diff --git a/.changelog/6049-fix-activity-graph-catch-up.md b/.changelog/6049-fix-activity-graph-catch-up.md
new file mode 100644
index 0000000000..cc1507ed4d
--- /dev/null
+++ b/.changelog/6049-fix-activity-graph-catch-up.md
@@ -0,0 +1,3 @@
+### Fixed
+
+- Fixed development builds throwing when a React 19.2 `<Activity>` shows Web Chat again, in PR [#6049](https://github.com/microsoft/BotFramework-WebChat/pull/6049), by [@OEvgeny](https://github.com/OEvgeny)
diff --git a/__tests__/html2/react19/activity.hideShow.transcript.html b/__tests__/html2/react19/activity.hideShow.transcript.html
new file mode 100644
index 0000000000..1f45e9cc0a
--- /dev/null
+++ b/__tests__/html2/react19/activity.hideShow.transcript.html
@@ -0,0 +1,98 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <title>React 19: Web Chat keeps its transcript when an &lt;Activity&gt; hides and shows it</title>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script type="importmap">
+      {
+        "imports": {
+          "@testduet/wait-for": "https://esm.sh/@testduet/wait-for",
+          "botframework-webchat": "/__dist__/packages/bundle/static/botframework-webchat.js",
+          "react": "https://esm.sh/react@19.3.1?dev",
+          "react-dom": "https://esm.sh/react-dom@19.3.1?dev",
+          "react-dom/client": "https://esm.sh/react-dom@19.3.1/client?dev"
+        }
+      }
+    </script>
+    <script type="module">
+      import '/test-harness.mjs';
+      import '/test-page-object.mjs';
+
+      import { waitFor } from '@testduet/wait-for';
+      import { createStoreWithOptions, ReactWebChat, testIds } from 'botframework-webchat';
+      import { Activity, createElement, version } from 'react';
+      import { createRoot } from 'react-dom/client';
+
+      run(async function () {
+        // TODO: This is for `createDirectLineEmulator` and `pageElements` only, should find ways to eliminate this line.
+        window.WebChat = { createStoreWithOptions, testIds };
+
+        const { directLine, store } = testHelpers.createDirectLineEmulator();
+
+        // GIVEN: React 19.2 or later: a hidden <Activity> runs the cleanup of the effects of its children, and keeps their state.
+        expect(version).toBe('19.3.1');
+
+        const root = createRoot(document.getElementById('webchat'));
+        // A new element each time, as JSX does: showing Web Chat again renders it before its effects run again.
+        const render = mode =>
+          root.render(
+            createElement(
+              Activity,
+              { mode },
+              createElement(ReactWebChat, { directLine, store, styleOptions: { hideSendBox: true } })
+            )
+          );
+
+        // WHEN: Web Chat is rendered in a visible <Activity> and the bot sends a message.
+        render('visible');
+
+        await pageConditions.uiConnected();
+        await directLine.emulateIncomingActivity('Hello, World!');
+        await pageConditions.numActivitiesShown(1);
+        await pageConditions.scrollStabilized();
+
+        const [activityElement] = pageElements.activities();
+
+        // THEN: Web Chat shows the message.
+        await host.snapshot('local');
+
+        // WHEN: The <Activity> hides Web Chat.
+        render('hidden');
+
+        await waitFor(() => expect(activityElement).toHaveProperty('offsetParent', null));
+
+        // WHEN: The host adds a message to the store while Web Chat is hidden.
+        // Not through the Direct Line emulator: a hidden Web Chat is disconnected, as when unmounted.
+        store.dispatch({
+          payload: {
+            activity: {
+              from: { id: 'bot', role: 'bot' },
+              id: 'a-00002',
+              text: 'Added while hidden.',
+              timestamp: new Date().toISOString(),
+              type: 'message'
+            }
+          },
+          type: 'DIRECT_LINE/INCOMING_ACTIVITY'
+        });
+
+        // WHEN: The <Activity> shows Web Chat again.
+        render('visible');
+
+        // THEN: Web Chat shows the element of the first message, which it kept while hidden, and the message added while hidden.
+        // No snapshot: after it is shown again, Web Chat loses some styles (React 19.2 <Activity> support is not complete yet).
+        await waitFor(() => expect(activityElement).not.toHaveProperty('offsetParent', null));
+        await pageConditions.numActivitiesShown(2);
+
+        expect(pageElements.activities()[0]).toBe(activityElement);
+        expect(pageElements.activityContents().map(({ textContent }) => textContent)).toEqual([
+          'Hello, World!',
+          'Added while hidden.'
+        ]);
+      });
+    </script>
+  </body>
+</html>
diff --git a/__tests__/html2/react19/activity.hideShow.transcript.html.snap-1.png b/__tests__/html2/react19/activity.hideShow.transcript.html.snap-1.png
new file mode 100644
index 0000000000..041fcd2500
