save — every location — into zipPath the way
+// a snapshot here is archived: files unchanged since the branch's newest
+// snapshot are copied from it rather than compressed again (reuse.go). For a
+// snapshot recorded under another device's id after a sync; compressing a save
+// of a quarter-million files from scratch kept a finished pull at 100% for
+// minutes.
+func (m *Manager) ArchiveSaveTo(gameID, branch, zipPath string) (skipped []string, captured []store.CapturedFile, err error) {
+	m.inFlight.Add()
+	defer m.inFlight.Done()
+	game, err := m.Store.GetGame(gameID)
+	if err != nil {
+		return nil, nil, err
+	}
+	extra, rootsErr := m.Store.GameRootPaths(gameID)
+	if rootsErr != nil {
+		extra = nil
+	}
+	reuse := m.openReuseSource(gameID, branch)
+	defer reuse.Close()
+	return zipRootsCapturing(game.SavePath, extra, zipPath, reuse)
+}
+
+// isNewest reports whether id is the newest snapshot on a game's branch.
+func (m *Manager) isNewest(gameID, branch, id string) bool {
+	snaps, err := m.Store.ListSnapshots(gameID, branch)
+	return err == nil && len(snaps) > 0 && snaps[0].ID == id
+}
+
+// MergeDuplicates names every snapshot of a game by its content and keeps each
+// content archived once per branch: of snapshots holding the same files, one
+// keeps its archive — a pinned one, else one taken by hand, else the newest,
+// which retention keeps longest —
+// and the others are removed, their ids kept as aliases of it so a device that
+// knows a snapshot by one of them still finds it. Pinned snapshots are never
+// removed. Returns how many were merged and the bytes freed.
+func (m *Manager) MergeDuplicates(gameID string) (merged int, freed int64, err error) {
+	missing, err := m.Store.SnapshotsWithoutContentHash(gameID)
+	if err != nil {
+		return 0, 0, err
+	}
+	for _, snap := range missing {
+		if !ArchiveExists(snap.ZipPath) {
+			continue
+		}
+		key, err := m.contentHashOf(snap)
+		if err != nil {
+			if m.Log != nil {
+				m.Log("warn", fmt.Sprintf("could not read snapshot %s to name its content: %v", snap.ID, err))
+			}
+			continue
+		}
+		_ = m.Store.SetSnapshotContentHash(snap.ID, key)
+	}
+
+	snaps, err := m.Store.AllSnapshotsOfGame(gameID)
+	if err != nil {
+		return 0, 0, err
+	}
+	groups := map[string][]store.Snapshot{}
+	for _, s := range snaps {
+		if s.ContentHash == "" {
+			continue
+		}
+		k := s.BranchName + "\x00" + s.ContentHash
+		groups[k] = append(groups[k], s)
+	}
+	for _, group := range groups {
+		if len(group) < 2 {
+			continue
+		}
+		keep := group[len(group)-1] // newest (oldest first)
+		for _, s := range group {
+			if s.Pinned && !keep.Pinned || s.Pinned == keep.Pinned && !s.IsSystemAuto && keep.IsSystemAuto {
+				keep = s
+			}
+		}
+		for _, s := range group {
+			if s.ID == keep.ID || s.Pinned {
+				continue
+			}
+			_ = m.Store.RepointSnapshotAliases(s.ID, keep.ID)
+			_ = m.Store.AddSnapshotAlias(s.ID, keep.ID)
+			f, err := m.DeleteSnapshot(gameID, s.ID)
+			if err != nil {
+				continue
+			}
+			merged++
+			freed += f
+			if m.Log != nil {
+				m.Log("info", fmt.Sprintf("snapshot %s held the same files as %s; kept once", s.ID, keep.ID))
+			}
+		}
+	}
+	return merged, freed, nil
+}
+
+// defaultAutoComment is what an automatic snapshot taken for no particular
+// reason is called.
+const defaultAutoComment = "Auto backup"
+
+// carryName keeps what an older snapshot is called when a newer one holding
+// exactly the same files takes its place: its reason ("After playing (1 h)",
+// "Put back") if the newer one has only the default an automatic snapshot
+// gets, and a note someone wrote if the newer one has none. A session's
+// named snapshot and the watcher's copy of the same save, taken a moment
+// later as the game's last write settled, otherwise left only "Auto backup".
+func (m *Manager) carryName(older store.Snapshot, newer *store.Snapshot) {
+	if newer.Comment == defaultAutoComment && older.Comment != "" && older.Comment != defaultAutoComment {
+		if m.Store.SetSnapshotComment(newer.ID, older.Comment) == nil {
+			newer.Comment = older.Comment
+		}
+	}
+	if newer.Note == "" && older.Note != "" {
+		if m.Store.SetSnapshotNote(newer.ID, older.Note) == nil {
+			newer.Note = older.Note
+		}
+	}
+}
diff --git a/internal/snapshot/content_test.go b/internal/snapshot/content_test.go
new file mode 100644
index 00000000..ab4db8ee
--- /dev/null
+++ b/internal/snapshot/content_test.go
@@ -0,0 +1,259 @@
+package snapshot
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+// The content a snapshot records and the content read back from its archive
+// are named alike — which is what lets a device that only has another's
+// archive, or only its file list, tell they hold the same save.
+func TestContentKey_FromFilesAndFromArchiveAgree(t *testing.T) {
+	env := setup(t)
+	writeSave(t, env.saveDir, "slot1.sav", "one")
+	writeSave(t, env.saveDir, "sub/slot2.sav", "two")
+	snap, err := env.mgr.Create("game1", "", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	files, err := env.store.SnapshotFiles(snap.ID)
+	if err != nil || len(files) == 0 {
+		t.Fatalf("no file list recorded: %v", err)
+	}
+	fromArchive, err := ContentKeyOfArchive(snap.ZipPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := ContentKey(files); got != fromArchive {
+		t.Errorf("from the file list %s, from the archive %s", got, fromArchive)
+	}
+	stored, _ := env.store.GetSnapshot(snap.ID)
+	if stored.ContentHash != fromArchive {
+		t.Errorf("recorded content %q, want %q", stored.ContentHash, fromArchive)
+	}
+}
+
+// Steps on the way to a newer snapshot — every file of them in it, unchanged —
+// go. A newer snapshot that lacks files (something was deleted) does not make
+// the older one redundant; pinned and hand-taken snapshots are never removed.
+func TestPruneContained(t *testing.T) {
+	env := setup(t)
+	game, err := env.store.GetGame("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	game.MaxSnapshots = 50
+	if err := env.store.UpdateGame(game); err != nil {
+		t.Fatal(err)
+	}
+	snap := func(auto bool) string {
+		t.Helper()
+		s, err := env.mgr.Create("game1", "", auto)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return s.ID
+	}
+	writeSave(t, env.saveDir, "a.bin", "a")
+	step1 := snap(true)
+	writeSave(t, env.saveDir, "b.bin", "b")
+	step2 := snap(true)
+	pinnedStep := snap(false) // same files as step2, by hand, then pinned below
+	_ = env.store.SetSnapshotPinned(pinnedStep, true)
+	writeSave(t, env.saveDir, "c.bin", "c")
+	full := snap(true)
+	// Then a file is deleted on purpose.
+	if err := os.Remove(filepath.Join(env.saveDir, "c.bin")); err != nil {
+		t.Fatal(err)
+	}
+	writeSave(t, env.saveDir, "a.bin", "a, played on")
+	after := snap(true)
+
+	removed, _, err := env.mgr.PruneContained("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	gone := func(id string) bool { _, err := env.store.GetSnapshot(id); return err != nil }
+	if !gone(step1) || !gone(step2) {
+		t.Errorf("steps on the way to a newer snapshot kept: step1 gone=%v step2 gone=%v", gone(step1), gone(step2))
+	}
+	if gone(pinnedStep) {
+		t.Error("a pinned snapshot was removed")
+	}
+	if gone(full) {
+		t.Error("removed a snapshot holding a file deleted later on purpose")
+	}
+	if gone(after) {
+		t.Error("removed the newest snapshot")
+	}
+	if removed != 2 {
+		t.Errorf("removed %d, want 2", removed)
+	}
+}
+
+// With PruneOnCreate, a new snapshot takes the place of older automatic ones it
+// holds entirely, as it is taken; one that lacks a file of them does not.
+func TestPruneContainedBy_ANewSnapshotReplacesItsSteps(t *testing.T) {
+	env := setup(t)
+	env.mgr.PruneOnCreate = true
+	game, _ := env.store.GetGame("game1")
+	game.MaxSnapshots = 50
+	_ = env.store.UpdateGame(game)
+
+	writeSave(t, env.saveDir, "a.bin", "a")
+	first, err := env.mgr.Create("game1", "", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	writeSave(t, env.saveDir, "b.bin", "b")
+	if _, err := env.mgr.Create("game1", "", true); err != nil {
+		t.Fatal(err)
+	}
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		if _, err := env.store.GetSnapshot(first.ID); err != nil {
+			break
+		}
+		time.Sleep(50 * time.Millisecond)
+	}
+	if _, err := env.store.GetSnapshot(first.ID); err == nil {
+		t.Fatal("the step the new snapshot holds entirely was kept")
+	}
+
+	// Now a file goes: the newer snapshot does not hold the older one.
+	before, _ := env.store.ListSnapshots("game1", "main")
+	if err := os.Remove(filepath.Join(env.saveDir, "b.bin")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := env.mgr.Create("game1", "", true); err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(500 * time.Millisecond)
+	if _, err := env.store.GetSnapshot(before[0].ID); err != nil {
+		t.Error("removed the snapshot holding a file deleted afterwards")
+	}
+}
+
+// A copy taken before a sync replaces a save already archived is that archive,
+// not a second one.
+func TestBeforeReplacing_AnArchivedSaveIsNotArchivedAgain(t *testing.T) {
+	env := setup(t)
+	writeSave(t, env.saveDir, "slot1.sav", "progress")
+	first, err := env.mgr.Create("game1", "", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	copy, err := env.mgr.CreateBeforeReplacing("game1", "before sync")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if copy.ID != first.ID {
+		t.Errorf("a second archive %s of the save %s already holds", copy.ID, first.ID)
+	}
+	snaps, _ := env.store.ListSnapshots("game1", "main")
+	if len(snaps) != 1 {
+		t.Errorf("%d snapshots, want 1", len(snaps))
+	}
+}
+
+// Several snapshots of the same files — one per device the save was compared
+// with — are kept once: the newest unless one is pinned or taken by hand;
+// pinned ones are never removed; the others' ids still lead to it.
+func TestMergeDuplicates(t *testing.T) {
+	env := setup(t)
+	// Room for every snapshot below: this is about merging, not retention.
+	game, err := env.store.GetGame("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	game.MaxSnapshots = 50
+	if err := env.store.UpdateGame(game); err != nil {
+		t.Fatal(err)
+	}
+	writeSave(t, env.saveDir, "slot1.sav", "the save")
+	var same []string
+	for i := 0; i < 3; i++ {
+		s, err := env.mgr.Create("game1", "", true)
+		if err != nil {
+			t.Fatal(err)
+		}
+		same = append(same, s.ID)
+	}
+	pinned, err := env.mgr.Create("game1", "pinned copy", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := env.store.SetSnapshotPinned(pinned.ID, true); err != nil {
+		t.Fatal(err)
+	}
+	writeSave(t, env.saveDir, "slot1.sav", "played on")
+	other, err := env.mgr.Create("game1", "", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// As if taken before contents were recorded.
+	for _, id := range append(same, pinned.ID, other.ID) {
+		_ = env.store.SetSnapshotContentHash(id, "")
+	}
+
+	merged, _, err := env.mgr.MergeDuplicates("game1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if merged != 3 {
+		t.Errorf("merged %d, want the 3 unpinned copies of the pinned one", merged)
+	}
+	if _, err := env.store.GetSnapshot(pinned.ID); err != nil {
+		t.Error("the pinned snapshot was removed")
+	}
+	if _, err := env.store.GetSnapshot(other.ID); err != nil {
+		t.Error("a snapshot of different content was removed")
+	}
+	for _, id := range same {
+		if got, ok := env.store.ResolveSnapshotID(id); !ok || got != pinned.ID {
+			t.Errorf("%s leads to %q (%v), want the snapshot kept, %s", id, got, ok, pinned.ID)
+		}
+	}
+	// Run again: nothing more to do.
+	if again, _, _ := env.mgr.MergeDuplicates("game1"); again != 0 {
+		t.Errorf("a second pass merged %d more", again)
+	}
+}
+
+// A session's named snapshot, then the watcher's copy of the same save a
+// moment later: the copy takes its place and keeps its name.
+func TestPruneContainedBy_AnIdenticalCopyKeepsTheName(t *testing.T) {
+	env := setup(t)
+	env.mgr.PruneOnCreate = true
+	writeSave(t, env.saveDir, "slot1.sav", "after an evening")
+	named, err := env.mgr.Create("game1", "After playing (1 h)", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := env.store.SetSnapshotNote(named.ID, "boss beaten"); err != nil {
+		t.Fatal(err)
+	}
+	copied, err := env.mgr.Create("game1", "", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if copied.ID == named.ID {
+		t.Skip("the copy was not taken: nothing to replace")
+	}
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		if _, err := env.store.GetSnapshot(named.ID); err != nil {
+			break
+		}
+		time.Sleep(50 * time.Millisecond)
+	}
+	kept, err := env.store.GetSnapshot(copied.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if kept.Comment != "After playing (1 h)" || kept.Note != "boss beaten" {
+		t.Errorf("the snapshot kept is called %q (note %q), want the name and note of the one it replaced", kept.Comment, kept.Note)
+	}
+}
diff --git a/internal/snapshot/history.go b/internal/snapshot/history.go
new file mode 100644
index 00000000..b6ebe914
--- /dev/null
+++ b/internal/snapshot/history.go
@@ -0,0 +1,103 @@
+package snapshot
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"sort"
+)
+
+// How often files changed along with a game's save, read from its
+// snapshots: a settings file is changed now and then, when someone changes a
+// setting; a save changes with nearly every session. A file taken for a
+// game's settings that changes like a save is not its settings
+// (daemon/detectsettings.go).
+
+const (
+	historySnapshots = 40 // the most recent ones are read
+	historyMinSaves  = 4  // changes of the save needed before anything is said
+)
+
+// ChangesLikeASave returns, for each of paths (relative to the main save
