diff --git a/src/agents/embedded-agent-runner/post-compaction-loop-guard.test.ts b/src/agents/embedded-agent-runner/post-compaction-loop-guard.test.ts
index b7b42dd46e6d..d288299c7892 100644
--- a/src/agents/embedded-agent-runner/post-compaction-loop-guard.test.ts
+++ b/src/agents/embedded-agent-runner/post-compaction-loop-guard.test.ts
@@ -1,5 +1,18 @@
 // Coverage for detecting repeated tool loops immediately after compaction.
-import { describe, expect, it } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const logInfo = vi.hoisted(() => vi.fn());
+const logError = vi.hoisted(() => vi.fn());
+
+vi.mock("../../logging/subsystem.js", () => ({
+  createSubsystemLogger: vi.fn(() => ({
+    info: logInfo,
+    error: logError,
+    warn: vi.fn(),
+    debug: vi.fn(),
+  })),
+}));
+
 import {
   createPostCompactionLoopGuard,
   PostCompactionLoopPersistedError,
@@ -120,6 +133,64 @@ describe("createPostCompactionLoopGuard", () => {
   });
 });
 
+describe("post-compaction re-read instrumentation", () => {
+  beforeEach(() => {
+    logInfo.mockClear();
+    logError.mockClear();
+  });
+
+  it("summarizes post-compaction re-reads against the pre-compaction baseline", () => {
+    const guard = createPostCompactionLoopGuard();
+    guard.observe(callOutcome("read", { path: "/report.md" }, "content-v1"));
+    guard.observe(callOutcome("exec", { cmd: "ls" }, "ok"));
+    guard.armPostCompaction();
+    // The model immediately re-reads what compaction summarized away, then moves on.
+    guard.observe(callOutcome("read", { path: "/report.md" }, "content-v2"));
+    guard.observe(callOutcome("read", { path: "/other.md" }, "fresh"));
+    const last = guard.observe(callOutcome("gateway", { action: "probe" }, "r1"));
+    expect(last.shouldAbort).toBe(false);
+    expect(logInfo).toHaveBeenCalledWith(expect.stringContaining("post-compaction window closed"));
+    const summary = logInfo.mock.calls
+      .map((call) => call[0] as string)
+      .find((message) => message.includes("post-compaction window closed"));
+    expect(summary).toContain("toolCalls=3");
+    expect(summary).toContain("preCompactionRepeats=1");
+    expect(summary).toContain("read");
+    expect(logError).not.toHaveBeenCalled();
+  });
+
+  it("reports zero re-reads when the window introduces only fresh calls", () => {
+    const guard = createPostCompactionLoopGuard();
+    guard.observe(callOutcome("read", { path: "/a.md" }, "v1"));
+    guard.armPostCompaction();
+    guard.observe(callOutcome("read", { path: "/b.md" }, "v1"));
+    guard.observe(callOutcome("read", { path: "/c.md" }, "v1"));
+    guard.observe(callOutcome("exec", { cmd: "ls" }, "ok"));
+    const summary = logInfo.mock.calls
+      .map((call) => call[0] as string)
+      .find((message) => message.includes("post-compaction window closed"));
+    expect(summary).toContain("preCompactionRepeats=0");
+  });
+
+  it("does not count signatures evicted from the bounded baseline", () => {
+    const guard = createPostCompactionLoopGuard();
+    for (let i = 0; i < 20; i += 1) {
+      guard.observe(callOutcome("read", { path: `/old-${i}.md` }, "v1"));
+    }
+    guard.armPostCompaction();
+    // Signature from before the baseline window: no longer comparable.
+    guard.observe(callOutcome("read", { path: "/old-0.md" }, "v2"));
+    // Signature still inside the baseline window tail.
+    guard.observe(callOutcome("read", { path: "/old-19.md" }, "v2"));
+    guard.observe(callOutcome("gateway", { action: "probe" }, "r1"));
+    const summary = logInfo.mock.calls
+      .map((call) => call[0] as string)
+      .find((message) => message.includes("post-compaction window closed"));
+    expect(summary).toContain("toolCalls=3");
+    expect(summary).toContain("preCompactionRepeats=1");
+  });
+});
+
 describe("PostCompactionLoopPersistedError", () => {
   it("captures the detector, count, toolName, and message", () => {
     const err = new PostCompactionLoopPersistedError("loop persisted", {
diff --git a/src/agents/embedded-agent-runner/post-compaction-loop-guard.ts b/src/agents/embedded-agent-runner/post-compaction-loop-guard.ts
index 2375db8a2a34..4bb18eb3553b 100644
--- a/src/agents/embedded-agent-runner/post-compaction-loop-guard.ts
+++ b/src/agents/embedded-agent-runner/post-compaction-loop-guard.ts
@@ -13,6 +13,11 @@ const log = createSubsystemLogger("agents/post-compaction-guard");
 
 const DEFAULT_WINDOW_SIZE = 3;
 
