diff --git a/src/resources/extensions/gsd/auto-loop.ts b/src/resources/extensions/gsd/auto-loop.ts
index 7ce847d20..b2e5800ea 100644
--- a/src/resources/extensions/gsd/auto-loop.ts
+++ b/src/resources/extensions/gsd/auto-loop.ts
@@ -15,6 +15,7 @@ import type { ExtensionAPI, ExtensionContext } from "@gsd/pi-coding-agent";
 import type { AutoSession } from "./auto/session.js";
 import { NEW_SESSION_TIMEOUT_MS } from "./auto/session.js";
 import type { GSDPreferences } from "./preferences.js";
+import type { SessionLockStatus } from "./session-lock.js";
 import type { GSDState } from "./types.js";
 import type { CloseoutOptions } from "./auto-unit-closeout.js";
 import type { PostUnitContext } from "./auto-post-unit.js";
@@ -307,7 +308,7 @@ export interface LoopDeps {
   checkResourcesStale: (version: string | null) => string | null;
 
   // Session lock
-  validateSessionLock: (basePath: string) => boolean;
+  validateSessionLock: (basePath: string) => SessionLockStatus;
   updateSessionLock: (
     basePath: string,
     unitType: string,
@@ -315,7 +316,10 @@ export interface LoopDeps {
     completedUnits: number,
     sessionFile?: string,
   ) => void;
-  handleLostSessionLock: (ctx?: ExtensionContext) => void;
+  handleLostSessionLock: (
+    ctx?: ExtensionContext,
+    lockStatus?: SessionLockStatus,
+  ) => void;
 
   // Milestone transition functions
   sendDesktopNotification: (
@@ -559,10 +563,24 @@ export async function autoLoop(
     try {
       // ── Blanket try/catch: one bad iteration must not kill the session
 
-      if (deps.lockBase() && !deps.validateSessionLock(deps.lockBase())) {
-        deps.handleLostSessionLock(ctx);
-        debugLog("autoLoop", { phase: "exit", reason: "session-lock-lost" });
-        break;
+      const sessionLockBase = deps.lockBase();
+      if (sessionLockBase) {
+        const lockStatus = deps.validateSessionLock(sessionLockBase);
+        if (!lockStatus.valid) {
+          debugLog("autoLoop", {
+            phase: "session-lock-invalid",
+            reason: lockStatus.failureReason ?? "unknown",
+            existingPid: lockStatus.existingPid,
+            expectedPid: lockStatus.expectedPid,
+          });
+          deps.handleLostSessionLock(ctx, lockStatus);
+          debugLog("autoLoop", {
+            phase: "exit",
+            reason: "session-lock-lost",
+            detail: lockStatus.failureReason ?? "unknown",
+          });
+          break;
+        }
       }
 
       // ── Phase 1: Pre-dispatch ───────────────────────────────────────────
diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index c3de44263..191dc8e21 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -47,10 +47,11 @@ import {
 } from "./crash-recovery.js";
 import {
   acquireSessionLock,
-  validateSessionLock,
+  getSessionLockStatus,
   releaseSessionLock,
   updateSessionLock,
 } from "./session-lock.js";
+import type { SessionLockStatus } from "./session-lock.js";
 import {
   clearUnitRuntimeRecord,
   inspectExecuteTaskDurability,
@@ -461,15 +462,33 @@ function buildSnapshotOpts(
   };
 }
 
-function handleLostSessionLock(ctx?: ExtensionContext): void {
-  debugLog("session-lock-lost", { lockBase: lockBase() });
+function handleLostSessionLock(
+  ctx?: ExtensionContext,
+  lockStatus?: SessionLockStatus,
+): void {
+  debugLog("session-lock-lost", {
+    lockBase: lockBase(),
+    reason: lockStatus?.failureReason,
+    existingPid: lockStatus?.existingPid,
+    expectedPid: lockStatus?.expectedPid,
+  });
   s.active = false;
   s.paused = false;
   clearUnitTimeout();
   deregisterSigtermHandler();
   clearCmuxSidebar(loadEffectiveGSDPreferences()?.preferences);
+  const message =
+    lockStatus?.failureReason === "pid-mismatch"
+      ? lockStatus.existingPid
+        ? `Session lock moved to PID ${lockStatus.existingPid} — another GSD process appears to have taken over. Stopping gracefully.`
+        : "Session lock moved to a different process — another GSD process appears to have taken over. Stopping gracefully."
+      : lockStatus?.failureReason === "missing-metadata"
+        ? "Session lock metadata disappeared, so ownership could not be confirmed. Stopping gracefully."
+        : lockStatus?.failureReason === "compromised"
+          ? "Session lock was compromised or invalidated during heartbeat checks; takeover was not confirmed. Stopping gracefully."
+          : "Session lock lost. Stopping gracefully.";
   ctx?.ui.notify(
-    "Session lock lost — another GSD process appears to have taken over. Stopping gracefully.",
+    message,
     "error",
   );
   ctx?.ui.setStatus("gsd-auto", undefined);
@@ -736,7 +755,7 @@ function buildLoopDeps(): LoopDeps {
     checkResourcesStale,
 
