diff --git a/src/chrome/profile-manager.ts b/src/chrome/profile-manager.ts
index 1f8bb56..3c8a5f1 100644
--- a/src/chrome/profile-manager.ts
+++ b/src/chrome/profile-manager.ts
@@ -197,6 +197,25 @@ export class ProfileManager {
       return false;
     }
 
+    // Guard: if the persistent profile's Cookies file has been modified after
+    // the last sync, a headless session wrote cookies — do not overwrite them.
+    const persistentCookiesPath = path.join(
+      ProfileManager.PERSISTENT_PROFILE_DIR,
+      profileSubdir,
+      'Cookies'
+    );
+    try {
+      const persistentStat = fs.statSync(persistentCookiesPath);
+      if (persistentStat.mtimeMs > metadata.lastSyncTimestamp) {
+        console.error(
+          '[ProfileManager] Persistent profile cookies modified after last sync — skipping overwrite to preserve headless-acquired session'
+        );
+        return false;
+      }
+    } catch {
+      // Persistent Cookies file doesn't exist yet — sync is needed
+    }
+
     if (currentHash !== metadata.sourceProfileHash) {
       return true; // Source has changed
     }
diff --git a/tests/chrome/persistent-profile.test.ts b/tests/chrome/persistent-profile.test.ts
index 8954c21..fcf7c5f 100644
--- a/tests/chrome/persistent-profile.test.ts
+++ b/tests/chrome/persistent-profile.test.ts
@@ -180,6 +180,35 @@ describe('ProfileManager', () => {
       const manager = new ProfileManager();
       expect(manager.needsSync(sourceDir)).toBe(false);
     });
+
+    it('should return false when persistent profile Cookies were modified after last sync (headless guard)', () => {
+      // Set up source Cookies file and compute its hash
+      const cookiesPath = path.join(sourceDir, 'Default', 'Cookies');
+      fs.writeFileSync(cookiesPath, 'source-cookie-data');
+      const stat = fs.statSync(cookiesPath);
+      const hash = `${stat.mtimeMs}:${stat.size}`;
+
+      // Write metadata with an old timestamp (simulating last sync happened in the past)
+      const oldTimestamp = Date.now() - (40 * 60 * 1000); // 40 minutes ago
+      const metadata: SyncMetadata = {
+        lastSyncTimestamp: oldTimestamp,
+        sourceProfileHash: hash,
+        syncCount: 1,
+        sourceProfileDir: sourceDir,
+      };
+      fs.writeFileSync(ProfileManager.SYNC_METADATA_PATH, JSON.stringify(metadata));
+
+      // Create the persistent profile's Cookies file with a recent mtime
+      // (simulating a headless session that wrote cookies after the last sync)
+      const persistentCookiesDir = path.join(ProfileManager.PERSISTENT_PROFILE_DIR, 'Default');
+      fs.mkdirSync(persistentCookiesDir, { recursive: true });
+      fs.writeFileSync(path.join(persistentCookiesDir, 'Cookies'), 'headless-acquired-cookies');
+      // The persistent Cookies file was just written — its mtime is newer than oldTimestamp
+
+      const manager = new ProfileManager();
+      // Should NOT sync: persistent cookies were written after the last sync by a headless session
+      expect(manager.needsSync(sourceDir)).toBe(false);
+    });
   });
 
   // =========================================================================