+// folder) that changed in more than half of the snapshots in which the rest
+// of the save changed, why it is judged a save. Paths with too little history
+// to tell are not in it.
+func (m *Manager) ChangesLikeASave(gameID string, paths []string) map[string]string {
+	out := map[string]string{}
+	if len(paths) == 0 {
+		return out
+	}
+	watched := make(map[string]bool, len(paths))
+	for _, p := range paths {
+		watched[p] = true
+	}
+	snaps, err := m.Store.AllSnapshotsOfGame(gameID)
+	if err != nil {
+		return out
+	}
+	if len(snaps) > historySnapshots {
+		snaps = snaps[len(snaps)-historySnapshots:]
+	}
+
+	// Per snapshot, the hashes of the watched files and one value for the
+	// rest of the save.
+	type state struct {
+		watched map[string]string
+		rest    string
+	}
+	var states []state
+	for _, s := range snaps {
+		files, err := m.filesOf(s)
+		if err != nil || len(files) == 0 {
+			continue
+		}
+		st := state{watched: map[string]string{}}
+		var rest []string
+		for _, f := range files {
+			if f.Root != "" {
+				continue
+			}
+			if watched[f.Path] {
+				st.watched[f.Path] = f.Hash
+			} else {
+				rest = append(rest, f.Path+"\x00"+f.Hash)
+			}
+		}
+		st.rest = restKey(rest)
+		states = append(states, st)
+	}
+
+	saves := 0
+	moved := map[string]int{}
+	for i := 1; i < len(states); i++ {
+		a, b := states[i-1], states[i]
+		if a.rest == b.rest {
+			continue // the save did not change: nothing to compare with
+		}
+		saves++
+		for p := range watched {
+			if a.watched[p] != b.watched[p] {
+				moved[p]++
+			}
+		}
+	}
+	if saves < historyMinSaves {
+		return out
+	}
+	for p, n := range moved {
+		if 2*n > saves {
+			out[p] = fmt.Sprintf("changed in %d of the %d snapshots in which the save changed", n, saves)
+		}
+	}
+	return out
+}
+
+func restKey(entries []string) string {
+	sort.Strings(entries)
+	h := sha256.New()
+	for _, e := range entries {
+		h.Write([]byte(e))
+		h.Write([]byte{0})
+	}
+	return hex.EncodeToString(h.Sum(nil))
+}
diff --git a/internal/snapshot/history_test.go b/internal/snapshot/history_test.go
new file mode 100644
index 00000000..1c7ea812
--- /dev/null
+++ b/internal/snapshot/history_test.go
@@ -0,0 +1,52 @@
+package snapshot
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// A file that changes with every change of the save is a save; one changed
+// once among many is settings; too little history says nothing.
+func TestChangesLikeASave(t *testing.T) {
+	env := setup(t)
+	write := func(name, data string) {
+		if err := os.WriteFile(filepath.Join(env.saveDir, name), []byte(data), 0o666); err != nil {
+			t.Fatal(err)
+		}
+	}
+	snap := func() {
+		if _, err := env.mgr.Create("game1", "", false); err != nil {
+			t.Fatal(err)
+		}
+	}
+	write("graphics.cfg", "resolution=1080")
+	write("progress.cfg", "level=0")
+	write("slot1.sav", "0")
+	snap()
+	ask := []string{"graphics.cfg", "progress.cfg"}
+	for i := 1; i <= 3; i++ {
+		write("slot1.sav", strings.Repeat("x", i))
+		write("progress.cfg", "level="+strings.Repeat("1", i))
+		snap()
+	}
+	if got := env.mgr.ChangesLikeASave("game1", ask); len(got) != 0 {
+		t.Fatalf("3 changes of the save already judged: %v", got)
+	}
+	for i := 4; i <= 6; i++ {
+		write("slot1.sav", strings.Repeat("x", i))
+		write("progress.cfg", "level="+strings.Repeat("1", i))
+		if i == 5 {
+			write("graphics.cfg", "resolution=2160")
+		}
+		snap()
+	}
+	got := env.mgr.ChangesLikeASave("game1", ask)
+	if _, ok := got["progress.cfg"]; !ok {
+		t.Errorf("a file that changed with every save is not judged a save: %v", got)
+	}
+	if _, ok := got["graphics.cfg"]; ok {
+		t.Errorf("a file changed once in six saves is judged a save: %v", got)
+	}
+}
diff --git a/internal/snapshot/reuse.go b/internal/snapshot/reuse.go
new file mode 100644
index 00000000..90da079e
--- /dev/null
+++ b/internal/snapshot/reuse.go
@@ -0,0 +1,124 @@
+package snapshot
+
+import (
+	"archive/zip"
+	"os"
+	"sync/atomic"
+
+	"github.com/opensave/opensave/internal/delta"
+)
+
+// Snapshots that re-pack only what changed.
+//
+// Every snapshot is a complete zip, restorable on its own — that stays. What
+// changes is how it is written: a file whose content is exactly what the
+// previous snapshot of the same branch holds is copied into the new archive
+// as its already-compressed entry (zip.Writer.Copy), not read and deflated
+// again. Only new and changed files are compressed.
+//
+// For a save of a quarter-million small files (Project Zomboid map chunks)
+// where a play session touches a few hundred, that turns a snapshot from a
+// minute of reading and deflating 0.76GB into writing mostly copied bytes.
+//
+// Content is identified by SHA-256 — the hash each snapshot already records
+// per file (store.SnapshotFiles) and the one the manifest's hash cache holds,
+// so an unchanged file is recognised without being read. Size is checked as
+// well, cheaply, before the hash is asked for.
+type reuseSource struct {
+	zr     *zip.ReadCloser
+	files  map[string]*zip.File // entry name -> previous entry
+	hashes map[string]string    // entry name -> sha256 recorded for it
+	reused int                  // entries copied, for tests
+}
+
+// lastReused is how many entries the most recent snapshot copied from its
+// predecessor. Tests read it; nothing else does.
+var lastReused atomic.Int64
+
+// openReuseSource prepares the newest snapshot of a branch for reuse. Any
+// problem — no snapshot, no recorded file list (an older snapshot, or a
+// mirror), a zip that does not open — means no reuse, never a failed snapshot.
+func (m *Manager) openReuseSource(gameID, branch string) *reuseSource {
+	prev, err := m.LatestSnapshot(gameID, branch)
+	if err != nil || prev.ZipPath == "" {
+		return nil
+	}
+	recorded, err := m.Store.SnapshotFiles(prev.ID)
+	if err != nil || len(recorded) == 0 {
+		return nil
+	}
+	zr, err := zip.OpenReader(prev.ZipPath)
+	if err != nil {
+		return nil
+	}
+	r := &reuseSource{
+		zr:     zr,
+		files:  make(map[string]*zip.File, len(zr.File)),
+		hashes: make(map[string]string, len(recorded)),
+	}
+	for _, f := range zr.File {
+		r.files[f.Name] = f
+	}
+	for _, c := range recorded {
+		name := c.Path
+		if c.Root != "" {
+			name = RootPrefix + c.Root + "/" + c.Path
+		}
+		r.hashes[name] = c.Hash
+	}
+	return r
+}
+
+func (r *reuseSource) Close() {
+	if r == nil {
+		lastReused.Store(0)
+		return
+	}
+	lastReused.Store(int64(r.reused))
+	if r.zr != nil {
+		r.zr.Close()
+	}
+}
+
+// addFileEntry adds filePath under entryName, copying the previous
+// snapshot's entry when the content is the same, and archiving it afresh
+// otherwise. Returns the file's sha256 either way. A nil receiver archives
+// afresh, as every snapshot did before.
+func (r *reuseSource) addFileEntry(w *zip.Writer, filePath, entryName string, info os.FileInfo) (string, error) {
+	if r != nil {
+		if hash, ok := r.reusable(filePath, entryName, info); ok {
+			if err := w.Copy(r.files[entryName]); err != nil {
+				return "", err
+			}
+			r.reused++
+			return hash, nil
+		}
+	}
+	return addFileEntry(w, filePath, entryName)
+}
+
+// reusable reports whether the previous snapshot holds exactly this file's
+// current content under the same name.
+func (r *reuseSource) reusable(filePath, entryName string, info os.FileInfo) (string, bool) {
+	prevHash, ok := r.hashes[entryName]
+	prev := r.files[entryName]
+	if !ok || prev == nil || prev.Mode().IsDir() {
+		return "", false
+	}
+	if info == nil {
+		var err error
+		if info, err = os.Stat(filePath); err != nil {
+			return "", false
+		}
+	}
+	if uint64(info.Size()) != prev.UncompressedSize64 {
+		return "", false
+	}
+	// From the hash cache when the file is unchanged since it was last
+	// hashed, which after the watcher's manifest build it nearly always is.
+	entry, err := delta.FileEntryForInfo(filePath, info)
+	if err != nil || entry.Hash != prevHash {
+		return "", false
+	}
+	return prevHash, true
+}
diff --git a/internal/snapshot/reuse_bench_test.go b/internal/snapshot/reuse_bench_test.go
new file mode 100644
index 00000000..641a0335
--- /dev/null
+++ b/internal/snapshot/reuse_bench_test.go
@@ -0,0 +1,61 @@
+package snapshot
+
+import (
+	"archive/zip"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/internal/delta"
+)
+
+// TestBench_SnapshotReuse times a full snapshot of a real save folder and a
+// second one that reuses it. Opt-in: OPENSAVE_BENCH_SAVE=<folder>.
+// Reads the folder only; archives go to a temp dir.
+func TestBench_SnapshotReuse(t *testing.T) {
+	src := os.Getenv("OPENSAVE_BENCH_SAVE")
+	if src == "" {
+		t.Skip("set OPENSAVE_BENCH_SAVE to run")
+	}
+	dir := t.TempDir()
+
+	first := filepath.Join(dir, "first.zip")
+	start := time.Now()
+	skipped, captured, err := zipRootsCapturing(src, nil, first, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	full := time.Since(start)
+	t.Logf("full snapshot skipped %d unreadable files", len(skipped))
+
+	// As in the app: the watcher's manifest build has hashed the save just
+	// before a snapshot is taken, so the cache is warm.
+	if _, err := delta.BuildManifest(src); err != nil {
+		t.Fatal(err)
+	}
+
+	zr, err := zip.OpenReader(first)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := &reuseSource{zr: zr, files: map[string]*zip.File{}, hashes: map[string]string{}}
+	for _, f := range zr.File {
+		r.files[f.Name] = f
+	}
+	for _, c := range captured {
+		r.hashes[c.Path] = c.Hash
+	}
+	second := filepath.Join(dir, "second.zip")
+	start = time.Now()
+	if _, _, err := zipRootsCapturing(src, nil, second, r); err != nil {
+		t.Fatal(err)
+	}
+	incr := time.Since(start)
+	r.Close()
+
+	fi1, _ := os.Stat(first)
+	fi2, _ := os.Stat(second)
+	t.Logf("%d files: full snapshot %s (%d MB), reusing snapshot %s (%d MB), %d entries copied",
+		len(captured), full.Round(time.Millisecond), fi1.Size()>>20, incr.Round(time.Millisecond), fi2.Size()>>20, lastReused.Load())
+}
diff --git a/internal/snapshot/reuse_test.go b/internal/snapshot/reuse_test.go
new file mode 100644
index 00000000..104a8efd
--- /dev/null
+++ b/internal/snapshot/reuse_test.go
@@ -0,0 +1,69 @@
+package snapshot
+
+import (
+	"fmt"
+	"testing"
+)
+
+// A snapshot copies the files that did not change from the previous one, and
+// still restores to exactly the save it was taken of.
+func TestSnapshotReusesUnchangedFiles(t *testing.T) {
+	env := setup(t)
+	for i := 0; i < 50; i++ {
+		write(t, env.saveDir, fmt.Sprintf("map/chunk_%02d.bin", i), fmt.Sprintf("chunk %d %s", i, "................................................"))
+	}
+	first, err := env.mgr.Create("game1", "first", false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := lastReused.Load(); got != 0 {
+		t.Errorf("first snapshot reused %d entries from nothing", got)
+	}
+
+	// A session changes two chunks and adds one.
+	write(t, env.saveDir, "map/chunk_03.bin", "changed three")
+	write(t, env.saveDir, "map/chunk_40.bin", "changed forty")
+	write(t, env.saveDir, "map/chunk_99.bin", "new")
+	second, err := env.mgr.Create("game1", "second", false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := lastReused.Load(); got != 48 {
+		t.Errorf("second snapshot copied %d unchanged entries, want 48", got)
+	}
+
+	// Each snapshot restores its own state, independently of the other.
+	write(t, env.saveDir, "map/chunk_03.bin", "ruined")
+	if _, err := env.mgr.Restore("game1", second.ID); err != nil {
+		t.Fatal(err)
+	}
+	for name, want := range map[string]string{
+		"map/chunk_03.bin": "changed three",
+		"map/chunk_40.bin": "changed forty",
+		"map/chunk_99.bin": "new",
+		"map/chunk_07.bin": "chunk 7 ................................................",
+	} {
+		if got := read(env.saveDir, name); got != want {
+			t.Errorf("after restoring the second snapshot, %s = %q, want %q", name, got, want)
+		}
+	}
+	if _, err := env.mgr.Restore("game1", first.ID); err != nil {
+		t.Fatal(err)
+	}
+	if got := read(env.saveDir, "map/chunk_03.bin"); got != "chunk 3 ................................................" {
+		t.Errorf("after restoring the first snapshot, chunk_03 = %q", got)
+	}
+	if got := read(env.saveDir, "map/chunk_99.bin"); got != "" {
+		t.Errorf("restoring the first snapshot left the later file: %q", got)
+	}
+
+	// And the recorded contents of the second snapshot are right for the
+	// copied entries too.
+	files, err := env.store.SnapshotFiles(second.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) != 51 {
+		t.Errorf("second snapshot records %d files, want 51", len(files))
+	}
+}
diff --git a/internal/snapshot/shared_test.go b/internal/snapshot/shared_test.go
index f125591e..73049548 100644
--- a/internal/snapshot/shared_test.go
+++ b/internal/snapshot/shared_test.go
@@ -558,7 +558,9 @@ func TestSwitchingToABranchWhoseNewestIsCompacted(t *testing.T) {
 	env := setup(t)
 	snaps, originals := threeSnapshots(t, env)
 	// Over to another branch: the switch keeps the save on main first, and
-	// that copy is main's newest.
+	// that copy is main's newest. Changed first, or the copy would hold the
+	// same files as the third snapshot, and be that snapshot (content.go).
+	writeSave(t, env.saveDir, "slot_switch.sav", "before the switch")
 	if _, err := env.mgr.CreateBranch("game1", "other", true); err != nil {
 		t.Fatal(err)
 	}
diff --git a/internal/snapshot/snapshot.go b/internal/snapshot/snapshot.go
index 77d0cf6f..8a82e379 100644
--- a/internal/snapshot/snapshot.go
+++ b/internal/snapshot/snapshot.go
@@ -41,6 +41,18 @@ import (
 // before starting one.
 type UploadHook func(zipPath, remoteFileName string)
 
