diff --git a/src/agents/pi-embedded-runner/post-compaction-loop-guard.test.ts b/src/agents/pi-embedded-runner/post-compaction-loop-guard.test.ts
new file mode 100644
index 00000000000..a665c964665
--- /dev/null
+++ b/src/agents/pi-embedded-runner/post-compaction-loop-guard.test.ts
@@ -0,0 +1,113 @@
+import { describe, expect, it } from "vitest";
+import {
+  createPostCompactionLoopGuard,
+  type PostCompactionLoopGuard,
+} from "./post-compaction-loop-guard.js";
+
+function callOutcome(toolName: string, args: unknown, result: string) {
+  return { toolName, argsHash: JSON.stringify(args), resultHash: result };
+}
+
+describe("createPostCompactionLoopGuard", () => {
+  it("is dormant when never armed", () => {
+    const guard = createPostCompactionLoopGuard();
+    const verdict = guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    expect(verdict.shouldAbort).toBe(false);
+    expect(verdict.armed).toBe(false);
+  });
+
+  it("arms for the configured window after compaction", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 3 });
+    guard.armPostCompaction();
+    expect(guard.snapshot().armed).toBe(true);
+    expect(guard.snapshot().remainingAttempts).toBe(3);
+  });
+
+  it("decrements remainingAttempts on each observation", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 3 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    expect(guard.snapshot().remainingAttempts).toBe(2);
+    guard.observe(callOutcome("read", { path: "/y" }, "r2"));
+    expect(guard.snapshot().remainingAttempts).toBe(1);
+    guard.observe(callOutcome("read", { path: "/z" }, "r3"));
+    expect(guard.snapshot().remainingAttempts).toBe(0);
+    expect(guard.snapshot().armed).toBe(false);
+  });
+
+  it("aborts on the windowSize-th identical (tool,args,result) call within the window", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 3 });
+    guard.armPostCompaction();
+    expect(
+      guard.observe(callOutcome("gateway", { action: "lookup", path: "x" }, "r1")).shouldAbort,
+    ).toBe(false);
+    expect(
+      guard.observe(callOutcome("gateway", { action: "lookup", path: "x" }, "r1")).shouldAbort,
+    ).toBe(false);
+    const third = guard.observe(callOutcome("gateway", { action: "lookup", path: "x" }, "r1"));
+    expect(third.shouldAbort).toBe(true);
+    if (third.shouldAbort) {
+      expect(third.detector).toBe("compaction_loop_persisted");
+      expect(third.count).toBe(3);
+      expect(third.toolName).toBe("gateway");
+    }
+  });
+
+  it("does NOT abort when the result hash changes (progress was made)", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 3 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    guard.observe(callOutcome("read", { path: "/x" }, "r2"));
+    const third = guard.observe(callOutcome("read", { path: "/x" }, "r3"));
+    expect(third.shouldAbort).toBe(false);
+  });
+
+  it("does NOT abort when the args hash changes", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 3 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/a" }, "r1"));
+    guard.observe(callOutcome("read", { path: "/b" }, "r1"));
+    const third = guard.observe(callOutcome("read", { path: "/c" }, "r1"));
+    expect(third.shouldAbort).toBe(false);
+  });
+
+  it("does NOT abort outside the window", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 2 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    expect(guard.snapshot().armed).toBe(false);
+    const after = guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    expect(after.shouldAbort).toBe(false);
+  });
+
+  it("re-arms when armPostCompaction is called again (multiple compactions per run)", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 2 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    guard.observe(callOutcome("read", { path: "/x" }, "r1"));
+    expect(guard.snapshot().armed).toBe(false);
+    guard.armPostCompaction();
+    expect(guard.snapshot().armed).toBe(true);