Binary files /dev/null and b/__tests__/html2/react19/activity.hideShow.transcript.html.snap-1.png differ
diff --git a/__tests__/html2/react19/activity.hideShow.transcript.sameElement.html b/__tests__/html2/react19/activity.hideShow.transcript.sameElement.html
new file mode 100644
index 0000000000..4d8debae0f
--- /dev/null
+++ b/__tests__/html2/react19/activity.hideShow.transcript.sameElement.html
@@ -0,0 +1,92 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <title>React 19: Web Chat shows activities added while an &lt;Activity&gt; hid it, without rendering on show</title>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script type="importmap">
+      {
+        "imports": {
+          "@testduet/wait-for": "https://esm.sh/@testduet/wait-for",
+          "botframework-webchat": "/__dist__/packages/bundle/static/botframework-webchat.js",
+          "react": "https://esm.sh/react@19.3.1?dev",
+          "react-dom": "https://esm.sh/react-dom@19.3.1?dev",
+          "react-dom/client": "https://esm.sh/react-dom@19.3.1/client?dev"
+        }
+      }
+    </script>
+    <script type="module">
+      import '/test-harness.mjs';
+      import '/test-page-object.mjs';
+
+      import { waitFor } from '@testduet/wait-for';
+      import { createStoreWithOptions, ReactWebChat, testIds } from 'botframework-webchat';
+      import { Activity, createElement, version } from 'react';
+      import { createRoot } from 'react-dom/client';
+
+      run(async function () {
+        // TODO: This is for `createDirectLineEmulator` and `pageElements` only, should find ways to eliminate this line.
+        window.WebChat = { createStoreWithOptions, testIds };
+
+        const { directLine, store } = testHelpers.createDirectLineEmulator();
+
+        // GIVEN: React 19.2 or later: a hidden <Activity> runs the cleanup of the effects of its children, and keeps their state.
+        expect(version).toBe('19.3.1');
+
+        const root = createRoot(document.getElementById('webchat'));
+        // The same element each time: showing Web Chat again does not render it, only its effects run again.
+        const webChat = createElement(ReactWebChat, { directLine, store, styleOptions: { hideSendBox: true } });
+        const render = mode => root.render(createElement(Activity, { mode }, webChat));
+
+        // WHEN: Web Chat is rendered in a visible <Activity> and the bot sends a message.
+        render('visible');
+
+        await pageConditions.uiConnected();
+        await directLine.emulateIncomingActivity('Hello, World!');
+        await pageConditions.numActivitiesShown(1);
+        await pageConditions.scrollStabilized();
+
+        const [activityElement] = pageElements.activities();
+
+        // THEN: Web Chat shows the message.
+        await host.snapshot('local');
+
+        // WHEN: The <Activity> hides Web Chat.
+        render('hidden');
+
+        await waitFor(() => expect(activityElement).toHaveProperty('offsetParent', null));
+
+        // WHEN: The host adds a message to the store while Web Chat is hidden.
+        // Not through the Direct Line emulator: a hidden Web Chat is disconnected, as when unmounted.
+        store.dispatch({
+          payload: {
+            activity: {
+              from: { id: 'bot', role: 'bot' },
+              id: 'a-00002',
+              text: 'Added while hidden.',
+              timestamp: new Date().toISOString(),
+              type: 'message'
+            }
+          },
+          type: 'DIRECT_LINE/INCOMING_ACTIVITY'
+        });
+
+        // WHEN: The <Activity> shows Web Chat again.
+        render('visible');
+
+        // THEN: Web Chat shows the element of the first message, which it kept while hidden, and the message added while hidden.
+        // No snapshot: after it is shown again, Web Chat loses some styles (React 19.2 <Activity> support is not complete yet).
+        await waitFor(() => expect(activityElement).not.toHaveProperty('offsetParent', null));
+        await pageConditions.numActivitiesShown(2);
+
+        expect(pageElements.activities()[0]).toBe(activityElement);
+        expect(pageElements.activityContents().map(({ textContent }) => textContent)).toEqual([
+          'Hello, World!',
+          'Added while hidden.'
+        ]);
+      });
+    </script>
+  </body>
+</html>
diff --git a/__tests__/html2/react19/activity.hideShow.transcript.sameElement.html.snap-1.png b/__tests__/html2/react19/activity.hideShow.transcript.sameElement.html.snap-1.png
new file mode 100644
index 0000000000..041fcd2500
Binary files /dev/null and b/__tests__/html2/react19/activity.hideShow.transcript.sameElement.html.snap-1.png differ
diff --git a/packages/core/src/graph/createGraphFromStore.spec.ts b/packages/core/src/graph/createGraphFromStore.spec.ts
index 00f7362666..ced808113a 100644
--- a/packages/core/src/graph/createGraphFromStore.spec.ts
+++ b/packages/core/src/graph/createGraphFromStore.spec.ts
@@ -15,7 +15,7 @@ const incoming = (id: string) => ({
 
