diff --git a/src/agents/subagent-registry-lifecycle.test.ts b/src/agents/subagent-registry-lifecycle.test.ts
index 18ae4daa051..92ec0199330 100644
--- a/src/agents/subagent-registry-lifecycle.test.ts
+++ b/src/agents/subagent-registry-lifecycle.test.ts
@@ -114,8 +114,16 @@ function expectFields(value: unknown, expected: Record<string, unknown>): void {
   }
 }
 
+function firstCall(mock: ReturnType<typeof vi.fn>): ReadonlyArray<unknown> {
+  const call = mock.mock.calls[0];
+  if (!call) {
+    throw new Error("expected first mock call");
+  }
+  return call;
+}
+
 function firstCallArg(mock: ReturnType<typeof vi.fn>): Record<string, unknown> {
-  const [arg] = mock.mock.calls.at(0) ?? [];
+  const [arg] = firstCall(mock);
   if (!arg || typeof arg !== "object") {
     throw new Error("expected first call argument object");
   }
@@ -206,8 +214,9 @@ describe("subagent registry lifecycle hardening", () => {
     ).resolves.toBeUndefined();
 
     expect(warn).toHaveBeenCalledTimes(1);
-    expect(warn.mock.calls.at(0)?.[0]).toBe("failed to finalize subagent background task state");
-    expectFields(warn.mock.calls.at(0)?.[1], {
+    const [warning, warningFields] = firstCall(warn);
+    expect(warning).toBe("failed to finalize subagent background task state");
+    expectFields(warningFields, {
       error: { name: "Error", message: "task store boom" },
       runId: "***",
       childSessionKey: "agent:main:…",
@@ -250,10 +259,9 @@ describe("subagent registry lifecycle hardening", () => {
     ).resolves.toBeUndefined();
 
     expect(warn).toHaveBeenCalledTimes(1);
-    expect(warn.mock.calls.at(0)?.[0]).toBe(
-      "failed to update subagent background task delivery state",
-    );
-    expectFields(warn.mock.calls.at(0)?.[1], {
+    const [warning, warningFields] = firstCall(warn);
+    expect(warning).toBe("failed to update subagent background task delivery state");
+    expectFields(warningFields, {
       error: { name: "Error", message: "delivery state boom" },
       runId: "***",
       childSessionKey: "agent:main:…",
@@ -677,10 +685,9 @@ describe("subagent registry lifecycle hardening", () => {
     ).resolves.toBeUndefined();
 
     expect(warn).toHaveBeenCalledTimes(1);
-    expect(warn.mock.calls.at(0)?.[0]).toBe(
-      "failed to update subagent background task delivery state",
-    );
-    expectFields(warn.mock.calls.at(0)?.[1], {
+    const [warning, warningFields] = firstCall(warn);
+    expect(warning).toBe("failed to update subagent background task delivery state");
+    expectFields(warningFields, {
       error: { name: "Error", message: "delivery status boom" },
       deliveryStatus: "delivered",
     });