fix(agents): escalate LLM idle timeout to model fallback after profile rotation

When the LLM idle watchdog fires (model produced no tokens for N seconds),
idleTimedOut is set in handleAssistantFailover but was never passed into
resolveRunFailoverDecision. As a result, shouldRotateAssistant saw neither
failoverReason nor timedOut (the run-budget timeout) set, returned false,
and the decision fell through to continue_normal -- the agent silently froze
without surfacing an error or advancing the fallback chain.

Fixes #76877 (regression since 2026.4.24).

Changes:
- failover-policy.ts: add idleTimedOut to AssistantDecisionParams; include it
  in shouldRotateAssistant and reason selection in resolveRunFailoverDecision
- assistant-failover.ts: pass idleTimedOut into resolveRunFailoverDecision
- failover-policy.test.ts: 4 new cases for idle timeout path; update existing
  assistant stage cases with the new required field (idleTimedOut: false)

src/agents/pi-embedded-runner/run/assistant-failover.ts

@@ -190,6 +190,7 @@ export async function handleAssistantFailover(params: {
       failoverFailure: params.failoverFailure,
       failoverReason: params.failoverReason,
       timedOut: params.timedOut,
+      idleTimedOut: params.idleTimedOut,
       timedOutDuringCompaction: params.timedOutDuringCompaction,
       timedOutDuringToolExecution: params.timedOutDuringToolExecution,
       profileRotated: true,

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

@@ -106,6 +106,7 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: "rate_limit",
         timedOut: false,
+        idleTimedOut: false,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: false,
         profileRotated: false,
@@ -167,6 +168,7 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: "rate_limit",
         timedOut: false,
+        idleTimedOut: false,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: false,
         profileRotated: true,
@@ -187,6 +189,7 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: null,
         timedOut: false,
+        idleTimedOut: false,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: false,
         profileRotated: false,
@@ -223,6 +226,7 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: null,
         timedOut: true,
+        idleTimedOut: false,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: true,
         profileRotated: false,
@@ -242,6 +246,7 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: null,
         timedOut: true,
+        idleTimedOut: false,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: true,
         profileRotated: true,
@@ -261,6 +266,7 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: null,
         timedOut: true,
+        idleTimedOut: false,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: false,
         profileRotated: false,
@@ -281,6 +287,95 @@ describe("resolveRunFailoverDecision", () => {
         failoverFailure: false,
         failoverReason: null,
         timedOut: true,
+        idleTimedOut: false,
+        timedOutDuringCompaction: false,
+        timedOutDuringToolExecution: false,
+        profileRotated: false,
+      }),
+    ).toEqual({
+      action: "surface_error",
+      reason: null,
+    });
+  });
+});
+
+  it("rotates profile on LLM idle timeout before falling back", () => {
+    // idleTimedOut = model produced no tokens; no provider API error was classified.
+    // Before this fix, failoverReason=null + timedOut=false → shouldRotateAssistant=false
+    // → continue_normal, causing a silent agent freeze.
+    expect(
+      resolveRunFailoverDecision({
+        stage: "assistant",
+        aborted: false,
+        externalAbort: false,
+        fallbackConfigured: true,
+        failoverFailure: false,
+        failoverReason: null,
+        timedOut: false,
+        idleTimedOut: true,
+        timedOutDuringCompaction: false,
+        timedOutDuringToolExecution: false,
+        profileRotated: false,
+      }),
+    ).toEqual({
+      action: "rotate_profile",
+      reason: null,
+    });
+  });
+
+  it("escalates LLM idle timeout to fallback_model after profile rotation is exhausted", () => {
+    expect(
+      resolveRunFailoverDecision({
+        stage: "assistant",
+        aborted: false,
+        externalAbort: false,
+        fallbackConfigured: true,
+        failoverFailure: false,
+        failoverReason: null,
+        timedOut: false,
+        idleTimedOut: true,
+        timedOutDuringCompaction: false,
+        timedOutDuringToolExecution: false,
+        profileRotated: true,
+      }),
+    ).toEqual({
+      action: "fallback_model",
+      reason: "timeout",
+    });
+  });
+
+  it("surfaces error on LLM idle timeout when no fallback is configured and rotation is exhausted", () => {
+    expect(
+      resolveRunFailoverDecision({
+        stage: "assistant",
+        aborted: false,
+        externalAbort: false,
+        fallbackConfigured: false,
+        failoverFailure: false,
+        failoverReason: null,
+        timedOut: false,
+        idleTimedOut: true,
+        timedOutDuringCompaction: false,
+        timedOutDuringToolExecution: false,
+        profileRotated: true,
+      }),
+    ).toEqual({
+      action: "surface_error",
+      reason: null,
+    });
+  });
+
+  it("does not escalate LLM idle timeout after an external abort", () => {
+    expect(
+      resolveRunFailoverDecision({
+        stage: "assistant",
+        aborted: false,
+        externalAbort: true,
+        fallbackConfigured: true,
+        failoverFailure: false,
+        failoverReason: null,
+        timedOut: false,
+        idleTimedOut: true,
         timedOutDuringCompaction: false,
         timedOutDuringToolExecution: false,
         profileRotated: false,

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

@@ -57,6 +57,7 @@ type AssistantDecisionParams = {
   failoverFailure: boolean;
   failoverReason: FailoverReason | null;
   timedOut: boolean;
+  idleTimedOut: boolean;
   timedOutDuringCompaction: boolean;
   timedOutDuringToolExecution: boolean;
   profileRotated: boolean;
@@ -98,7 +99,8 @@ function shouldRotateAssistant(params: AssistantDecisionParams): boolean {
   }
   return (
     (!params.aborted && (params.failoverFailure || params.failoverReason !== null)) ||
-    (params.timedOut && !params.timedOutDuringCompaction && !params.timedOutDuringToolExecution)
+    (params.timedOut && !params.timedOutDuringCompaction && !params.timedOutDuringToolExecution) ||
+    params.idleTimedOut
   );
 }
 
@@ -178,7 +180,8 @@ export function resolveRunFailoverDecision(params: RunFailoverDecisionParams): R
   if (assistantShouldRotate && params.fallbackConfigured) {
     return {
       action: "fallback_model",
-      reason: params.timedOut ? "timeout" : (params.failoverReason ?? "unknown"),
+      reason:
+        params.timedOut || params.idleTimedOut ? "timeout" : (params.failoverReason ?? "unknown"),
     };
   }
   if (!assistantShouldRotate) {