Safari Web Extensions can only be distributed via the AppStore or ran locally to test. I am (successfully) running this locally. As above, the app works, