diff --git a/extensions/googlechat/src/auth.ts b/extensions/googlechat/src/auth.ts
index 9845aefb043..6454e88bda1 100644
--- a/extensions/googlechat/src/auth.ts
+++ b/extensions/googlechat/src/auth.ts
@@ -1,4 +1,5 @@
 import { GoogleAuth, OAuth2Client } from "google-auth-library";
+import { fetchWithSsrFGuard } from "../runtime-api.js";
 import type { ResolvedGoogleChatAccount } from "./accounts.js";
 
 const CHAT_SCOPE = "https://www.googleapis.com/auth/chat.bot";
@@ -83,13 +84,20 @@ async function fetchChatCerts(): Promise<Record<string, string>> {
   if (cachedCerts && now - cachedCerts.fetchedAt < 10 * 60 * 1000) {
     return cachedCerts.certs;
   }
-  const res = await fetch(CHAT_CERTS_URL);
-  if (!res.ok) {
-    throw new Error(`Failed to fetch Chat certs (${res.status})`);
+  const { response, release } = await fetchWithSsrFGuard({
+    url: CHAT_CERTS_URL,
+    auditContext: "googlechat.auth.certs",
+  });
+  try {
+    if (!response.ok) {
+      throw new Error(`Failed to fetch Chat certs (${response.status})`);
+    }
+    const certs = (await response.json()) as Record<string, string>;
+    cachedCerts = { fetchedAt: now, certs };
+    return certs;
+  } finally {
+    await release();
   }
-  const certs = (await res.json()) as Record<string, string>;
-  cachedCerts = { fetchedAt: now, certs };
-  return certs;
 }
 
 export type GoogleChatAudienceType = "app-url" | "project-number";
diff --git a/extensions/googlechat/src/targets.test.ts b/extensions/googlechat/src/targets.test.ts
index 00f29a98792..724cd8aa399 100644
--- a/extensions/googlechat/src/targets.test.ts
+++ b/extensions/googlechat/src/targets.test.ts
@@ -14,6 +14,7 @@ const mocks = vi.hoisted(() => ({
     response: await fetch(params.url, params.init),
     release: async () => {},
   })),
+  verifySignedJwtWithCertsAsync: vi.fn(),
   verifyIdToken: vi.fn(),
   getGoogleChatAccessToken: vi.fn().mockResolvedValue("token"),
 }));
@@ -28,6 +29,7 @@ vi.mock("google-auth-library", () => ({
   GoogleAuth: function GoogleAuth() {},
   OAuth2Client: class {
     verifyIdToken = mocks.verifyIdToken;
+    verifySignedJwtWithCertsAsync = mocks.verifySignedJwtWithCertsAsync;
   },
 }));
 
@@ -293,4 +295,34 @@ describe("verifyGoogleChatRequest", () => {
       reason: "unexpected add-on principal: principal-2",
     });
   });
+
+  it("fetches Chat certs through the guarded fetch for project-number tokens", async () => {
+    const release = vi.fn();
+    mocks.fetchWithSsrFGuard.mockClear();
+    mocks.fetchWithSsrFGuard.mockResolvedValueOnce({
+      response: new Response(JSON.stringify({ "kid-1": "cert-body" }), { status: 200 }),
+      release,
+    });
+    mocks.verifySignedJwtWithCertsAsync.mockReset().mockResolvedValue(undefined);
+
+    await expect(
+      verifyGoogleChatRequest({
+        bearer: "token",
+        audienceType: "project-number",
+        audience: "123456789",
+      }),
+    ).resolves.toEqual({ ok: true });
+
+    expect(mocks.fetchWithSsrFGuard).toHaveBeenCalledWith({
+      url: "https://www.googleapis.com/service_accounts/v1/metadata/x509/chat@system.gserviceaccount.com",
+      auditContext: "googlechat.auth.certs",
+    });
+    expect(mocks.verifySignedJwtWithCertsAsync).toHaveBeenCalledWith(
+      "token",
+      { "kid-1": "cert-body" },
+      "123456789",
+      ["chat@system.gserviceaccount.com"],
+    );
+    expect(release).toHaveBeenCalledOnce();
+  });
 });
diff --git a/extensions/matrix/src/matrix/sdk/transport.test.ts b/extensions/matrix/src/matrix/sdk/transport.test.ts
index 26095563c22..ab85767573b 100644
--- a/extensions/matrix/src/matrix/sdk/transport.test.ts
+++ b/extensions/matrix/src/matrix/sdk/transport.test.ts
@@ -8,6 +8,15 @@ function clearTestUndiciRuntimeDepsOverride(): void {
   Reflect.deleteProperty(globalThis as object, TEST_UNDICI_RUNTIME_DEPS_KEY);
 }
 
