diff --git a/lib/federation.ts b/lib/federation.ts
index aca388f..54ebbe3 100644
--- a/lib/federation.ts
+++ b/lib/federation.ts
@@ -19,7 +19,7 @@
 // for the transport — a real deployment swaps in a second connection
 // (data-level federation) or an MCP client to a remote cbio-lab
 // (document-level federation) behind the same interface.
-import { chQuery, DB, PRIMARY, connections, loopbackOnlyConn, isAbortError, LoopbackDirectError, type ChConn } from "./ch.ts";
+import { chQuery, DB, PRIMARY, connections, loopbackOnlyConn, isAbortError, LoopbackDirectError, withChContext, type ChConn } from "./ch.ts";
 import { sq } from "./compiler.ts";
 import { canonicalEventName, nameFoldSql, foldedEqualsSql } from "./event-names.ts";
 import { linesCtes, lineGapOf, inLineClassesSql, TREATMENT_EVENT } from "./treatment-lines.ts";
@@ -183,6 +183,15 @@ export const routingCollisions: string[] = [];
 export const restrictedStudies = new Set<string>();
 export async function studyRouting(): Promise<Map<string, ChConn>> {
   if (_routing) return _routing;
+  // One build at a time, shared by every caller that arrives while it runs: a cold request asks
+  // for hundreds of studies' routes at once, and each starting its own build sent hundreds of
+  // queries (and, with a simulated peer, hundreds of MCP calls) for one answer. The build answers
+  // them all, so no one reader's cancellation stops it.
+  return _routingBuild ??= withChContext({}, buildRouting).finally(() => { _routingBuild = null; });
+}
+let _routingBuild: Promise<Map<string, ChConn>> | null = null;
+
+async function buildRouting(): Promise<Map<string, ChConn>> {
   const map = new Map<string, ChConn>();
   let complete = true;
   for (const conn of connections()) {
diff --git a/test/routing.test.ts b/test/routing.test.ts
new file mode 100644
index 0000000..f2fdd77
--- /dev/null
+++ b/test/routing.test.ts
@@ -0,0 +1,31 @@
+// The routing table (lib/federation.ts studyRouting) is built once however many callers ask for
+// it at once: a cold /api/studies looks up every study's route together, and each lookup starting
+// its own build sent hundreds of queries — over a simulated peer, hundreds of MCP calls.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { studyRouting } from "../lib/federation.ts";
+
+test("concurrent lookups on a cold process share one routing build", async () => {
+  const env = { LOOPBACK_PEER: "b", LOOPBACK_PEER_URL: "http://loop.invalid/api/mcp", CLICKHOUSE2_URL: "http://ch.invalid", CLICKHOUSE2_USER: "u",
+    CLICKHOUSE2_PASSWORD: "p", CLICKHOUSE2_DB: "peer_db", CLICKHOUSE2_LABEL: "PEER" };
+  const saved = Object.fromEntries(Object.keys(env).map((k) => [k, process.env[k]]));
+  Object.assign(process.env, env);
+  const realFetch = globalThis.fetch;
+  let studyLists = 0;
+  globalThis.fetch = (async (_url: string, init: RequestInit) => {
+    const body = JSON.parse(String(init.body));
+    if (body.method === "tools/call" && body.params.name === "federated_studies") studyLists++;
+    await new Promise((r) => setTimeout(r, 20));   // an answer takes a moment, so the callers overlap
+    const result = body.method === "tools/call" ? { structuredContent: { studies: [{ id: "s1", name: "S1" }] } } : {};
+    return new Response(JSON.stringify({ jsonrpc: "2.0", id: body.id, result }), { headers: { "content-type": "application/json" } });
+  }) as typeof fetch;
+  try {
+    const maps = await Promise.all(Array.from({ length: 50 }, () => studyRouting()));
+    assert.equal(studyLists, 1, "one build asked the peer for its studies once, not once per caller");
+    assert.ok(maps.every((m) => m === maps[0]), "every caller got the same answer");
+    assert.equal(maps[0].get("s1")?.id, "b");
+  } finally {
+    globalThis.fetch = realFetch;
+    for (const [k, v] of Object.entries(saved)) { if (v == null) delete process.env[k]; else process.env[k] = v; }
+  }
+});