+// IncompleteError is a snapshot discarded because files of the save could not
+// be read — in use by the game, or changing while it was archived.
+type IncompleteError struct {
+	Game    string
+	Skipped []string
+}
+
+func (e *IncompleteError) Error() string {
+	return fmt.Sprintf("snapshot of %q discarded: %d file(s) could not be read (e.g. %s) — taken again at the next chance",
+		e.Game, len(e.Skipped), e.Skipped[0])
+}
+
 // Manager performs snapshot/branch operations against the store and
 // filesystem.
 type Manager struct {
@@ -80,6 +92,10 @@ type Manager struct {
 	// running past shutdown it wrote into a directory that was being deleted
 	// and recorded against a database that was already closed.
 	inFlight drain.Group
+	// PruneOnCreate checks each new snapshot against the older ones of its
+	// branch and removes those it holds entirely (content.go,
+	// PruneContainedBy). Off unless set: the daemon sets it.
+	PruneOnCreate bool
 	// sharedMu is held by a compaction and by the clean-up of shared files
 	// (shared.go): a compaction names shared files before any list does, and
 	// a clean-up running then would take them for unused. pendingRoots are
@@ -236,20 +252,48 @@ func (m *Manager) createOnBranchFrom(gameID, branch, from, comment string, isSys
 	} else {
 		src = from
 	}
-	skipped, captured, err := ZipRootsCapturing(src, extraRoots, stagingPath)
+	// The game's own save, onto its branch: unchanged files are copied from
+	// the branch's previous snapshot rather than compressed again (reuse.go).
+	var reuse *reuseSource
+	if from == "" {
+		reuse = m.openReuseSource(gameID, branch)
+	}
+	skipped, captured, err := zipRootsCapturing(src, extraRoots, stagingPath, reuse)
+	reuse.Close()
 	if err != nil {
 		os.Remove(stagingPath)
 		return store.Snapshot{}, fmt.Errorf("zip save data: %w", err)
 	}
 	defer os.Remove(stagingPath) // no-op once renamed
-	if len(skipped) > 0 && m.Log != nil {
-		sample := skipped[0]
-		m.Log("warn", fmt.Sprintf("snapshot of %q skipped %d unreadable file(s), e.g. %s", game.Name, len(skipped), sample))
+	// A snapshot missing files is not kept: it would stand in the history as
+	// a save that never existed — and, counted against retention, push out a
+	// complete one. The caller tries again at the next chance (the watcher
+	// leaves the change unrecorded, so its next pass takes it again; a sync
+	// that needed the copy before replacing files does not replace them).
+	if len(skipped) > 0 {
+		return store.Snapshot{}, &IncompleteError{Game: game.Name, Skipped: skipped}
+	}
+
+	// A copy taken before a sync replaces the save, of files a snapshot on this
+	// branch already holds: that one is the copy, and the save is not archived
+	// again (content.go). Syncs took one of these each time, mostly of a save
+	// already archived. Only when that snapshot is the branch's newest: a
+	// branch switch puts back the newest, and retention removes older ones
+	// first — an older copy standing in would leave the save behind either way. Snapshots the
+	// game's changes or a person ask for are always taken: the watcher takes
+	// them only when something changed, and a person asked.
+	contentKey := ContentKey(captured)
+	if existing, ok, _ := m.Store.SnapshotByContent(gameID, branch, contentKey); !current && ok && ArchiveExists(existing.ZipPath) &&
+		m.isNewest(gameID, branch, existing.ID) {
+		if m.Log != nil {
+			m.Log("info", fmt.Sprintf("%q holds the same files as snapshot %s; not archived again", game.Name, existing.ID))
+		}
+		return existing, nil
 	}
 