     // Session lock
-    validateSessionLock,
+    validateSessionLock: getSessionLockStatus,
     updateSessionLock,
     handleLostSessionLock,
 
diff --git a/src/resources/extensions/gsd/session-lock.ts b/src/resources/extensions/gsd/session-lock.ts
index 2275ff29d..b2b722388 100644
--- a/src/resources/extensions/gsd/session-lock.ts
+++ b/src/resources/extensions/gsd/session-lock.ts
@@ -40,6 +40,19 @@ export type SessionLockResult =
   | { acquired: true }
   | { acquired: false; reason: string; existingPid?: number };
 
+export type SessionLockFailureReason =
+  | "compromised"
+  | "missing-metadata"
+  | "pid-mismatch";
+
+export interface SessionLockStatus {
+  valid: boolean;
+  failureReason?: SessionLockFailureReason;
+  existingPid?: number;
+  expectedPid?: number;
+  recovered?: boolean;
+}
+
 // ─── Module State ───────────────────────────────────────────────────────────
 
 /** Release function from proper-lockfile — calling it releases the OS lock. */
@@ -368,7 +381,7 @@ export function updateSessionLock(
  *
  * This is called periodically during the dispatch loop.
  */
-export function validateSessionLock(basePath: string): boolean {
+export function getSessionLockStatus(basePath: string): SessionLockStatus {
   // Lock was compromised by proper-lockfile (mtime drift from sleep, stall, etc.)
   if (_lockCompromised) {
     // Recovery gate (#1512): Before declaring the lock lost, check if the lock
@@ -385,18 +398,23 @@ export function validateSessionLock(basePath: string): boolean {
           process.stderr.write(
             `[gsd] Lock recovered after onCompromised — lock file PID matched, re-acquired.\n`,
           );
-          return true;
+          return { valid: true, recovered: true };
         }
       } catch {
         // Re-acquisition failed — fall through to return false
       }
     }
-    return false;
+    return {
+      valid: false,
+      failureReason: "compromised",
+      existingPid: existing?.pid,
+      expectedPid: process.pid,
+    };
   }
 
   // If we have an OS-level lock, we're still the owner
   if (_releaseFunction && _lockedPath === basePath) {
-    return true;
+    return { valid: true };
   }
 
   // Fallback: check the lock file PID
@@ -404,10 +422,27 @@ export function validateSessionLock(basePath: string): boolean {
   const existing = readExistingLockData(lp);
   if (!existing) {
     // Lock file was deleted — we lost ownership
-    return false;
+    return {
+      valid: false,
+      failureReason: "missing-metadata",
+      expectedPid: process.pid,
+    };
   }
 
-  return existing.pid === process.pid;
+  if (existing.pid !== process.pid) {
+    return {
+      valid: false,
+      failureReason: "pid-mismatch",
+      existingPid: existing.pid,
+      expectedPid: process.pid,
+    };
+  }
+
+  return { valid: true };
+}
+
+export function validateSessionLock(basePath: string): boolean {
+  return getSessionLockStatus(basePath).valid;
 }
 
 /**
diff --git a/src/resources/extensions/gsd/tests/auto-loop.test.ts b/src/resources/extensions/gsd/tests/auto-loop.test.ts
index f5dfe4db3..dc34f1952 100644
--- a/src/resources/extensions/gsd/tests/auto-loop.test.ts
+++ b/src/resources/extensions/gsd/tests/auto-loop.test.ts
@@ -14,6 +14,7 @@ import {
   type AgentEndEvent,
   type LoopDeps,
 } from "../auto-loop.js";
+import type { SessionLockStatus } from "../session-lock.js";
 
