fix(agents): stop injecting heartbeat system prompt on non-heartbeat runs (#69079)

src/agents/pi-embedded-runner/run/trigger-policy.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import { shouldInjectHeartbeatPromptForTrigger } from "./trigger-policy.js";
+
+describe("shouldInjectHeartbeatPromptForTrigger", () => {
+  it("injects the heartbeat prompt on heartbeat-triggered runs", () => {
+    expect(shouldInjectHeartbeatPromptForTrigger("heartbeat")).toBe(true);
+  });
+
+  // Regression: the heartbeat system prompt instructs the model to reply
+  // exactly "HEARTBEAT_OK" when nothing is pending. If that prompt leaks into
+  // a user-triggered turn, the model can pattern-match the literal HEARTBEAT_OK
+  // token (which the delivery runtime then suppresses, so the user sees
+  // silence) or hallucinate a "[object Object]" serialization error as it
+  // tries to reconcile the heartbeat instruction with a real user message.
+  // See issue #69079 and its parent #50797.
+  it.each([
+    ["user"] as const,
+    ["manual"] as const,
+    ["cron"] as const,
+    ["memory"] as const,
+    ["overflow"] as const,
+  ])("does not inject the heartbeat prompt on %s-triggered runs", (trigger) => {
+    expect(shouldInjectHeartbeatPromptForTrigger(trigger)).toBe(false);
+  });
+
+  it("does not inject the heartbeat prompt when no trigger is supplied", () => {
+    // Defense-in-depth: if a new call site lands without a trigger, it should
+    // fall through to the safe default rather than spuriously injecting
+    // heartbeat instructions.
+    expect(shouldInjectHeartbeatPromptForTrigger(undefined)).toBe(false);
+  });
+});

src/agents/pi-embedded-runner/run/trigger-policy.ts

@@ -4,13 +4,21 @@ type EmbeddedRunTriggerPolicy = {
   injectHeartbeatPrompt: boolean;
 };
 
+// The heartbeat system prompt tells the model to reply exactly "HEARTBEAT_OK"
+// when nothing needs attention. It is only meaningful on heartbeat-triggered
+// runs; injecting it on user/manual/memory/overflow runs confuses smaller
+// models into pattern-matching the HEARTBEAT_OK output on real user messages
+// (delivery then suppresses the "reply", so the user sees silence) or into
+// fabricating "[object Object]" serialization errors as they try to reconcile
+// the heartbeat instruction with a non-heartbeat turn. See issue #69079 and
+// its parent #50797. Default off; heartbeat trigger explicitly opts in.
 const DEFAULT_EMBEDDED_RUN_TRIGGER_POLICY: EmbeddedRunTriggerPolicy = {
-  injectHeartbeatPrompt: true,
+  injectHeartbeatPrompt: false,
 };
 
 const EMBEDDED_RUN_TRIGGER_POLICY: Partial<Record<EmbeddedRunTrigger, EmbeddedRunTriggerPolicy>> = {
-  cron: {
-    injectHeartbeatPrompt: false,
+  heartbeat: {
+    injectHeartbeatPrompt: true,
   },
 };