 	if comment == "" {
 		if isSystemAuto {
-			comment = "Auto backup"
+			comment = defaultAutoComment
 		} else {
 			comment = "Manual snapshot"
 		}
@@ -260,6 +304,11 @@ func (m *Manager) createOnBranchFrom(gameID, branch, from, comment string, isSys
 		return store.Snapshot{}, err
 	}
 	snapshotID := snap.ID
+	_ = m.Store.SetSnapshotContentHash(snapshotID, contentKey)
+	snap.ContentHash = contentKey
+	// Older automatic snapshots this one holds entirely are steps on the way
+	// to it (content.go).
+	defer m.PruneContainedBy(snap, captured)
 
 	// What this snapshot holds, file by file, recorded once and never
 	// revisited. It is what lets a later caller ask whether some exact content
@@ -730,7 +779,18 @@ func (m *Manager) Restore(gameID, snapshotID string) (store.Snapshot, error) {
 	if err != nil {
 		return store.Snapshot{}, err
 	}
+	// An id kept as an alias of the snapshot holding the same files
+	// (content.go) restores that snapshot.
+	if resolved, ok := m.Store.ResolveSnapshotID(snapshotID); ok {
+		snapshotID = resolved
+	}
 	snap, err := m.Store.GetSnapshot(snapshotID)
+	if err != nil {
+		// Removed in favour of the same files just now: its alias is there.
+		if resolved, ok := m.Store.ResolveSnapshotID(snapshotID); ok {
+			snap, err = m.Store.GetSnapshot(resolved)
+		}
+	}
 	if err != nil || snap.GameID != gameID {
 		return store.Snapshot{}, fmt.Errorf("snapshot %q not found for game %q", snapshotID, gameID)
 	}
diff --git a/internal/snapshot/zip.go b/internal/snapshot/zip.go
index 1ea2920c..b60ea395 100644
--- a/internal/snapshot/zip.go
+++ b/internal/snapshot/zip.go
@@ -8,6 +8,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"strings"
@@ -38,6 +39,10 @@ func ZipPath(sourcePath, outPath string) (skipped []string, err error) {
 // archived so the caller can record what the snapshot holds. The hashes come
 // free: the bytes pass through a hasher on their way into the archive.
 func ZipPathCapturing(sourcePath, outPath string) (skipped []string, captured []store.CapturedFile, err error) {
+	return zipPathCapturing(sourcePath, outPath, nil)
+}
+
+func zipPathCapturing(sourcePath, outPath string, reuse *reuseSource) (skipped []string, captured []store.CapturedFile, err error) {
 	info, err := os.Stat(sourcePath)
 	if err != nil {
 		return nil, nil, fmt.Errorf("source path does not exist: %w", err)
@@ -54,7 +59,7 @@ func ZipPathCapturing(sourcePath, outPath string) (skipped []string, captured []
 
 	if !info.IsDir() {
 		name := filepath.Base(sourcePath)
-		hash, addErr := addFileEntry(w, sourcePath, name)
+		hash, addErr := reuse.addFileEntry(w, sourcePath, name, info)
 		if addErr != nil {
 			return nil, nil, addErr
 		}
@@ -62,7 +67,10 @@ func ZipPathCapturing(sourcePath, outPath string) (skipped []string, captured []
 	}
 
 	archived := 0
-	walkErr := filepath.Walk(sourcePath, func(path string, walkInfo os.FileInfo, walkErr error) error {
+	// WalkDir, not Walk: Walk's extra Lstat per entry opens every file on
+	// Windows, which on a save of a quarter-million files cost more than the
+	// archiving itself once unchanged files are copied (reuse.go).
+	walkErr := filepath.WalkDir(sourcePath, func(path string, d fs.DirEntry, walkErr error) error {
 		if walkErr != nil {
 			// Unreadable subtree — record it and move on.
 			if path != sourcePath {
@@ -80,14 +88,24 @@ func ZipPathCapturing(sourcePath, outPath string) (skipped []string, captured []
 		}
 		rel = filepath.ToSlash(rel)
 
-		if walkInfo.IsDir() {
+		if d.IsDir() {
 			// Explicit directory entries keep empty dirs restorable.
 			if _, err := w.CreateHeader(&zip.FileHeader{Name: rel + "/", Method: zip.Store}); err != nil {
 				return err
 			}
 			return nil
 		}
-		hash, addErr := addFileEntry(w, path, rel)
+		// OpenSave's own temporary file for a file being pulled: never part
+		// of the save, and gone by the time it is read.
+		if strings.HasSuffix(rel, delta.TmpSuffix) || delta.NeverSynced(rel) {
+			return nil
+		}
+		info, infoErr := d.Info()
+		if infoErr != nil {
+			skipped = append(skipped, path)
+			return nil
+		}
+		hash, addErr := reuse.addFileEntry(w, path, rel, info)
 		if addErr != nil {
 			skipped = append(skipped, path)
 			return nil
diff --git a/internal/snapshot/zip_roots.go b/internal/snapshot/zip_roots.go
index 8f70ff2f..5100f6ea 100644
--- a/internal/snapshot/zip_roots.go
+++ b/internal/snapshot/zip_roots.go
@@ -3,6 +3,7 @@ package snapshot
 import (
 	"archive/zip"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"sort"
@@ -44,8 +45,14 @@ func ZipRoots(primary string, extra map[string]string, outPath string) (skipped
 // instead of inferred from a single whole-save hash that nothing reliably
 // kept current.
 func ZipRootsCapturing(primary string, extra map[string]string, outPath string) (skipped []string, captured []store.CapturedFile, err error) {
+	return zipRootsCapturing(primary, extra, outPath, nil)
+}
+
+// zipRootsCapturing is ZipRootsCapturing, copying unchanged files straight
+// out of reuse (the previous snapshot) when it is given. See reuse.go.
+func zipRootsCapturing(primary string, extra map[string]string, outPath string, reuse *reuseSource) (skipped []string, captured []store.CapturedFile, err error) {
 	if len(extra) == 0 {
-		return ZipPathCapturing(primary, outPath)
+		return zipPathCapturing(primary, outPath, reuse)
 	}
 
 	// One archive, written in a single pass.
@@ -69,7 +76,7 @@ func ZipRootsCapturing(primary string, extra map[string]string, outPath string)
 	defer f.Close()
 	w := newSnapshotWriter(f)
 
-	primarySkipped, primaryFiles, err := archiveInto(w, primary, "", "")
+	primarySkipped, primaryFiles, err := archiveInto(w, primary, "", "", reuse)
 	skipped = append(skipped, primarySkipped...)
 	captured = append(captured, primaryFiles...)
 	if err != nil {
@@ -82,7 +89,7 @@ func ZipRootsCapturing(primary string, extra map[string]string, outPath string)
 		if strings.TrimSpace(path) == "" {
 			continue
 		}
-		sub, subFiles, subErr := archiveInto(w, path, RootPrefix+name+"/", name)
+		sub, subFiles, subErr := archiveInto(w, path, RootPrefix+name+"/", name, reuse)
 		skipped = append(skipped, sub...)
 		captured = append(captured, subFiles...)
 		if subErr != nil {
@@ -94,14 +101,14 @@ func ZipRootsCapturing(primary string, extra map[string]string, outPath string)
 }
 
 // archiveInto writes one directory tree into an open zip under prefix.
-func archiveInto(w *zip.Writer, sourcePath, prefix, root string) (skipped []string, captured []store.CapturedFile, err error) {
+func archiveInto(w *zip.Writer, sourcePath, prefix, root string, reuse *reuseSource) (skipped []string, captured []store.CapturedFile, err error) {
 	info, err := os.Stat(sourcePath)
 	if err != nil {
 		return nil, nil, err
 	}
 	if !info.IsDir() {
 		name := filepath.Base(sourcePath)
-		hash, addErr := addFileEntry(w, sourcePath, prefix+name)
+		hash, addErr := reuse.addFileEntry(w, sourcePath, prefix+name, info)
 		if addErr != nil {
 			return nil, nil, addErr
 		}
@@ -117,7 +124,7 @@ func archiveInto(w *zip.Writer, sourcePath, prefix, root string) (skipped []stri
 		}
 	}
 
-	walkErr := filepath.Walk(sourcePath, func(path string, walkInfo os.FileInfo, walkErr error) error {
+	walkErr := filepath.WalkDir(sourcePath, func(path string, d fs.DirEntry, walkErr error) error {
 		if walkErr != nil {
 			if path != sourcePath {
 				skipped = append(skipped, path)
@@ -133,11 +140,20 @@ func archiveInto(w *zip.Writer, sourcePath, prefix, root string) (skipped []stri
 			return err
 		}
 		rel = filepath.ToSlash(rel)
-		if walkInfo.IsDir() {
+		if d.IsDir() {
 			_, err := w.CreateHeader(&zip.FileHeader{Name: prefix + rel + "/", Method: zip.Store})
 			return err
 		}
-		hash, addErr := addFileEntry(w, path, prefix+rel)
+		// OpenSave's own temporary file for a file being pulled (zip.go).
+		if strings.HasSuffix(rel, delta.TmpSuffix) || delta.NeverSynced(rel) {
+			return nil
+		}
+		info, infoErr := d.Info()
+		if infoErr != nil {
+			skipped = append(skipped, path)
+			return nil
+		}
+		hash, addErr := reuse.addFileEntry(w, path, prefix+rel, info)
 		if addErr != nil {
 			skipped = append(skipped, path)
 			return nil
diff --git a/internal/store/games.go b/internal/store/games.go
index 52d9c928..18b23ee6 100644
--- a/internal/store/games.go
+++ b/internal/store/games.go
@@ -72,7 +72,11 @@ type Game struct {
 	// else — snapshots keep capturing every file, so a restore can never be
 	// the thing that deletes an excluded config.
 	SyncIgnore string `db:"sync_ignore" json:"syncIgnore"`
-	CreatedAt  string `db:"created_at" json:"createdAt"`
+	// SyncDeviceSettings syncs the files the game database names as the
+	// game's settings, which are otherwise left out of syncing
+	// (presets/devicesettings.go). Off unless the user turns it on.
+	SyncDeviceSettings bool   `db:"sync_device_settings" json:"syncDeviceSettings"`
+	CreatedAt          string `db:"created_at" json:"createdAt"`
 }
 
