Browsers cap the cache lifetime of a service worker script at 24 hours during update checks

A browser will not honor a Cache-Control freshness longer than 24 hours on the service worker script itself, so a broken worker cannot pin its own version in place forever. The updateViaCache option on navigator.serviceWorker.register controls this precisely: the default value bypasses the HTTP cache for the top-level worker script while still allowing it for scripts pulled in with importScripts, one setting bypasses it for both, and another allows it for both.

Serve the worker with Cache-Control: no-cache so every update check is a cheap revalidation. The check runs when an in-scope page is navigated to, and after a functional event when the last check was more than 24 hours ago, so a tab left open for days does not pick up a new worker on its own. A worker counts as updated when the fetched bytes differ from the installed script, which is why changing only a hashed asset URL inside the file triggers an update.