diff --git a/src/cron/isolated-agent/run.announce-suppressed-dedupe.test.ts b/src/cron/isolated-agent/run.announce-suppressed-dedupe.test.ts
index 530a3477a5c..fe106012be2 100644
--- a/src/cron/isolated-agent/run.announce-suppressed-dedupe.test.ts
+++ b/src/cron/isolated-agent/run.announce-suppressed-dedupe.test.ts
@@ -10,10 +10,10 @@
  * proceed normally.
  */
 
-import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-import { deliverOutboundPayloads } from "../../infra/outbound/deliver.js";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import {
   clearFastTestEnv,
+  deliverOutboundPayloadsMock,
   loadRunCronIsolatedAgentTurn,
   resetRunCronIsolatedAgentTurnHarness,
   resolveCronDeliveryPlanMock,
@@ -83,9 +83,7 @@ describe("runCronIsolatedAgentTurn -- announce suppressed send dedupe (#42244)",
       source: "delivery",
     });
 
-    // The harness mock returns undefined which causes TypeError in delivery
-    // dispatch. Override to return a proper result array.
-    vi.mocked(deliverOutboundPayloads).mockResolvedValue([{ ok: true } as never]);
+    deliverOutboundPayloadsMock.mockResolvedValue([{ ok: true } as never]);
   });
 
   afterEach(() => {
diff --git a/src/cron/isolated-agent/run.test-harness.ts b/src/cron/isolated-agent/run.test-harness.ts
index 81e4c8b902b..df5c8fb0f0f 100644
--- a/src/cron/isolated-agent/run.test-harness.ts
+++ b/src/cron/isolated-agent/run.test-harness.ts
@@ -43,6 +43,7 @@ export const logWarnMock = createMock();
 export const countActiveDescendantRunsMock = createMock();
 export const listDescendantRunsForRequesterMock = createMock();
 export const pickLastNonEmptyTextFromPayloadsMock = createMock();
+export const deliverOutboundPayloadsMock = createMock();
 export const resolveCronDeliveryPlanMock = createMock();
 export const resolveDeliveryTargetMock = createMock();
 
@@ -240,7 +241,7 @@ vi.mock("../../infra/outbound/deliver.js", async (importOriginal) => {
   const actual = await importOriginal<typeof import("../../infra/outbound/deliver.js")>();
   return {
     ...actual,
-    deliverOutboundPayloads: vi.fn().mockResolvedValue(undefined),
+    deliverOutboundPayloads: deliverOutboundPayloadsMock,
   };
 });
 
@@ -387,6 +388,8 @@ export function resetRunCronIsolatedAgentTurnHarness(): void {
   listDescendantRunsForRequesterMock.mockReturnValue([]);
   pickLastNonEmptyTextFromPayloadsMock.mockReset();
   pickLastNonEmptyTextFromPayloadsMock.mockReturnValue("test output");
+  deliverOutboundPayloadsMock.mockReset();
+  deliverOutboundPayloadsMock.mockResolvedValue([]);
   resolveCronDeliveryPlanMock.mockReset();
   resolveCronDeliveryPlanMock.mockReturnValue({ requested: false, mode: "none" });
   resolveDeliveryTargetMock.mockReset();