 // CreateGame inserts a new game and its default "main" branch in one
@@ -159,7 +163,8 @@ func (s *Store) UpdateGame(g Game) error {
 			exe_path = :exe_path,
 			cover_url = :cover_url,
 			last_manifest_hash = :last_manifest_hash,
-			sync_ignore = :sync_ignore
+			sync_ignore = :sync_ignore,
+			sync_device_settings = :sync_device_settings
 		WHERE id = :id`, g)
 	if err != nil {
 		return fmt.Errorf("update game %s: %w", g.ID, err)
diff --git a/internal/store/marks.go b/internal/store/marks.go
new file mode 100644
index 00000000..5928757a
--- /dev/null
+++ b/internal/store/marks.go
@@ -0,0 +1,22 @@
+package store
+
+import "fmt"
+
+// Mark returns the value a one-time step was taken for, empty if never.
+// See migrations/0040_marks.sql.
+func (s *Store) Mark(name string) string {
+	var v string
+	if err := s.db.Get(&v, `SELECT value FROM marks WHERE name = ?`, name); err != nil {
+		return ""
+	}
+	return v
+}
+
+// SetMark records that a one-time step was taken for value.
+func (s *Store) SetMark(name, value string) error {
+	if _, err := s.db.Exec(`INSERT INTO marks (name, value) VALUES (?, ?)
+		ON CONFLICT(name) DO UPDATE SET value = excluded.value`, name, value); err != nil {
+		return fmt.Errorf("set mark %s: %w", name, err)
+	}
+	return nil
+}
diff --git a/internal/store/migrations/0038_peer_links.sql b/internal/store/migrations/0038_peer_links.sql
new file mode 100644
index 00000000..4cff6cfb
--- /dev/null
+++ b/internal/store/migrations/0038_peer_links.sql
@@ -0,0 +1,15 @@
+-- How fast this device has measured its connection to each paired device:
+-- from transfers it made, and from a short speed test when there is nothing
+-- recent (see internal/p2p/syncengine/linkspeed.go). Syncs go to the fastest
+-- connections first, so a save reaches the devices close to it before it
+-- crawls over a slow relay — and those can then hand it on.
+--
+-- bytes_per_sec is a running average; measured_ms when it last changed;
+-- probed_ms when a speed test last ran, so a slow link is not tested over and
+-- over.
+CREATE TABLE peer_links (
+    peer_id       TEXT PRIMARY KEY,
+    bytes_per_sec REAL NOT NULL DEFAULT 0,
+    measured_ms   INTEGER NOT NULL DEFAULT 0,
+    probed_ms     INTEGER NOT NULL DEFAULT 0
+);
diff --git a/internal/store/migrations/0039_game_versions.sql b/internal/store/migrations/0039_game_versions.sql
new file mode 100644
index 00000000..3c0c2ce1
--- /dev/null
+++ b/internal/store/migrations/0039_game_versions.sql
@@ -0,0 +1,42 @@
+-- Which version of a game's save this device holds, as a version vector, and
+-- whether it knows of a newer one it has not finished taking yet (see
+-- internal/p2p/syncengine/version.go).
+--
+-- vector is a JSON object of writer key to counter. A writer key belongs to
+-- one device for one game (key below), and its counter moves only when the
+-- save really changes there — the game or the user wrote it, never a sync.
+-- Comparing two vectors says, without guessing from files or clocks, whether
+-- one device is simply behind the other or both changed independently.
+--
+-- target is the newest version this device has heard of that is newer than
+-- its own, '' when none. While it is set this device is outdated: it takes
+-- the save only from a device holding that version, and is a source for
+-- nobody — its files are not handed out, and what it lacks is not read as
+-- deleted.
+--
+-- counter is this device's own counter for the game. It never goes back, even
+-- when the device adopts another's version, so the same key and number never
+-- name two different saves.
+--
+-- pulling is set while a pull towards target is under way, so the files it
+-- writes are not mistaken for a new local version if the app stops half-way.
+--
+-- hash is the save's content hash when vector was last set ('' unknown). Two
+-- devices on one version holding different files means a change went
+-- unnoticed on one of them; this says which.
+--
+-- answered is the other device's version this one kept its own save over when
+-- someone answered a conflict here ('' none), and answered_at when (Unix ms).
+-- It travels with the version, so the other device is asked in turn rather
+-- than overwritten, and this one is not asked again.
+CREATE TABLE game_versions (
+    game_id  TEXT PRIMARY KEY REFERENCES games(id) ON DELETE CASCADE,
+    key      TEXT NOT NULL,
+    counter  INTEGER NOT NULL DEFAULT 0,
+    vector   TEXT NOT NULL DEFAULT '{}',
+    target   TEXT NOT NULL DEFAULT '',
+    pulling  INTEGER NOT NULL DEFAULT 0,
+    hash     TEXT NOT NULL DEFAULT '',
+    answered TEXT NOT NULL DEFAULT '',
+    answered_at INTEGER NOT NULL DEFAULT 0
+);
diff --git a/internal/store/migrations/0040_snapshot_content.sql b/internal/store/migrations/0040_snapshot_content.sql
new file mode 100644
index 00000000..46e0a102
--- /dev/null
+++ b/internal/store/migrations/0040_snapshot_content.sql
@@ -0,0 +1,16 @@
+-- What a snapshot holds, as one value: a hash over every file's name in the
+-- archive and its content hash (snapshot.ContentKey). The same files give the
+-- same value on every device, so two snapshots of one save — taken here, or
+-- taken on another device and recorded here as "Synced from peer" — are known
+-- to be the same, and the archive is kept once. '' until computed.
+ALTER TABLE snapshots ADD COLUMN content_hash TEXT NOT NULL DEFAULT '';
+
+CREATE INDEX IF NOT EXISTS idx_snapshots_content ON snapshots(game_id, branch_name, content_hash);
+
+-- Another snapshot id that names the same content as a snapshot kept here: a
+-- peer's snapshot whose save this device already has archived. Known, so it is
+-- not archived again, and not asked about again on every sync.
+CREATE TABLE snapshot_aliases (
+    alias_id    TEXT PRIMARY KEY,
+    snapshot_id TEXT NOT NULL REFERENCES snapshots(id) ON DELETE CASCADE
+);
diff --git a/internal/store/migrations/0041_marks.sql b/internal/store/migrations/0041_marks.sql
new file mode 100644
index 00000000..e9992819
--- /dev/null
+++ b/internal/store/migrations/0041_marks.sql
@@ -0,0 +1,8 @@
+-- One-time steps this device has already taken, by name, with the value they
+-- were taken for. A step whose value changes in a later build runs again.
+-- Used for delta.NeverSyncedList: when the files no save is made of change,
+-- the hashes recorded under the old list are re-taken once (daemon.go).
+CREATE TABLE marks (
+    name  TEXT PRIMARY KEY,
+    value TEXT NOT NULL
+);
diff --git a/internal/store/migrations/0042_sync_device_settings.sql b/internal/store/migrations/0042_sync_device_settings.sql
new file mode 100644
index 00000000..a5f9d82e
--- /dev/null
+++ b/internal/store/migrations/0042_sync_device_settings.sql
@@ -0,0 +1,5 @@
+-- Whether a game's device settings (the files the Ludusavi manifest tags as
+-- its settings and not its save, presets/devicesettings.go) sync like the rest
+-- of its save. Off by default: one PC's resolution and graphics quality are
+-- not another's.
+ALTER TABLE games ADD COLUMN sync_device_settings INTEGER NOT NULL DEFAULT 0;
diff --git a/internal/store/migrations/0043_settings_files.sql b/internal/store/migrations/0043_settings_files.sql
new file mode 100644
index 00000000..46dee16b
--- /dev/null
+++ b/internal/store/migrations/0043_settings_files.sql
@@ -0,0 +1,31 @@
+-- Files in a game's main save folder judged to be its settings rather than its
+-- save, by what they hold (presets/detectsettings.go), and files shown to be a
+-- save after all — by changing with the save, in this device's snapshots or in
+-- what another device sent (daemon/detectsettings.go).
+--
+-- verdict 'settings' leaves the file out of syncing, as the game database's
+-- settings files are. verdict 'save' keeps it syncing for good: detection never
+-- takes it again, and a game database entry that named it is overruled.
+-- path is relative to the main save folder, slash-separated.
+CREATE TABLE settings_files (
+    game_id TEXT NOT NULL REFERENCES games(id) ON DELETE CASCADE,
+    path    TEXT NOT NULL,
+    verdict TEXT NOT NULL,
+    reason  TEXT NOT NULL DEFAULT '',
+    at_ms   INTEGER NOT NULL,
+    PRIMARY KEY (game_id, path)
+);
+
+-- What another device's copy of an excluded settings file did, sync by sync:
+-- moved counts syncs in which the save changed and so did that copy, still
+-- those in which the save changed and it did not. A settings file is changed
+-- now and then; a save that was taken for one changes with every session.
+CREATE TABLE settings_observations (
+    game_id   TEXT NOT NULL REFERENCES games(id) ON DELETE CASCADE,
+    peer_id   TEXT NOT NULL,
+    path      TEXT NOT NULL,
+    last_hash TEXT NOT NULL DEFAULT '',
+    moved     INTEGER NOT NULL DEFAULT 0,
+    still     INTEGER NOT NULL DEFAULT 0,
+    PRIMARY KEY (game_id, peer_id, path)
+);
diff --git a/internal/store/peer_links.go b/internal/store/peer_links.go
new file mode 100644
index 00000000..d65b3a37
--- /dev/null
+++ b/internal/store/peer_links.go
@@ -0,0 +1,37 @@
+package store
+
+import "fmt"
+
+// PeerLink is how fast this device has measured its connection to a peer.
+// See migrations/0038_peer_links.sql.
+type PeerLink struct {
+	PeerID      string  `db:"peer_id"`
+	BytesPerSec float64 `db:"bytes_per_sec"`
+	MeasuredMs  int64   `db:"measured_ms"`
+	ProbedMs    int64   `db:"probed_ms"`
+}
+
+// PeerLinks returns every recorded link, by peer.
+func (s *Store) PeerLinks() (map[string]PeerLink, error) {
+	var rows []PeerLink
+	if err := s.db.Select(&rows, `SELECT * FROM peer_links`); err != nil {
+		return nil, fmt.Errorf("list peer links: %w", err)
+	}
+	out := make(map[string]PeerLink, len(rows))
+	for _, r := range rows {
+		out[r.PeerID] = r
+	}
+	return out, nil
+}
+
+// SavePeerLink records a peer's link, replacing what was there.
+func (s *Store) SavePeerLink(l PeerLink) error {
+	_, err := s.db.Exec(`INSERT INTO peer_links (peer_id, bytes_per_sec, measured_ms, probed_ms) VALUES (?, ?, ?, ?)
+		ON CONFLICT(peer_id) DO UPDATE SET bytes_per_sec = excluded.bytes_per_sec,
+			measured_ms = excluded.measured_ms, probed_ms = excluded.probed_ms`,
+		l.PeerID, l.BytesPerSec, l.MeasuredMs, l.ProbedMs)
+	if err != nil {
+		return fmt.Errorf("save peer link %s: %w", l.PeerID, err)
+	}
+	return nil
+}
diff --git a/internal/store/settings_files.go b/internal/store/settings_files.go
new file mode 100644
index 00000000..94794a57
--- /dev/null
+++ b/internal/store/settings_files.go
@@ -0,0 +1,83 @@
+package store
+
+import (
+	"fmt"
+	"time"
+)
+
+// Verdicts on a file in a game's save folder (migrations/0042_settings_files.sql).
+const (
+	VerdictSettings = "settings"
+	VerdictSave     = "save"
+)
+
+// SettingsFile is a verdict on one file of a game's main save folder.
+type SettingsFile struct {
+	GameID  string `db:"game_id" json:"-"`
+	Path    string `db:"path" json:"path"`
+	Verdict string `db:"verdict" json:"verdict"`
+	Reason  string `db:"reason" json:"reason"`
+	AtMs    int64  `db:"at_ms" json:"atMs"`
+}
+
+// SettingsFiles returns every verdict recorded for a game.
+func (s *Store) SettingsFiles(gameID string) ([]SettingsFile, error) {
+	var out []SettingsFile
+	if err := s.db.Select(&out, `SELECT * FROM settings_files WHERE game_id = ? ORDER BY path`, gameID); err != nil {
+		return nil, fmt.Errorf("list settings files of %s: %w", gameID, err)
+	}
+	return out, nil
+}
+
+// SetSettingsFile records a verdict, replacing any earlier one.
+func (s *Store) SetSettingsFile(f SettingsFile) error {
+	if f.AtMs == 0 {
+		f.AtMs = time.Now().UnixMilli()
+	}
+	if _, err := s.db.NamedExec(`INSERT INTO settings_files (game_id, path, verdict, reason, at_ms)
+		VALUES (:game_id, :path, :verdict, :reason, :at_ms)
+		ON CONFLICT(game_id, path) DO UPDATE SET verdict = excluded.verdict, reason = excluded.reason, at_ms = excluded.at_ms`, f); err != nil {
+		return fmt.Errorf("set settings file %s/%s: %w", f.GameID, f.Path, err)
+	}
+	return nil
+}
+
+// SettingsObservation is what one device's copy of an excluded settings file
+// has done so far.
+type SettingsObservation struct {
+	GameID   string `db:"game_id"`
+	PeerID   string `db:"peer_id"`
+	Path     string `db:"path"`
+	LastHash string `db:"last_hash"`
+	Moved    int    `db:"moved"`
+	Still    int    `db:"still"`
+}
+
+// SettingsObservations returns what every device's copy of path has done.
+func (s *Store) SettingsObservations(gameID, path string) ([]SettingsObservation, error) {
+	var out []SettingsObservation
+	if err := s.db.Select(&out, `SELECT * FROM settings_observations WHERE game_id = ? AND path = ?`, gameID, path); err != nil {
+		return nil, fmt.Errorf("list settings observations: %w", err)
+	}
+	return out, nil
+}
+
+// GetSettingsObservation returns one device's record ({} when none).
+func (s *Store) GetSettingsObservation(gameID, peerID, path string) SettingsObservation {
+	var o SettingsObservation
+	if err := s.db.Get(&o, `SELECT * FROM settings_observations WHERE game_id = ? AND peer_id = ? AND path = ?`, gameID, peerID, path); err != nil {
+		return SettingsObservation{GameID: gameID, PeerID: peerID, Path: path}
+	}
+	return o
+}
+
+// SaveSettingsObservation writes one device's record.
+func (s *Store) SaveSettingsObservation(o SettingsObservation) error {
+	if _, err := s.db.NamedExec(`INSERT INTO settings_observations (game_id, peer_id, path, last_hash, moved, still)
+		VALUES (:game_id, :peer_id, :path, :last_hash, :moved, :still)
+		ON CONFLICT(game_id, peer_id, path) DO UPDATE SET last_hash = excluded.last_hash,
+			moved = excluded.moved, still = excluded.still`, o); err != nil {
+		return fmt.Errorf("save settings observation: %w", err)
+	}
+	return nil
+}
diff --git a/internal/store/snapshot_content.go b/internal/store/snapshot_content.go
new file mode 100644
index 00000000..f9df1be2
--- /dev/null
+++ b/internal/store/snapshot_content.go
@@ -0,0 +1,92 @@
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+// Snapshot content: which snapshots hold the same files, so a save is
+// archived once however many times — and on however many devices — it is
+// snapshotted. See migrations/0039_snapshot_content.sql.
+
+// SetSnapshotContentHash records what a snapshot holds.
+func (s *Store) SetSnapshotContentHash(id, hash string) error {
+	if _, err := s.db.Exec(`UPDATE snapshots SET content_hash = ? WHERE id = ?`, hash, id); err != nil {
+		return fmt.Errorf("set content hash of %s: %w", id, err)
+	}
+	return nil
+}
+
+// SnapshotByContent finds a snapshot of a game's branch holding exactly the
+// content hash names; ok is false when there is none. The newest, so a copy
+// kept in its place is the one retention keeps longest.
+func (s *Store) SnapshotByContent(gameID, branch, hash string) (snap Snapshot, ok bool, err error) {
+	if hash == "" {
+		return Snapshot{}, false, nil
+	}
+	err = s.db.Get(&snap, `SELECT * FROM snapshots WHERE game_id = ? AND branch_name = ? AND content_hash = ?
+		ORDER BY timestamp DESC LIMIT 1`, gameID, branch, hash)
+	if errors.Is(err, sql.ErrNoRows) {
+		return Snapshot{}, false, nil
+	}
+	if err != nil {
+		return Snapshot{}, false, fmt.Errorf("find snapshot by content: %w", err)
+	}
+	return snap, true, nil
+}
+
+// AddSnapshotAlias records that alias (another device's snapshot id) names the
+// same content as snapshotID here.
+func (s *Store) AddSnapshotAlias(alias, snapshotID string) error {
+	if alias == "" || alias == snapshotID {
+		return nil
+	}
+	if _, err := s.db.Exec(`INSERT INTO snapshot_aliases (alias_id, snapshot_id) VALUES (?, ?)
+		ON CONFLICT(alias_id) DO UPDATE SET snapshot_id = excluded.snapshot_id`, alias, snapshotID); err != nil {
+		return fmt.Errorf("add snapshot alias %s: %w", alias, err)
+	}
+	return nil
+}
+
+// RepointSnapshotAliases moves every alias of from onto to, before from is
+// removed in favour of to.
+func (s *Store) RepointSnapshotAliases(from, to string) error {
+	if _, err := s.db.Exec(`UPDATE snapshot_aliases SET snapshot_id = ? WHERE snapshot_id = ?`, to, from); err != nil {
+		return fmt.Errorf("repoint aliases of %s: %w", from, err)
+	}
+	return nil
+}
+
+// ResolveSnapshotID says which snapshot here an id names: itself when it is
+// one, the snapshot it is an alias of when it is that; ok false when neither.
+func (s *Store) ResolveSnapshotID(id string) (string, bool) {
+	var found string
+	if err := s.db.Get(&found, `SELECT id FROM snapshots WHERE id = ?`, id); err == nil {
+		return found, true
+	}
+	if err := s.db.Get(&found, `SELECT snapshot_id FROM snapshot_aliases WHERE alias_id = ?`, id); err == nil {
+		return found, true
+	}
+	return "", false
+}
+
+// SnapshotsWithoutContentHash lists a game's snapshots whose content has not
+// been computed yet.
+func (s *Store) SnapshotsWithoutContentHash(gameID string) ([]Snapshot, error) {
+	var out []Snapshot
+	if err := s.db.Select(&out, `SELECT * FROM snapshots WHERE game_id = ? AND content_hash = '' ORDER BY timestamp`, gameID); err != nil {
+		return nil, fmt.Errorf("list snapshots without content hash: %w", err)
+	}
+	return out, nil
+}
+
+// AllSnapshotsOfGame lists every snapshot of a game, on every branch, oldest
+// first.
+func (s *Store) AllSnapshotsOfGame(gameID string) ([]Snapshot, error) {
+	var out []Snapshot
+	if err := s.db.Select(&out, `SELECT * FROM snapshots WHERE game_id = ? ORDER BY timestamp`, gameID); err != nil {
+		return nil, fmt.Errorf("list snapshots of %s: %w", gameID, err)
+	}
+	return out, nil
+}
diff --git a/internal/store/snapshots.go b/internal/store/snapshots.go
index bf0efc23..19ed403c 100644
--- a/internal/store/snapshots.go
+++ b/internal/store/snapshots.go
@@ -26,13 +26,16 @@ type Snapshot struct {
 	// if it was not (migration 0032; internal/snapshot/verify.go).
 	CheckedMs int64  `db:"checked_ms" json:"checkedMs"`
 	Problem   string `db:"problem" json:"problem"`
+	// ContentHash names what the snapshot holds (snapshot.ContentKey), the
+	// same on every device for the same files; '' until computed.
+	ContentHash string `db:"content_hash" json:"contentHash"`
 }
 
