diff --git a/changelog.d/render-cancel-test-wait.md b/changelog.d/render-cancel-test-wait.md
new file mode 100644
index 00000000000..1b92f0aa003
--- /dev/null
+++ b/changelog.d/render-cancel-test-wait.md
@@ -0,0 +1,9 @@
+### Fixed
+
+- **Render cancellation test waits longer and fails clearly.**
+  `test_local_backend_kills_the_container_on_cancellation` waited at most 10 s
+  for both fake theme containers and then cancelled anyway, so a loaded runner
+  could fail with a misleading "only one container killed" assertion. It now
+  waits up to 25 s and raises a clear timeout error when the containers never
+  start. Written by Google Jules; the race could not be reproduced locally.
+  (#12449)
diff --git a/tests/unit/agents/runtime/test_render.py b/tests/unit/agents/runtime/test_render.py
index b8ae25ec22f..f1257a38107 100644
--- a/tests/unit/agents/runtime/test_render.py
+++ b/tests/unit/agents/runtime/test_render.py
@@ -866,10 +866,15 @@ async def test_local_backend_kills_the_container_on_cancellation(self, tmp_path:
 
         task = asyncio.create_task(backend.render(job(job_id="cancelme")))
         # Cancel once both theme containers run: a fixed sleep lost the race on a loaded CI runner.
-        for _ in range(200):
+        for _ in range(500):
             if started.exists() and len(started.read_text().split()) == 2:
                 break
             await asyncio.sleep(0.05)
+        else:
+            # Do not leave the render task and its fake containers to event-loop teardown.
+            task.cancel()
+            await asyncio.gather(task, return_exceptions=True)
+            raise RuntimeError("Timed out waiting for containers to start")
         task.cancel()
         with pytest.raises(asyncio.CancelledError):
             await task