 type RawJSON = readonly { readonly '@value': { readonly id: string } }[] | undefined;
 
-function activityIdsOf(graph: ReturnType<typeof createGraphFromStore>): readonly string[] {
+function activityIdsOf(graph: Pick<ReturnType<typeof createGraphFromStore>, 'getState'>): readonly string[] {
   return Array.from(graph.getState().values()).flatMap(
     node =>
       (node['urn:microsoft:webchat:direct-line-activity:raw-json'] as RawJSON)?.map(({ '@value': { id } }) => id) ?? []
@@ -83,17 +83,15 @@ scenario('createGraphFromStore()', bdd => {
 
       return outcome;
     })
-    .then('the graph should not follow the store anymore', ({ subscriptions }, { graph, subscriber1 }) => {
+    .then('the graph should not follow the store anymore', ({ subscriptions }, { subscriber1 }) => {
+      // Does not read the graph: a read would catch up with the store, before the subscription below does.
       expect(subscriptions.size).toBe(0);
-      expect(activityIdsOf(graph)).toEqual(['a-1', 'a-2']);
       expect(subscriber1).toHaveBeenCalledTimes(1);
     })
     .when('a subscriber subscribes again', (_, { graph }) => {
       const subscriber = jest.fn();
 
-      graph.subscribe(subscriber);
-
-      return { graph, subscriber };
+      return { graph, subscriber, unsubscribe: graph.subscribe(subscriber) };
     })
     .then(
       'the graph should catch up with the store and notify the subscriber',
@@ -102,5 +100,53 @@ scenario('createGraphFromStore()', bdd => {
         expect(subscriber).toHaveBeenCalledTimes(1);
         expect(subscriptions.size).toBe(1);
       }
+    )
+    // A hidden <Activity> (React 19.2+) runs the cleanup of effects, so the graph has no subscribers. When the host shows
+    // Web Chat again with a new element, it renders and reads the graph before its effects subscribe again.
+    .when('the subscriber unsubscribes, then an activity is received', ({ store }, outcome) => {
+      outcome.unsubscribe();
+      store.dispatch(incoming('a-4'));
+
+      return outcome;
+    })
+    .then(
+      'reading the graph should catch up with the store, without subscribing to it',
+      ({ subscriptions }, { graph, subscriber }) => {
+        expect(activityIdsOf(graph)).toEqual(['a-1', 'a-2', 'a-3', 'a-4']);
+        expect(subscriptions.size).toBe(0);
+        expect(subscriber).toHaveBeenCalledTimes(1);
+      }
+    )
+    .and('reading the graph again should return the same state', (_, { graph }) =>
+      expect(graph.getState()).toBe(graph.getState())
     );
+
+  bdd
+    .given('a graph without subscribers, created from a store with an activity', () => {
+      const store = createStore();
+
+      store.dispatch(incoming('a-1'));
+
+      return { graph: createGraphFromStore(store), store };
+    })
+    .when('an activity is received, then a transaction is made on the graph', ({ graph, store }) => {
+      let activityIdsInTransaction: readonly string[] = [];
+
+      store.dispatch(incoming('a-2'));
+
+      // The transaction reads the graph (auto-inversion) before the graph caught up with the store.
+      graph.act(graph => {
+        activityIdsInTransaction = activityIdsOf(graph);