 // CreateSnapshot inserts a new snapshot record.
 func (s *Store) CreateSnapshot(snap Snapshot) error {
 	_, err := s.db.NamedExec(`
-		INSERT INTO snapshots (id, game_id, branch_name, timestamp, comment, is_system_auto, zip_path, size_bytes)
-		VALUES (:id, :game_id, :branch_name, :timestamp, :comment, :is_system_auto, :zip_path, :size_bytes)`,
+		INSERT INTO snapshots (id, game_id, branch_name, timestamp, comment, is_system_auto, zip_path, size_bytes, content_hash)
+		VALUES (:id, :game_id, :branch_name, :timestamp, :comment, :is_system_auto, :zip_path, :size_bytes, :content_hash)`,
 		snap)
 	if err != nil {
 		return fmt.Errorf("create snapshot %s: %w", snap.ID, err)
diff --git a/internal/store/versions.go b/internal/store/versions.go
new file mode 100644
index 00000000..3803db67
--- /dev/null
+++ b/internal/store/versions.go
@@ -0,0 +1,71 @@
+package store
+
+import (
+	"crypto/rand"
+	"database/sql"
+	"encoding/hex"
+	"errors"
+	"fmt"
+)
+
+// Save versions: which version of a game's save this device holds. See
+// migrations/0037_game_versions.sql for the columns and
+// internal/p2p/syncengine/version.go for the rules. The vectors are kept as
+// the JSON the sync engine writes; this layer does not interpret them.
+
+// GameVersion is one game's version record on this device.
+type GameVersion struct {
+	GameID  string `db:"game_id"`
+	Key     string `db:"key"`
+	Counter int64  `db:"counter"`
+	Vector  string `db:"vector"`
+	Target  string `db:"target"`
+	Pulling bool   `db:"pulling"`
+	Hash    string `db:"hash"`
+	// Answered is the JSON vector this device kept its save over in a
+	// conflict, AnsweredAt when.
+	Answered   string `db:"answered"`
+	AnsweredAt int64  `db:"answered_at"`
+}
+
+// GetGameVersion returns a game's version record, creating an empty one —
+// with a fresh writer key — the first time it is asked for. A game that is not
+// tracked gets a record that is not stored.
+func (s *Store) GetGameVersion(gameID string) (GameVersion, error) {
+	var v GameVersion
+	err := s.db.Get(&v, `SELECT * FROM game_versions WHERE game_id = ?`, gameID)
+	if err == nil {
+		return v, nil
+	}
+	if !errors.Is(err, sql.ErrNoRows) {
+		return GameVersion{}, fmt.Errorf("get game version %s: %w", gameID, err)
+	}
+	var b [8]byte
+	if _, err := rand.Read(b[:]); err != nil {
+		return GameVersion{}, err
+	}
+	v = GameVersion{GameID: gameID, Key: hex.EncodeToString(b[:]), Vector: "{}"}
+	// OR IGNORE: two callers creating it at once keep the first key.
+	if _, err := s.db.Exec(`INSERT OR IGNORE INTO game_versions (game_id, key, vector)
+		SELECT ?, ?, '{}' WHERE EXISTS (SELECT 1 FROM games WHERE id = ?)`,
+		gameID, v.Key, gameID); err != nil {
+		return GameVersion{}, fmt.Errorf("create game version %s: %w", gameID, err)
+	}
+	if err := s.db.Get(&v, `SELECT * FROM game_versions WHERE game_id = ?`, gameID); err != nil &&
+		!errors.Is(err, sql.ErrNoRows) {
+		return GameVersion{}, fmt.Errorf("get game version %s: %w", gameID, err)
+	}
+	return v, nil
+}
+
+// SaveGameVersion writes a game's version record. The writer key is never
+// changed by this: it is the record's identity.
+func (s *Store) SaveGameVersion(v GameVersion) error {
+	_, err := s.db.Exec(`UPDATE game_versions SET counter = ?, vector = ?, target = ?, pulling = ?, hash = ?,
+		answered = ?, answered_at = ? WHERE game_id = ?`,
+		v.Counter, v.Vector, v.Target, v.Pulling, v.Hash, v.Answered, v.AnsweredAt, v.GameID)
+	if err != nil {
+		return fmt.Errorf("save game version %s: %w", v.GameID, err)
+	}
+	return nil
+}
diff --git a/internal/watcher/neversynced_test.go b/internal/watcher/neversynced_test.go
new file mode 100644
index 00000000..f3d19a75
--- /dev/null
+++ b/internal/watcher/neversynced_test.go
@@ -0,0 +1,26 @@
+package watcher
+
+import (
+	"testing"
+
+	"github.com/opensave/opensave/internal/delta"
+)
+
+// Steam rewriting remotecache.vdf is not a change of the save; a value
+// recorded by an older build, which counted it, is still recognisable.
+func TestContentHashLeavesOutNeverSynced(t *testing.T) {
+	save := delta.Manifest{Files: map[string]delta.FileEntry{"save.sav": {Hash: "a"}}}
+	withCache := delta.Manifest{Files: map[string]delta.FileEntry{
+		"save.sav":        {Hash: "a"},
+		"remotecache.vdf": {Hash: "steam"},
+	}}
+	if ContentHash(withCache, "") != ContentHash(save, "") {
+		t.Error("remotecache.vdf counts toward the content hash")
+	}
+	if ContentHash(save, "") != save.ContentHash() {
+		t.Error("a save without such files hashes differently than before")
+	}
+	if ContentHashBeforeNeverSynced(withCache, "") != withCache.ContentHash() {
+		t.Error("the hash an older build recorded cannot be reproduced")
+	}
+}
diff --git a/internal/watcher/owntouch_test.go b/internal/watcher/owntouch_test.go
new file mode 100644
index 00000000..d984bc29
--- /dev/null
+++ b/internal/watcher/owntouch_test.go
@@ -0,0 +1,157 @@
+package watcher
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/opensave/opensave/internal/owntouch"
+)
+
+// Files OpenSave writes or deletes itself — a pull, a peer's deletions — are
+// not a new save. Only the recorded hash moves; no auto-snapshot, and no
+// OnChanged echoing the sync back out.
+func TestOwnChanges_AreNotSnapshotted(t *testing.T) {
+	saveDir := t.TempDir()
+	col := newCollector()
+	eng := New(col.callbacks())
+	defer eng.Stop()
+	if err := eng.Watch("game1", saveDir); err != nil {
+		t.Fatal(err)
+	}
+
+	// A peer's deletions arriving one at a time, as they do, plus pulled files.
+	for i := 0; i < 5; i++ {
+		p := filepath.Join(saveDir, fmt.Sprintf("map_%d.bin", i))
+		owntouch.Mark(p)
+		if err := os.WriteFile(p, []byte(fmt.Sprint(i)), 0o666); err != nil {
+			t.Fatal(err)
+		}
+		time.Sleep(300 * time.Millisecond)
+	}
+	// Past the debounce, with room to spare.
+	if !waitFor(t, 8*time.Second, func() bool {
+		col.mu.Lock()
+		defer col.mu.Unlock()
+		return col.manifestHashes["game1"] != ""
+	}) {
+		t.Fatal("the recorded hash was not moved to the new content")
+	}
+	time.Sleep(3 * time.Second)
+	if n := col.snapshotCount(); n != 0 {
+		t.Errorf("%d auto-snapshot(s) for changes OpenSave made itself, want 0", n)
+	}
+	if n := col.changedCount(); n != 0 {
+		t.Errorf("OnChanged fired %d time(s) for changes OpenSave made itself", n)
+	}
+
+	// The game then saves: that is snapshotted, as always.
+	if err := os.WriteFile(filepath.Join(saveDir, "slot1.sav"), []byte("played"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if !waitFor(t, 10*time.Second, func() bool { return col.snapshotCount() >= 1 }) {
+		t.Fatal("a change the game made was not snapshotted")
+	}
+}
+
+// A pull that creates folders and writes into them before the watcher has put
+// them under watch is still the pull: the rescan that follows registering
+// them is not someone else's change.
+func TestOwnNewFolders_AreNotSnapshotted(t *testing.T) {
+	saveDir := t.TempDir()
+	col := newCollector()
+	eng := New(col.callbacks())
+	defer eng.Stop()
+	if err := eng.Watch("game1", saveDir); err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 20; i++ {
+		dir := filepath.Join(saveDir, fmt.Sprintf("slot%02d", i))
+		owntouch.Mark(dir)
+		if err := os.MkdirAll(dir, 0o777); err != nil {
+			t.Fatal(err)
+		}
+		p := filepath.Join(dir, "data.sav")
+		owntouch.Mark(p)
+		if err := os.WriteFile(p, []byte(fmt.Sprint(i)), 0o666); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if !waitFor(t, 10*time.Second, func() bool {
+		col.mu.Lock()
+		defer col.mu.Unlock()
+		return col.manifestHashes["game1"] != ""
+	}) {
+		t.Fatal("the recorded hash was not moved to the new content")
+	}
+	time.Sleep(4 * time.Second)
+	if n := col.snapshotCount(); n != 0 {
+		t.Errorf("%d auto-snapshot(s) for folders and files a pull created", n)
+	}
+
+	// A folder the game creates is a change, as before.
+	if err := os.MkdirAll(filepath.Join(saveDir, "newslot"), 0o777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(saveDir, "newslot", "data.sav"), []byte("played"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if !waitFor(t, 10*time.Second, func() bool { return col.snapshotCount() >= 1 }) {
+		t.Fatal("a folder the game created was not snapshotted")
+	}
+}
+
+// While a sync is writing the game, the watcher looks at what changed only
+// once it has finished — never at a save half-way between two states — and a
+// change of the game's is still snapshotted then.
+func TestWhileASyncWrites_TheWatcherWaits(t *testing.T) {
+	saveDir := t.TempDir()
+	col := newCollector()
+	var writing atomic.Bool
+	cb := col.callbacks()
+	cb.SyncWriting = func(string) bool { return writing.Load() }
+	cb.SyncWritingNow = cb.SyncWriting
+	eng := New(cb)
+	defer eng.Stop()
+	if err := eng.Watch("game1", saveDir); err != nil {
+		t.Fatal(err)
+	}
+	writing.Store(true)
+	if err := os.WriteFile(filepath.Join(saveDir, "slot1.sav"), []byte("played"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(5 * time.Second)
+	if n := col.snapshotCount(); n != 0 {
+		t.Fatalf("%d snapshot(s) while a sync was writing the game", n)
+	}
+	writing.Store(false)
+	if !waitFor(t, 10*time.Second, func() bool { return col.snapshotCount() >= 1 }) {
+		t.Fatal("the game's change was not snapshotted once the sync had finished")
+	}
+}
+
+// A burst that mixes OpenSave's own changes with one from the game is a real
+// change and is snapshotted.
+func TestMixedChanges_AreSnapshotted(t *testing.T) {
+	saveDir := t.TempDir()
+	col := newCollector()
+	eng := New(col.callbacks())
+	defer eng.Stop()
+	if err := eng.Watch("game1", saveDir); err != nil {
+		t.Fatal(err)
+	}
+	own := filepath.Join(saveDir, "pulled.bin")
+	owntouch.Mark(own)
+	if err := os.WriteFile(own, []byte("from a peer"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(saveDir, "slot1.sav"), []byte("played"), 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if !waitFor(t, 10*time.Second, func() bool { return col.snapshotCount() >= 1 }) {
+		t.Fatal("a burst including the game's own change was not snapshotted")
+	}
+}
diff --git a/internal/watcher/watcher.go b/internal/watcher/watcher.go
index 612ebfb6..786abb34 100644
--- a/internal/watcher/watcher.go
+++ b/internal/watcher/watcher.go
@@ -32,6 +32,7 @@ import (
 	"github.com/opensave/opensave/internal/delta"
 	"github.com/opensave/opensave/internal/ignore"
 	"github.com/opensave/opensave/internal/logging"
+	"github.com/opensave/opensave/internal/owntouch"
 )
 
