diff --git a/src/test-hooks/offscreen-hooks.ts b/src/test-hooks/offscreen-hooks.ts
index bf2463b..c0c7a90 100644
--- a/src/test-hooks/offscreen-hooks.ts
+++ b/src/test-hooks/offscreen-hooks.ts
@@ -300,6 +300,10 @@ globalThis.__mokoshTest = {
 //   op='install-fake-display-media' → { ok: true } OR { ok: false, error }
 //   op='dispatch-ended' → { ok: true } OR { ok: false, error: 'no stream' }
 //   op='has-stream' → { hasStream: boolean }
+//   op='get-display-surface' → { displaySurface: string|null } OR { ok: false, error }
+//     — Plan 01-13 Wave 3A A3 contract. Returns the active track's
+//       `getSettings().displaySurface` value (monkey-patched to 'monitor'
+//       by `installFakeDisplayMedia`); returns null when no stream is live.
 // Unknown ops respond { ok: false, error: 'unknown-op' }.
 //
 // The bridge handler MUST run BEFORE the production offscreen bridge
@@ -349,6 +353,35 @@ chrome.runtime.onMessage.addListener((rawMessage, _sender, sendResponse) => {
     sendResponse({ hasStream: currentStream !== null });
     return false;
   }
+  if (op === 'get-display-surface') {
+    // Plan 01-13 Wave 3A A3 contract — return the active track's
+    // displaySurface so the harness can verify the offscreen-hooks
+    // monkey-patched getSettings() correctly reports 'monitor'.
+    // Production code in src/offscreen/recorder.ts enforces this same
+    // invariant (tears down + throws 'wrong-display-surface' otherwise),
+    // so if recording is live the value is guaranteed monitor — the
+    // harness asserts === 'monitor' for explicit empirical verification.
+    try {
+      if (currentStream === null) {
+        sendResponse({ displaySurface: null });
+        return false;
+      }
+      const track = currentStream.getVideoTracks()[0];
+      if (track === undefined) {
+        sendResponse({ displaySurface: null });
+        return false;
+      }
+      const settings = track.getSettings();
+      const displaySurface = settings.displaySurface ?? null;
+      sendResponse({ displaySurface });
+    } catch (err) {
+      sendResponse({
+        ok: false,
+        error: err instanceof Error ? err.message : String(err),
+      });
+    }
+    return false;
+  }
   sendResponse({ ok: false, error: 'unknown-op' });
   return false;
 });
diff --git a/tests/background/no-test-hooks-in-prod-bundle.test.ts b/tests/background/no-test-hooks-in-prod-bundle.test.ts
index 4eb41ee..1073b0b 100644
--- a/tests/background/no-test-hooks-in-prod-bundle.test.ts
+++ b/tests/background/no-test-hooks-in-prod-bundle.test.ts
@@ -58,8 +58,9 @@
 //                                     (replaces Approach-A `simulateUserStop`)
 //   - `getSegmentCount`             — Plan 01-11 Task 7 segments-count getter (retained)
 //   - `__mokoshOffscreenQuery`      — 01-13 page→offscreen bridge message type
+//   - `get-display-surface`         — 01-13 Wave 3A bridge op string (A3 contract)
 //
-// Total: 8 surface strings. Each MUST be absent from EVERY file under
+// Total: 9 surface strings. Each MUST be absent from EVERY file under
 // `dist/` post-build. The list is mirrored by the harness's A0
 // assertion (tests/uat/harness.test.ts in Wave 3A) so the same
 // invariant is enforced at unit-test time (fast, every CI run) AND
@@ -109,6 +110,7 @@ const FORBIDDEN_HOOK_STRINGS: ReadonlyArray<string> = [
   'dispatchEndedOnTrack',
   'getSegmentCount',
   '__mokoshOffscreenQuery',
+  'get-display-surface',
 ];
 
 /** How long the build child has to finish (`npm run build` is ~10s).