From 7168896fdfed027cdb5da995bd18c358aab39b66 Mon Sep 17 00:00:00 2001
From: Peter Steinberger <steipete@gmail.com>
Date: Tue, 5 May 2026 01:27:17 +0100
Subject: [PATCH] fix(agents): abort post-compaction loops out-of-band

---
 .../run.compaction-loop-guard.test.ts         | 22 +++++++++----
 src/agents/pi-embedded-runner/run.ts          | 32 +++++++++++++++++--
 2 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/src/agents/pi-embedded-runner/run.compaction-loop-guard.test.ts b/src/agents/pi-embedded-runner/run.compaction-loop-guard.test.ts
index 59e3e245298..348ee9c0a80 100644
--- a/src/agents/pi-embedded-runner/run.compaction-loop-guard.test.ts
+++ b/src/agents/pi-embedded-runner/run.compaction-loop-guard.test.ts
@@ -153,20 +153,26 @@ describe("post-compaction loop guard wired into runEmbeddedPiAgent", () => {
     });
   });
 
-  it("aborts the run with PostCompactionLoopPersistedError when identical (tool, args, result) repeats windowSize times after compaction", async () => {
+  it("aborts the attempt out-of-band when identical (tool, args, result) repeats windowSize times after compaction", async () => {
     const overflowError = makeOverflowError();
     let attemptReturned = false;
+    let attemptSignalAborted = false;
+    let attemptSignalReason: unknown;
 
     // Attempt 1: overflow → triggers compaction.
     mockedRunEmbeddedAttempt.mockImplementationOnce(async () =>
       makeAttemptResult({ promptError: overflowError }),
     );
     // Attempt 2: post-compaction. The live wrapped-tool path records each
-    // outcome while the prompt is still running; the third identical result
-    // aborts before the attempt can return.
+    // outcome while the prompt is still running. The third identical result
+    // must not rely on throwing out of tool execution (the dependency converts
+    // tool errors into tool results); instead it aborts the attempt signal and
+    // the runner raises the persisted-loop error after the attempt unwinds.
     mockedRunEmbeddedAttempt.mockImplementationOnce(async (attemptParams: unknown) => {
-      const onToolOutcome = (attemptParams as { onToolOutcome?: ToolOutcomeObserver })
-        .onToolOutcome;
+      const { abortSignal, onToolOutcome } = attemptParams as {
+        abortSignal?: AbortSignal;
+        onToolOutcome?: ToolOutcomeObserver;
+      };
       for (let i = 0; i < 3; i += 1) {
         await executeWrappedToolOutcome(
           "gateway",
@@ -175,6 +181,8 @@ describe("post-compaction loop guard wired into runEmbeddedPiAgent", () => {
           onToolOutcome,
         );
       }
+      attemptSignalAborted = abortSignal?.aborted ?? false;
+      attemptSignalReason = abortSignal?.reason;
       attemptReturned = true;
       return makeAttemptResult({
         promptError: null,
@@ -196,7 +204,9 @@ describe("post-compaction loop guard wired into runEmbeddedPiAgent", () => {
 
     expect(mockedCompactDirect).toHaveBeenCalledTimes(1);
     expect(mockedRunEmbeddedAttempt).toHaveBeenCalledTimes(2);
-    expect(attemptReturned).toBe(false);
+    expect(attemptReturned).toBe(true);
+    expect(attemptSignalAborted).toBe(true);
+    expect(attemptSignalReason).toBeInstanceOf(PostCompactionLoopPersistedError);
   });
 
   it("does not abort when the result hash changes across post-compaction attempts (progress was made)", async () => {
diff --git a/src/agents/pi-embedded-runner/run.ts b/src/agents/pi-embedded-runner/run.ts
index 81219bd8ee2..f81ee725bb4 100644
--- a/src/agents/pi-embedded-runner/run.ts
+++ b/src/agents/pi-embedded-runner/run.ts
@@ -799,12 +799,15 @@ export async function runEmbeddedPiAgent(
         resolvedLoopDetectionConfig?.postCompactionGuard,
         { enabled: resolvedLoopDetectionConfig?.enabled !== false },
       );
+      let postCompactionAbortController: AbortController | undefined;
+      let postCompactionAbortError: PostCompactionLoopPersistedError | undefined;
       const observePostCompactionToolOutcome = (
         observation: PostCompactionGuardObservation,
       ): void => {
         const verdict = postCompactionGuard.observe(observation);
         if (verdict.shouldAbort) {
-          throw PostCompactionLoopPersistedError.fromVerdict(verdict);
+          postCompactionAbortError ??= PostCompactionLoopPersistedError.fromVerdict(verdict);
+          postCompactionAbortController?.abort(postCompactionAbortError);
         }
       };
       let lastRetryFailoverReason: FailoverReason | null = null;
@@ -1099,6 +1102,17 @@ export async function runEmbeddedPiAgent(
             startupStagesEmitted = true;
           }
 
+          const attemptAbortController = new AbortController();
+          postCompactionAbortController = attemptAbortController;
+          const parentAbortSignal = params.abortSignal;
+          const relayParentAbort = (): void => {
+            attemptAbortController.abort(parentAbortSignal?.reason);
+          };
+          if (parentAbortSignal?.aborted) {
+            relayParentAbort();
+          } else {
+            parentAbortSignal?.addEventListener("abort", relayParentAbort, { once: true });
+          }
           const rawAttempt = await runEmbeddedAttemptWithBackend({
             sessionId: activeSessionId,
             sessionKey: resolvedSessionKey,
@@ -1177,7 +1191,7 @@ export async function runEmbeddedPiAgent(
             bashElevated: params.bashElevated,
             timeoutMs: params.timeoutMs,
             runId: params.runId,
-            abortSignal: params.abortSignal,
+            abortSignal: attemptAbortController.signal,
             replyOperation: params.replyOperation,
             shouldEmitToolResult: params.shouldEmitToolResult,
             shouldEmitToolOutput: params.shouldEmitToolOutput,
@@ -1216,7 +1230,19 @@ export async function runEmbeddedPiAgent(
               bootstrapPromptWarningSignaturesSeen[bootstrapPromptWarningSignaturesSeen.length - 1],
             suppressNextUserMessagePersistence,
             onUserMessagePersisted,
-          });
+          })
+            .catch((err: unknown): never => {
+              throw postCompactionAbortError ?? err;
+            })
+            .finally(() => {
+              parentAbortSignal?.removeEventListener?.("abort", relayParentAbort);
+              if (postCompactionAbortController === attemptAbortController) {
+                postCompactionAbortController = undefined;
+              }
+            });
+          if (postCompactionAbortError) {
+            throw postCompactionAbortError;
+          }
           const attempt = normalizeEmbeddedRunAttemptResult(rawAttempt);
 
           const {