+// Bounded recent-call tail kept across the whole run so arming can snapshot what the
+// model was doing right before compaction. Without it, re-reads of summarized content
+// inside the post-compaction window leave no recorded fact at all.
+const BASELINE_WINDOW_SIZE = 16;
+
 type PostCompactionGuardObservation = {
   toolName: string;
   argsHash: string;
@@ -42,8 +47,16 @@ type GuardState = {
   windowSize: number;
   remainingAttempts: number;
   history: PostCompactionGuardObservation[];
+  recentCalls: PostCompactionGuardObservation[];
+  baselineSignatures: Set<string> | undefined;
+  windowObserved: number;
+  windowRepeats: number;
+  repeatTools: Set<string>;
 };
 
+const observationSignature = (call: PostCompactionGuardObservation): string =>
+  `${call.toolName}\0${call.argsHash}`;
+
 /** Creates a stateful post-compaction loop detector for one embedded run. */
 export function createPostCompactionLoopGuard(options?: {
   enabled?: boolean;
@@ -53,24 +66,56 @@ export function createPostCompactionLoopGuard(options?: {
     windowSize: DEFAULT_WINDOW_SIZE,
     remainingAttempts: 0,
     history: [],
+    recentCalls: [],
+    baselineSignatures: undefined,
+    windowObserved: 0,
+    windowRepeats: 0,
+    repeatTools: new Set<string>(),
   };
 
   const armPostCompaction = (): void => {
+    // Snapshot the pre-compaction call tail before the new window starts. A re-arm
+    // mid-window replaces the unclosed window's counts; compaction success implies
+    // the prior attempt ended, so that loss is accepted.
+    state.baselineSignatures =
+      state.enabled && state.recentCalls.length > 0
+        ? new Set(state.recentCalls.map(observationSignature))
+        : undefined;
     state.remainingAttempts = state.windowSize;
     state.history = [];
+    state.windowObserved = 0;
+    state.windowRepeats = 0;
+    state.repeatTools = new Set<string>();
     if (state.enabled) {
       log.info(`post-compaction guard armed for ${state.windowSize} attempts`);
     }
   };
 
+  const logWindowSummary = (): void => {
+    const tools = [...state.repeatTools].toSorted().join(",");
+    log.info(
+      `post-compaction window closed: toolCalls=${state.windowObserved} ` +
+        `preCompactionRepeats=${state.windowRepeats}${tools ? ` tools=${tools}` : ""}`,
+    );
+  };
+
   const observe = (call: PostCompactionGuardObservation): PostCompactionGuardVerdict => {
     if (!state.enabled) {
       return { shouldAbort: false, armed: false, remainingAttempts: 0 };
     }
+    state.recentCalls.push(call);
+    if (state.recentCalls.length > BASELINE_WINDOW_SIZE) {
+      state.recentCalls.shift();
+    }
     if (state.remainingAttempts <= 0) {
       return { shouldAbort: false, armed: false, remainingAttempts: 0 };
     }
     state.remainingAttempts -= 1;
+    state.windowObserved += 1;
+    if (state.baselineSignatures?.has(observationSignature(call))) {
+      state.windowRepeats += 1;
+      state.repeatTools.add(call.toolName);
+    }
     state.history.push(call);
     const armedAfter = state.remainingAttempts > 0;
 
@@ -98,6 +143,14 @@ export function createPostCompactionLoopGuard(options?: {
       };
     }
 
+    if (!armedAfter) {
+      logWindowSummary();
+      state.baselineSignatures = undefined;
+      state.windowObserved = 0;
+      state.windowRepeats = 0;
+      state.repeatTools = new Set<string>();
+    }
+
     return { shouldAbort: false, armed: armedAfter, remainingAttempts: state.remainingAttempts };
   };