 // ─── Helpers ─────────────────────────────────────────────────────────────────
 
@@ -341,7 +342,7 @@ function makeMockDeps(
     preDispatchHealthGate: async () => ({ proceed: true, fixesApplied: [] }),
     syncProjectRootToWorktree: () => {},
     checkResourcesStale: () => null,
-    validateSessionLock: () => true,
+    validateSessionLock: () => ({ valid: true } as SessionLockStatus),
     updateSessionLock: () => {
       callLog.push("updateSessionLock");
     },
@@ -532,6 +533,41 @@ test("autoLoop exits on terminal complete state", async (t) => {
   );
 });
 
+test("autoLoop passes structured session-lock failure details to the handler", async () => {
+  _resetPendingResolve();
+
+  const ctx = makeMockCtx();
+  ctx.ui.setStatus = () => {};
+  const pi = makeMockPi();
+  const s = makeLoopSession();
+  let observedLockStatus: SessionLockStatus | undefined;
+
+  const deps = makeMockDeps({
+    validateSessionLock: () =>
+      ({
+        valid: false,
+        failureReason: "compromised",
+        expectedPid: process.pid,
+      }) as SessionLockStatus,
+    handleLostSessionLock: (_ctx, lockStatus) => {
+      observedLockStatus = lockStatus;
+      deps.callLog.push("handleLostSessionLock");
+    },
+  });
+
+  await autoLoop(ctx, pi, s, deps);
+
+  assert.deepEqual(observedLockStatus, {
+    valid: false,
+    failureReason: "compromised",
+    expectedPid: process.pid,
+  });
+  assert.ok(
+    !deps.callLog.includes("resolveDispatch"),
+    "should stop before dispatch after lock validation fails",
+  );
+});
+
 test("autoLoop exits on terminal blocked state", async (t) => {
   _resetPendingResolve();
 
diff --git a/src/resources/extensions/gsd/tests/session-lock-regression.test.ts b/src/resources/extensions/gsd/tests/session-lock-regression.test.ts
index f955ebf83..22bc3d397 100644
--- a/src/resources/extensions/gsd/tests/session-lock-regression.test.ts
+++ b/src/resources/extensions/gsd/tests/session-lock-regression.test.ts
@@ -17,6 +17,7 @@ import { tmpdir } from 'node:os';
 
 import {
   acquireSessionLock,
+  getSessionLockStatus,
   validateSessionLock,
   releaseSessionLock,
   readSessionLockData,
@@ -201,6 +202,50 @@ async function main(): Promise<void> {
     }
   }
 
+  // ─── 7b. getSessionLockStatus with missing metadata → reason surfaced ──
+  console.log('\n=== 7b. missing lock metadata → structured reason ===');
+  {
+    const base = mkdtempSync(join(tmpdir(), 'gsd-session-lock-'));
+    mkdirSync(join(base, '.gsd'), { recursive: true });
+
+    try {
+      const status = getSessionLockStatus(base);
+      assertEq(status.valid, false, 'missing lock metadata is invalid');
+      assertEq(status.failureReason, 'missing-metadata', 'missing metadata reason is surfaced');
+      assertEq(status.expectedPid, process.pid, 'expected PID is included');
+    } finally {
+      rmSync(base, { recursive: true, force: true });
+    }
+  }
+
+  // ─── 7c. getSessionLockStatus with foreign PID → reason surfaced ───────
+  console.log('\n=== 7c. foreign PID in lock file → structured reason ===');
+  {
+    const base = mkdtempSync(join(tmpdir(), 'gsd-session-lock-'));
+    mkdirSync(join(base, '.gsd'), { recursive: true });
+
+    try {
+      const foreignPid = process.pid + 1000;
+      const lockFile = join(gsdRoot(base), 'auto.lock');
+      writeFileSync(lockFile, JSON.stringify({
+        pid: foreignPid,
+        startedAt: new Date().toISOString(),
+        unitType: 'execute-task',
+        unitId: 'M001/S01/T01',
+        unitStartedAt: new Date().toISOString(),
+        completedUnits: 0,
+      }, null, 2));
+
+      const status = getSessionLockStatus(base);
+      assertEq(status.valid, false, 'foreign PID lock is invalid');
+      assertEq(status.failureReason, 'pid-mismatch', 'PID mismatch reason is surfaced');
+      assertEq(status.existingPid, foreignPid, 'existing PID is included');
+      assertEq(status.expectedPid, process.pid, 'expected PID is included');
+    } finally {
+      rmSync(base, { recursive: true, force: true });
+    }
+  }
+
   // ─── 8. Acquire after release is possible ─────────────────────────────
   console.log('\n=== 8. acquire after release → re-acquirable ===');
   {