+    expect(guard.snapshot().remainingAttempts).toBe(2);
+  });
+
+  it("respects enabled: false (always returns shouldAbort: false even when armed)", () => {
+    const guard = createPostCompactionLoopGuard({ enabled: false, windowSize: 3 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("gateway", { x: 1 }, "r1"));
+    guard.observe(callOutcome("gateway", { x: 1 }, "r1"));
+    const third = guard.observe(callOutcome("gateway", { x: 1 }, "r1"));
+    expect(third.shouldAbort).toBe(false);
+  });
+
+  it("disarms after observing windowSize calls regardless of verdict", () => {
+    const guard = createPostCompactionLoopGuard({ windowSize: 3 });
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/a" }, "r1"));
+    guard.observe(callOutcome("write", { path: "/b" }, "r2"));
+    guard.observe(callOutcome("exec", { cmd: "ls" }, "r3"));
+    expect(guard.snapshot().armed).toBe(false);
+    expect(guard.snapshot().remainingAttempts).toBe(0);
+  });
+});
diff --git a/src/agents/pi-embedded-runner/post-compaction-loop-guard.ts b/src/agents/pi-embedded-runner/post-compaction-loop-guard.ts
new file mode 100644
index 00000000000..ef8b5cc6120
--- /dev/null
+++ b/src/agents/pi-embedded-runner/post-compaction-loop-guard.ts
@@ -0,0 +1,128 @@
+import type { ToolLoopPostCompactionGuardConfig } from "../../config/types.tools.js";
+import { createSubsystemLogger } from "../../logging/subsystem.js";
+
+const log = createSubsystemLogger("agents/post-compaction-guard");
+
+const DEFAULT_WINDOW_SIZE = 3;
+
+export type PostCompactionGuardObservation = {
+  toolName: string;
+  argsHash: string;
+  resultHash: string;
+};
+
+export type PostCompactionGuardVerdict =
+  | { shouldAbort: false; armed: boolean; remainingAttempts: number }
+  | {
+      shouldAbort: true;
+      armed: boolean;
+      remainingAttempts: number;
+      detector: "compaction_loop_persisted";
+      count: number;
+      toolName: string;
+      message: string;
+    };
+
+export type PostCompactionLoopGuard = {
+  armPostCompaction: () => void;
+  observe: (call: PostCompactionGuardObservation) => PostCompactionGuardVerdict;
+  snapshot: () => { armed: boolean; remainingAttempts: number };
+};
+
+type GuardState = {
+  enabled: boolean;
+  windowSize: number;
+  remainingAttempts: number;
+  history: PostCompactionGuardObservation[];
+};
+
+function asPositiveInt(value: number | undefined, fallback: number): number {
+  if (typeof value !== "number" || !Number.isInteger(value) || value <= 0) {
+    return fallback;
+  }
+  return value;
+}
+
+export function createPostCompactionLoopGuard(
+  config?: ToolLoopPostCompactionGuardConfig,
+): PostCompactionLoopGuard {
+  const state: GuardState = {
+    enabled: config?.enabled ?? true,
+    windowSize: asPositiveInt(config?.windowSize, DEFAULT_WINDOW_SIZE),
+    remainingAttempts: 0,
+    history: [],
+  };
+
+  const armPostCompaction = (): void => {
+    state.remainingAttempts = state.windowSize;
+    state.history = [];
+    if (state.enabled) {
+      log.info(`post-compaction guard armed for ${state.windowSize} attempts`);
+    }
+  };
+
+  const observe = (call: PostCompactionGuardObservation): PostCompactionGuardVerdict => {
+    if (state.remainingAttempts <= 0) {
+      return { shouldAbort: false, armed: false, remainingAttempts: 0 };
+    }
+    state.remainingAttempts -= 1;
+    state.history.push(call);
+    const armedAfter = state.remainingAttempts > 0;
+
+    if (!state.enabled) {
+      return { shouldAbort: false, armed: armedAfter, remainingAttempts: state.remainingAttempts };
+    }
+
+    const matches = state.history.filter(
+      (entry) =>
+        entry.toolName === call.toolName &&
+        entry.argsHash === call.argsHash &&
+        entry.resultHash === call.resultHash,
+    );
+
+    if (matches.length >= state.windowSize) {
+      log.error(