+function stubRuntimeFetch(fetchImpl: typeof fetch): void {
+  (globalThis as Record<string, unknown>)[TEST_UNDICI_RUNTIME_DEPS_KEY] = {
+    Agent: function MockAgent() {},
+    EnvHttpProxyAgent: function MockEnvHttpProxyAgent() {},
+    ProxyAgent: function MockProxyAgent() {},
+    fetch: fetchImpl,
+  };
+}
+
 describe("performMatrixRequest", () => {
   beforeEach(() => {
     vi.unstubAllGlobals();
@@ -19,8 +28,7 @@ describe("performMatrixRequest", () => {
   });
 
   it("rejects oversized raw responses before buffering the whole body", async () => {
-    vi.stubGlobal(
-      "fetch",
+    stubRuntimeFetch(
       vi.fn(
         async () =>
           new Response("too-big", {
@@ -55,8 +63,7 @@ describe("performMatrixRequest", () => {
         controller.close();
       },
     });
-    vi.stubGlobal(
-      "fetch",
+    stubRuntimeFetch(
       vi.fn(
         async () =>
           new Response(stream, {
@@ -87,8 +94,7 @@ describe("performMatrixRequest", () => {
           controller.enqueue(new Uint8Array([1, 2, 3]));
         },
       });
-      vi.stubGlobal(
-        "fetch",
+      stubRuntimeFetch(
         vi.fn(
           async () =>
             new Response(stream, {
@@ -135,12 +141,7 @@ describe("performMatrixRequest", () => {
         },
       });
     });
-    (globalThis as Record<string, unknown>)[TEST_UNDICI_RUNTIME_DEPS_KEY] = {
-      Agent: function MockAgent() {},
-      EnvHttpProxyAgent: function MockEnvHttpProxyAgent() {},
-      ProxyAgent: function MockProxyAgent() {},
-      fetch: runtimeFetch,
-    };
+    stubRuntimeFetch(runtimeFetch);
 
     const result = await performMatrixRequest({
       homeserver: "http://127.0.0.1:8008",
diff --git a/extensions/matrix/src/matrix/sdk/transport.ts b/extensions/matrix/src/matrix/sdk/transport.ts
index 48630c4067e..f3dc802bbdf 100644
--- a/extensions/matrix/src/matrix/sdk/transport.ts
+++ b/extensions/matrix/src/matrix/sdk/transport.ts
@@ -89,13 +89,6 @@ function buildBufferedResponse(params: {
   return response;
 }
 
-function isMockedFetch(fetchImpl: typeof fetch | undefined): boolean {
-  if (typeof fetchImpl !== "function") {
-    return false;
-  }
-  return typeof (fetchImpl as typeof fetch & { mock?: unknown }).mock === "object";
-}
-
 async function fetchWithMatrixDispatcher(params: {
   url: string;
   init: MatrixDispatcherRequestInit;
@@ -104,10 +97,7 @@ async function fetchWithMatrixDispatcher(params: {
   // fetches must stay fail-closed unless a retry path can preserve the
   // validated pinned-address binding. Route dispatcher-attached requests
   // through undici runtime fetch so the pinned dispatcher is preserved.
-  if (params.init.dispatcher && !isMockedFetch(globalThis.fetch)) {
-    return await fetchWithRuntimeDispatcher(params.url, params.init);
-  }
-  return await fetch(params.url, params.init);
+  return await fetchWithRuntimeDispatcher(params.url, params.init);
 }
 
 async function fetchWithMatrixGuardedRedirects(params: {