 const (
@@ -118,6 +119,30 @@ type Callbacks struct {
 	OnChanged func(gameID string)
 	// Log receives human-readable watcher activity. May be nil.
 	Log func(level, msg string)
+	// SyncWriting reports whether a sync on this device is writing the game
+	// now, or was a moment ago: a burst the watcher cannot attribute — an
+	// overflow of file events — is then taken for the sync's. May be nil.
+	SyncWriting func(gameID string) bool
+	// SyncWritingNow reports whether a sync is writing the game at this
+	// moment: the watcher looks at the burst once it has finished. May be nil.
+	SyncWritingNow func(gameID string) bool
+	// HoldSnapshot reports whether a change should not be snapshotted now:
+	// the game is being played, and a checkpoint was taken recently. The
+	// change is not recorded as captured either, so the next one, or the end
+	// of the session, still sees it. May be nil.
+	HoldSnapshot func(gameID string) bool
+}
+
+// syncWriting reports whether a sync is or was just writing a game
+// (Callbacks.SyncWriting).
+func (e *Engine) syncWriting(gameID string) bool {
+	return e.cb.SyncWriting != nil && e.cb.SyncWriting(gameID)
+}
+
+// syncWritingNow reports whether a sync is writing a game at this moment
+// (Callbacks.SyncWritingNow).
+func (e *Engine) syncWritingNow(gameID string) bool {
+	return e.cb.SyncWritingNow != nil && e.cb.SyncWritingNow(gameID)
 }
 
 // Engine owns one watch goroutine per tracked game.
@@ -188,6 +213,20 @@ type gameWatch struct {
 	// registerFolders when an Add fails.
 	rewatch atomic.Bool
 
+	// What the burst since the last snapshot check was made of: changes
+	// OpenSave applied itself (owntouch), anything else, or both. Read and
+	// written only by the event loop.
+	ownEvents, otherEvents bool
+	// pendingChange is set while a change made here (not by a sync) waits
+	// out the debounce, for ChangePending: read from other goroutines.
+	pendingChange atomic.Bool
+	// foreignFolders: a folder queued for watching since the last rescan was
+	// not one OpenSave created, or the watch started on a busy folder. The
+	// rescan that follows registration counts as someone else's change only
+	// then — a pull creates folders and writes into them before they are
+	// watched, and that is still the pull. Event loop only.
+	foreignFolders bool
+
 	// folders carries directories to put under watch to registerFolders. See
 	// there for why the event loop does not register them itself.
 	folders chan string
@@ -450,6 +489,11 @@ func (e *Engine) WatchWithLocations(gameID, savePath string, extra map[string]st
 	}
 	if busy {
 		gw.rewatch.Store(true)
+		// What the folder holds is nobody's change OpenSave knows of: this
+		// rescan must not record it as its own, or a change made while the
+		// app was closed would become the baseline before the catch-up
+		// below could see it. Set before the event loop starts.
+		gw.foreignFolders = true
 		gw.rescan <- struct{}{} // buffered and empty: cannot block
 	}
 	e.games[gameID] = gw
@@ -507,6 +551,15 @@ func (e *Engine) Unwatch(gameID string) {
 	}
 }
 