+        `post-compaction loop persisted: tool=${call.toolName} repeated ${matches.length} times with identical args+result post-compaction`,
+      );
+      return {
+        shouldAbort: true,
+        armed: armedAfter,
+        remainingAttempts: state.remainingAttempts,
+        detector: "compaction_loop_persisted",
+        count: matches.length,
+        toolName: call.toolName,
+        message: `CRITICAL: tool ${call.toolName} repeated ${matches.length} times with identical arguments and identical results within ${state.windowSize} attempts after auto-compaction. The compaction did not break the loop. Aborting to prevent runaway resource use.`,
+      };
+    }
+
+    return { shouldAbort: false, armed: armedAfter, remainingAttempts: state.remainingAttempts };
+  };
+
+  const snapshot = () => ({
+    armed: state.remainingAttempts > 0,
+    remainingAttempts: state.remainingAttempts,
+  });
+
+  return { armPostCompaction, observe, snapshot };
+}
+
+export class PostCompactionLoopPersistedError extends Error {
+  readonly detector: "compaction_loop_persisted";
+  readonly count: number;
+  readonly toolName: string;
+
+  constructor(
+    message: string,
+    details: {
+      detector: "compaction_loop_persisted";
+      count: number;
+      toolName: string;
+    },
+  ) {
+    super(message);
+    this.name = "PostCompactionLoopPersistedError";
+    this.detector = details.detector;
+    this.count = details.count;
+    this.toolName = details.toolName;
+  }
+}
diff --git a/src/config/schema.help.ts b/src/config/schema.help.ts
index 620684fa27a..5606dc7303d 100644
--- a/src/config/schema.help.ts
+++ b/src/config/schema.help.ts
@@ -658,6 +658,10 @@ export const FIELD_HELP: Record<string, string> = {
   "tools.loopDetection.detectors.knownPollNoProgress":
     "Enable known poll tool no-progress loop detection (default: true).",
   "tools.loopDetection.detectors.pingPong": "Enable ping-pong loop detection (default: true).",
+  "tools.loopDetection.postCompactionGuard.enabled":
+    "Enable the post-compaction loop guard (default: true). When the runner has just retried a prompt after auto-compaction, this guard aborts the run if the agent emits the same (tool, args, result) windowSize times. Targets the failure mode where context-overflow + compaction does not break a tool-call loop.",
+  "tools.loopDetection.postCompactionGuard.windowSize":
+    "Number of post-compaction attempts during which the guard stays armed (default: 3). Lower values are stricter; higher values give the agent more attempts before abort.",
   "tools.exec.notifyOnExit":
     "When true (default), backgrounded exec sessions on exit and node exec lifecycle events enqueue a system event and request a heartbeat.",
   "tools.exec.notifyOnExitEmptySuccess":
diff --git a/src/config/types.tools.ts b/src/config/types.tools.ts
index ae4dd596283..1e1a96ccacb 100644
--- a/src/config/types.tools.ts
+++ b/src/config/types.tools.ts
@@ -166,6 +166,13 @@ export type ToolLoopDetectionDetectorConfig = {
   pingPong?: boolean;
 };
 
+export type ToolLoopPostCompactionGuardConfig = {
+  /** Enable a strict guard that aborts when the agent re-enters the same tool-call loop immediately after a successful auto-compaction-retry (default: true). */
+  enabled?: boolean;
+  /** How many attempts post-compaction the guard remains armed (default: 3). */
+  windowSize?: number;
+};
+
 export type ToolLoopDetectionConfig = {
   /** Enable tool-loop protection (default: false). */
   enabled?: boolean;
@@ -181,6 +188,8 @@ export type ToolLoopDetectionConfig = {
   globalCircuitBreakerThreshold?: number;
   /** Detector toggles. */
   detectors?: ToolLoopDetectionDetectorConfig;
+  /** Post-compaction loop guard: aborts when the agent repeats the same (tool, args, result) immediately after auto-compaction-retry. */
+  postCompactionGuard?: ToolLoopPostCompactionGuardConfig;
 };
 
 export type SessionsToolsVisibility = "self" | "tree" | "agent" | "all";