diff --git a/src/cron/isolated-agent/run.fast-mode.test.ts b/src/cron/isolated-agent/run.fast-mode.test.ts
index 12214b1487d..59f8a0995e6 100644
--- a/src/cron/isolated-agent/run.fast-mode.test.ts
+++ b/src/cron/isolated-agent/run.fast-mode.test.ts
@@ -34,6 +34,14 @@ function mockSuccessfulModelFallback() {
   });
 }
 
+function requireFirstMockCall<T>(mock: { mock: { calls: T[][] } }, label: string): T[] {
+  const call = mock.mock.calls.at(0);
+  if (!call) {
+    throw new Error(`expected ${label} call`);
+  }
+  return call;
+}
+
 async function runFastModeCase(params: {
   configFastMode: boolean;
   expectedFastMode: boolean;
@@ -96,7 +104,7 @@ async function runFastModeCase(params: {
 
   expect(result.status).toBe("ok");
   expect(runEmbeddedPiAgentMock).toHaveBeenCalledOnce();
-  const [embeddedRunParams] = runEmbeddedPiAgentMock.mock.calls[0];
+  const [embeddedRunParams] = requireFirstMockCall(runEmbeddedPiAgentMock, "embedded run");
   expect(embeddedRunParams.provider).toBe("openai");
   expect(embeddedRunParams.model).toBe(EXPECTED_OPENAI_MODEL);
   expect(embeddedRunParams.fastMode).toBe(params.expectedFastMode);
@@ -106,7 +114,10 @@ async function runFastModeCase(params: {
   expect(embeddedRunParams.allowGatewaySubagentBinding).toBe(true);
   if (params.expectedRetiredSessionId) {
     expect(retireSessionMcpRuntimeMock).toHaveBeenCalledOnce();
-    const [retireParams] = retireSessionMcpRuntimeMock.mock.calls[0];
+    const [retireParams] = requireFirstMockCall(
+      retireSessionMcpRuntimeMock,
+      "retire session mcp runtime",
+    );
     expect(retireParams.sessionId).toBe(params.expectedRetiredSessionId);
     expect(retireParams.reason).toBe("cron-session-rollover");
     return;