diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 495f25d486..e0616946e5 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -2984,7 +2984,10 @@ export async function handleChatCore({
 
   const dedupRequestBody = { ...translatedBody, model: `${provider}/${model}`, stream };
   const dedupEnabled = shouldDeduplicate(dedupRequestBody);
-  const dedupHash = dedupEnabled ? computeRequestHash(dedupRequestBody) : null;
+  // Namespaced by the calling API key: dedup hands the SAME response object to
+  // every joiner, so a shared hash across keys is a cross-principal response
+  // leak (GHSA-6c7w-56xp-wpc6).
+  const dedupHash = dedupEnabled ? computeRequestHash(dedupRequestBody, apiKeyInfo?.id) : null;
 
   const executeProviderRequest = async (modelToCall = effectiveModel, allowDedup = false) => {
     const execute = async () => {
diff --git a/open-sse/services/requestDedup.ts b/open-sse/services/requestDedup.ts
index 1a39216197..b09b501164 100644
--- a/open-sse/services/requestDedup.ts
+++ b/open-sse/services/requestDedup.ts
@@ -129,8 +129,26 @@ function extractSystemContent(body: Record<string, unknown>): unknown {
  * `translatedBody`), so the body shape here is whatever the target provider
  * format produced — see `extractPromptContent`/`extractSystemContent` for the
  * full list of shapes this must cover (#10249, #10438).
+ *
+ * `tenantId` (the calling API key's id) namespaces the hash. Dedup shares ONE
+ * upstream call, and therefore one response, between everyone landing on the
+ * same hash — so the hash has to answer "who is asking", not just "what is
+ * being asked". Without it, two distinct API keys issuing the same request
+ * joined the same in-flight promise: the response was produced with the
+ * initiator's provider connection, under the initiator's per-key policy
+ * (allowedConnections / allowedModels), billed to the initiator, and handed to
+ * a different authenticated principal (GHSA-6c7w-56xp-wpc6).
+ *
+ * It is a PLAINTEXT prefix rather than digest input, matching
+ * `semanticCache.generateSignature` (#3740): the id is an internal namespace
+ * key, not a credential, and keeping it out of the digest avoids the
+ * false-positive CodeQL js/insufficient-password-hash on a cache/dedup key.
+ *
+ * Omitting `tenantId` keeps the un-namespaced hash. Keyless local-first
+ * deployments have no tenant boundary to preserve, and every such install would
+ * otherwise silently lose dedup.
  */
-export function computeRequestHash(requestBody: unknown): string {
+export function computeRequestHash(requestBody: unknown, tenantId?: string | null): string {
   const body = requestBody as Record<string, unknown>;
   const canonical = {
     model: body.model ?? null,
@@ -145,7 +163,8 @@ export function computeRequestHash(requestBody: unknown): string {
     frequency_penalty: body.frequency_penalty ?? null,
     presence_penalty: body.presence_penalty ?? null,
   };
-  return createHash("sha256").update(JSON.stringify(canonical)).digest("hex").slice(0, 16);
+  const digest = createHash("sha256").update(JSON.stringify(canonical)).digest("hex").slice(0, 16);
+  return tenantId ? `${tenantId}.${digest}` : digest;
 }
 
 /** Determine whether a request should be deduplicated */
diff --git a/tests/unit/request-dedup-tenant-isolation.test.ts b/tests/unit/request-dedup-tenant-isolation.test.ts
new file mode 100644
index 0000000000..15f4e5b10f
--- /dev/null
+++ b/tests/unit/request-dedup-tenant-isolation.test.ts
@@ -0,0 +1,97 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import {
+  computeRequestHash,
+  deduplicate,
+  clearInflight,
+} from "../../open-sse/services/requestDedup.ts";
+
+// GHSA-6c7w-56xp-wpc6 follow-up. The dedup layer shares ONE upstream call — and
+// therefore one response object — between every concurrent caller landing on the
+// same hash. The hash canonicalized model + prompt + sampling params and nothing
+// about WHO was asking, so two distinct OmniRoute API keys issuing the same
+// request joined the same in-flight promise: the response was produced with the
+// initiator's provider connection, under the initiator's per-key policy, and
+// handed to a different authenticated principal.
+//
+// #10438 fixed the *prompt* half of this class (translated bodies hashing the
+// prompt as `null`). This is the *identity* half.
+
+const body = {
+  messages: [{ role: "user", content: "Summarize the incident report." }],
+  temperature: 0,
+  model: "codex/gpt-5.6-terra",
+  stream: false,
+};
+
+test("the same request from two different API keys does NOT share a dedup hash", () => {
+  const hashKey1 = computeRequestHash(body, "apikey-1");
+  const hashKey2 = computeRequestHash(body, "apikey-2");
+  assert.notEqual(
+    hashKey1,
+    hashKey2,
+    "an identical request from a different API key must not join the first key's in-flight call"
+  );
+});
+
+test("the same request from the same API key still dedups", () => {
+  assert.equal(computeRequestHash(body, "apikey-1"), computeRequestHash(body, "apikey-1"));
+});
+
+test("concurrent identical requests on two keys each get their own response", async () => {
+  clearInflight();
+  const hash1 = computeRequestHash(body, "apikey-1");
+  const hash2 = computeRequestHash(body, "apikey-2");
+
+  let released!: () => void;
+  const gate = new Promise<void>((resolve) => {
+    released = resolve;
+  });
+
+  const first = deduplicate(hash1, async () => {
+    await gate;
+    return "RESPONSE_FOR_KEY_1";
+  });
+  const second = deduplicate(hash2, async () => {
+    await gate;
+    return "RESPONSE_FOR_KEY_2";
+  });
+  released();
+
+  const [a, b] = await Promise.all([first, second]);
+  assert.equal(a.result, "RESPONSE_FOR_KEY_1");
+  assert.equal(b.result, "RESPONSE_FOR_KEY_2");
+  assert.equal(b.wasDeduplicated, false, "key 2 must not have joined key 1's in-flight call");
+});
+
+test("an unkeyed (anonymous) caller keeps the un-namespaced hash", () => {
+  // Keyless local-first deployments have no tenant boundary to preserve, so the
+  // behaviour there is unchanged — and must stay stable, or every such install
+  // silently loses dedup.
+  const anonymous = computeRequestHash(body);
+  assert.equal(computeRequestHash(body, undefined), anonymous);
+  assert.equal(computeRequestHash(body, null as unknown as undefined), anonymous);
+  assert.notEqual(computeRequestHash(body, "apikey-1"), anonymous);
+});
+
+test("the tenant namespace cannot be forged by a colliding request body", () => {
+  // The namespace is a plaintext prefix, so it must not be possible to move
+  // between namespaces by crafting a body — the separator has to survive.
+  const h1 = computeRequestHash(body, "a");
+  const h2 = computeRequestHash(body, "a.b");
+  assert.notEqual(h1, h2);
+  assert.ok(h1.startsWith("a."), "namespace must prefix the digest");
+});
+
+test("chatCore passes the caller's API key id into the dedup hash", async () => {
+  const { readFileSync } = await import("node:fs");
+  const { fileURLToPath } = await import("node:url");
+  const source = readFileSync(
+    fileURLToPath(new URL("../../open-sse/handlers/chatCore.ts", import.meta.url)),
+    "utf8"
+  );
+  assert.ok(
+    /computeRequestHash\(\s*dedupRequestBody\s*,\s*apiKeyInfo\?\.id/.test(source),
+    "the dedup hash must be namespaced by the calling API key (GHSA-6c7w-56xp-wpc6)"
+  );
+});