+        graph.upsert({ '@id': '_:m1', '@type': 'MessageLike', text: 'Hello, World!' });
+      });
+
+      return { activityIdsInTransaction, graph };
+    })
+    .then('the transaction should see both activities', (_, { activityIdsInTransaction }) =>
+      expect(activityIdsInTransaction).toEqual(['a-1', 'a-2'])
+    )
+    .and('the graph should have both activities and the node', (_, { graph }) => {
+      expect(activityIdsOf(graph)).toEqual(['a-1', 'a-2']);
+      expect(graph.getState().has('_:m1')).toBe(true);
+    });
 });
diff --git a/packages/core/src/graph/createGraphFromStore.ts b/packages/core/src/graph/createGraphFromStore.ts
index 88e9d6b2b2..0847a87e82 100644
--- a/packages/core/src/graph/createGraphFromStore.ts
+++ b/packages/core/src/graph/createGraphFromStore.ts
@@ -7,7 +7,20 @@ import type { Activity } from '../reducers/activities/sort/types';
 
 function createGraphFromStore(store: ReturnType<typeof createStore>): SlantGraph {
   const graph = new SlantGraph();
+  const actOnGraph = graph.act.bind(graph);
   let prevActivities: readonly Activity[] | undefined;
+  let transacting = false;
+
+  // act() is not re-entrant: reads inside a transaction (e.g. by middleware) must not catch up with the store.
+  const transact: typeof graph.act = fn => {
+    transacting = true;
+
+    try {
+      actOnGraph(fn);
+    } finally {
+      transacting = false;
+    }
+  };
 
   // TODO: [P0] Except channel audience, we should be specific about all audience, e.g. name.
   const channelAudience = parse(SlantNodeSchema, {
@@ -46,7 +59,7 @@ function createGraphFromStore(store: ReturnType<typeof createStore>): SlantGraph
     // TODO: [P*] Supports deleting node from the graph.
     const addedActivities = activitySet.difference(prevActivitySet);
 
-    graph.act(graph => {
+    transact(graph => {
       for (const activity of addedActivities) {
         const {
           from: { role }
@@ -121,9 +134,24 @@ function createGraphFromStore(store: ReturnType<typeof createStore>): SlantGraph
   // Follows the store only while the graph has subscribers: a graph nobody subscribes to (e.g. after unmount, or from a
   // render that never committed) does not stay subscribed to the store.
   const subscribeToGraph = graph.subscribe.bind(graph);
+  const getGraphState = graph.getState.bind(graph);
   const subscribers = new Set<GraphSubscriber>();
   let unsubscribeFromStore: (() => void) | undefined;
 
+  // Catches up with changes made while the graph has no subscribers, e.g. when a hidden <Activity> (React 19.2+) ran the
+  // cleanup of effects and renders before subscribing again. Returns the same state until the store activities change.
+  graph.getState = () => {
+    unsubscribeFromStore || transacting || handleStoreChange();
+
+    return getGraphState();
+  };
+
+  // Catches up before a transaction, so its middleware sees the activities, e.g. to link nodes to them.
+  graph.act = fn => {
+    unsubscribeFromStore || handleStoreChange();
+    transact(fn);
+  };
+
   graph.subscribe = subscriber => {
     const unsubscribe = subscribeToGraph(subscriber);