I believe this is a bug in Safari. Safari MV3 supports the background.page key (so-called 'Event Pages') which you might be able to use as a workaround. Try something like In manifest: background: {page: background.html} In background.html: <script src="my_service_worker.js"></script> Since incorporating my "service worker" like this I have stopped seeing the error (touch wood).

You need to turn on the debug menu in Safari grant full disk access to Terminal run defaults write com.apple.Safari IncludeInternalDebugMenu -bool true In the debug menu select Extensions > reset web extensions state But I would like to know how to do this on iOS.