The fetch listener called `respondWith` for every request and re-issued the unrecognized ones itself. That routes the whole page through the worker, re-sends request bodies from a different context, and makes the worker's presence observable on requests it has no interest in. Classify synchronously and respond only for the shell and artifact URLs this worker actually owns. Not responding is what leaves the browser doing exactly what it would have done without a worker at all — which is the property the allowlist was written to preserve. Co-Authored-By: Claude Opus 5 <[email protected]>