+// ChangePending reports whether a change made here to a game's save is
+// waiting out the debounce: seen, and not yet judged.
+func (e *Engine) ChangePending(gameID string) bool {
+	e.mu.Lock()
+	gw, ok := e.games[gameID]
+	e.mu.Unlock()
+	return ok && gw.pendingChange.Load()
+}
+
 // Stop shuts down every watch goroutine.
 func (e *Engine) Stop() {
 	if e.stopCatch != nil {
@@ -676,10 +729,20 @@ func (e *Engine) run(ctx context.Context, gw *gameWatch) {
 			if !gw.eventRelevant(event) {
 				continue
 			}
+			own := owntouch.Recent(event.Name)
+			if own {
+				gw.ownEvents = true
+			} else {
+				gw.otherEvents = true
+				gw.pendingChange.Store(true)
+			}
 			// New subdirectory in directory mode: extend the watch — on
 			// registerFolders, never here. See registerFolders.
 			if !gw.isFile && event.Has(fsnotify.Create) {
 				if info, err := os.Stat(event.Name); err == nil && info.IsDir() {
+					if !own {
+						gw.foreignFolders = true
+					}
 					gw.queueFolder(event.Name)
 				}
 			}
@@ -689,7 +752,15 @@ func (e *Engine) run(ctx context.Context, gw *gameWatch) {
 			// Folders just came under watch, or the watch started on a folder
 			// that was busy. Anything written into a folder before its watch
 			// existed raised no event of its own, so read the tree again once
-			// things are quiet.
+			// things are quiet. Whose change that is follows from whose
+			// folders they were: what OpenSave created, it also filled.
+			if gw.foreignFolders {
+				gw.otherEvents = true
+				gw.pendingChange.Store(true)
+			} else {
+				gw.ownEvents = true
+			}
+			gw.foreignFolders = false
 			resetDebounce()
 
 		case err, ok := <-gw.fsw.Errors:
@@ -718,6 +789,17 @@ func (e *Engine) run(ctx context.Context, gw *gameWatch) {
 				// directory and allocates no second buffer for one it already
 				// has.
 				gw.rewatch.Store(true)
+				// The missed events could be anyone's — unless a sync is
+				// writing this game, which is what overflows the queue: a
+				// pull of a save of many files does, again and again, and
+				// each was taken for the game saving, an automatic snapshot
+				// of a half-pulled save every few seconds.
+				if e.syncWriting(gw.gameID) {
+					gw.ownEvents = true
+				} else {
+					gw.otherEvents = true
+					gw.pendingChange.Store(true)
+				}
 			} else {
 				e.log("warn", fmt.Sprintf("watching %q: %v", gw.gameID, err))
 				continue
@@ -727,6 +809,12 @@ func (e *Engine) run(ctx context.Context, gw *gameWatch) {
 		case <-debounceC:
 			debounce = nil
 			debounceC = nil
+			// A sync still writing this game: look at the burst once it has
+			// finished, not at a save half-way between two states.
+			if e.syncWritingNow(gw.gameID) {
+				resetDebounce()
+				continue
+			}
 			// Re-register, so a folder that went unwatched is watched from
 			// here on, and read the tree below, so it is found now. Doing only
 			// one of those leaves it correct today and silent tomorrow.
@@ -758,7 +846,10 @@ func (e *Engine) run(ctx context.Context, gw *gameWatch) {
 			for _, path := range gw.extra {
 				delta.InvalidateRoot(path)
 			}
-			e.handleChange(ctx, gw)
+			ownOnly := gw.ownEvents && !gw.otherEvents
+			gw.ownEvents, gw.otherEvents = false, false
+			gw.pendingChange.Store(false)
+			e.handleChangeFrom(ctx, gw, ownOnly)
 		}
 	}
 }
@@ -775,6 +866,12 @@ func (gw *gameWatch) eventRelevant(event fsnotify.Event) bool {
 	if strings.HasSuffix(name, ".opensave.tmp") {
 		return false
 	}
+	// A file the launcher rewrites on its own (Steam's remotecache.vdf, on
+	// every start) is not a save: it is never synced or snapshotted, so it
+	// does not wake the watcher either.
+	if delta.NeverSynced(name) {
+		return false
+	}
 	if gw.isFile {
 		return strings.EqualFold(
 			filepath.Clean(event.Name),
@@ -788,6 +885,22 @@ func (gw *gameWatch) eventRelevant(event fsnotify.Event) bool {
 // (gameplay guard), skip if content is unchanged since the last
 // auto-snapshot, then snapshot with retries and notify.
 func (e *Engine) handleChange(ctx context.Context, gw *gameWatch) {
+	e.handleChangeFrom(ctx, gw, false)
+}
+
+// handleChangeFrom is handleChange told whether every change in the burst was
+// one OpenSave applied itself — a pull, a deletion a peer asked for, mtimes a
+// conflict resolution touched (owntouch).
+//
+// Those are not a new save. They are a sync, which keeps its own history
+// (the mirror snapshot of a pull, the safety snapshot before files are
+// replaced). Snapshotting them as well took a full auto-snapshot every couple
+// of seconds while a peer deleted files one request at a time, each of a save
+// half-way between two states, until they had filled the retention budget and
+// pushed out the snapshots worth keeping. So the burst only moves the recorded
+// hash, which keeps the next real change measured against what is actually on
+// disk; anything not known to be ours is snapshotted as before.
+func (e *Engine) handleChangeFrom(ctx context.Context, gw *gameWatch, ownOnly bool) {
 	// Gameplay guard: the game may still be mid-write.
 	for anyFileLocked(gw.savePath) {
 		e.log("info", fmt.Sprintf("save files for %q are in use; waiting (gameplay guard)", gw.gameID))
@@ -821,6 +934,15 @@ func (e *Engine) handleChange(ctx context.Context, gw *gameWatch) {
 		e.log("info", fmt.Sprintf("no content change for %q; skipping auto-snapshot", gw.gameID))
 		return
 	}
+	if ownOnly {
+		if err := e.cb.SetLastManifestHash(gw.gameID, currentHash); err != nil {
+			e.log("warn", fmt.Sprintf("failed to record manifest hash for %q: %v", gw.gameID, err))
+		}
+		return
+	}
+	if e.cb.HoldSnapshot != nil && e.cb.HoldSnapshot(gw.gameID) {
+		return // being played: kept by the session (a checkpoint, or its end)
+	}
 
 	for attempt := 1; attempt <= snapshotMaxRetries; attempt++ {
 		err = e.cb.CreateSnapshot(gw.gameID)
@@ -1032,23 +1154,54 @@ func (e *Engine) log(level, msg string) {
 // against that recorded value, so it has to compute it this way — one
 // definition, or the question gets two answers.
 func ContentHash(m delta.Manifest, ignoreRules string) string {
-	if rules := ignore.Parse(ignoreRules); !rules.Empty() {
-		m = filterForHash(m, rules)
+	rules := ignore.Parse(ignoreRules)
+	leaveOut := func(p string) bool {
+		return delta.NeverSynced(p) || (!rules.Empty() && rules.Match(p))
+	}
+	if rules.Empty() && !leavesAnyOut(m, leaveOut) {
+		return m.ContentHash() // the same value, without the copy
+	}
+	return filterForHash(m, leaveOut).ContentHash()
+}
+
+func leavesAnyOut(m delta.Manifest, leaveOut func(string) bool) bool {
+	for p := range m.Files {
+		if leaveOut(p) {
+			return true
+		}
+	}
+	for _, root := range m.Extra {
+		for p := range root.Files {
+			if leaveOut(p) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// ContentHashBeforeNeverSynced is ContentHash as builds before
+// delta.NeverSynced took it, with those files counted: what a value recorded
+// by one of them is compared with, to tell whether the save changed since.
+func ContentHashBeforeNeverSynced(m delta.Manifest, ignoreRules string) string {
+	rules := ignore.Parse(ignoreRules)
+	if rules.Empty() {
+		return m.ContentHash()
 	}
-	return m.ContentHash()
+	return filterForHash(m, rules.Match).ContentHash()
 }
 
 // filterForHash drops excluded paths before the content hash is taken, so the
 // value recorded here means the same thing the sync engine means by it.
-func filterForHash(m delta.Manifest, rules ignore.Rules) delta.Manifest {
+func filterForHash(m delta.Manifest, leaveOut func(string) bool) delta.Manifest {
 	out := delta.Manifest{Files: make(map[string]delta.FileEntry, len(m.Files))}
 	for p, entry := range m.Files {
-		if !rules.Match(p) {
+		if !leaveOut(p) {
 			out.Files[p] = entry
 		}
 	}
 	for _, d := range m.Dirs {
-		if !rules.Match(d) {
+		if !leaveOut(d) {
 			out.Dirs = append(out.Dirs, d)
 		}
 	}
@@ -1057,12 +1210,12 @@ func filterForHash(m delta.Manifest, rules ignore.Rules) delta.Manifest {
 		for name, root := range m.Extra {
 			sub := delta.RootManifest{Files: make(map[string]delta.FileEntry, len(root.Files))}
 			for p, entry := range root.Files {
-				if !rules.Match(p) {
+				if !leaveOut(p) {
 					sub.Files[p] = entry
 				}
 			}
 			for _, d := range root.Dirs {
-				if !rules.Match(d) {
+				if !leaveOut(d) {
 					sub.Dirs = append(sub.Dirs, d)
 				}
 			}
diff --git a/testutil/harness.go b/testutil/harness.go
index 9323e7b4..c5d7d043 100644
--- a/testutil/harness.go
+++ b/testutil/harness.go
@@ -112,7 +112,7 @@ func NewTestDaemon(t *testing.T, name string) *TestDaemon {
 	if err := os.MkdirAll(home, 0o777); err != nil {
 		t.Fatal(err)
 	}
-	d, err := daemon.New(daemon.Options{HomeOverride: home, DisableDiscovery: true})
+	d, err := daemon.New(daemon.Options{HomeOverride: home, DisableDiscovery: true, SyncEveryChange: true})
 	if err != nil {
 		t.Fatalf("daemon.New(%s): %v", name, err)
 	}
@@ -175,6 +175,13 @@ func NewTestDaemon(t *testing.T, name string) *TestDaemon {
 	}
 
 	td := &TestDaemon{T: t, Daemon: d, Server: srv, Addr: addr, Port: port, SaveDir: saveDir}
+	t.Cleanup(func() {
+		if t.Failed() {
+			for _, e := range d.Log.History() {
+				t.Logf("[%s] %s %s", name, e.Level, e.Message)
+			}
+		}
+	})
 	t.Cleanup(func() {
 		srv.Stop()
 		d.Stop()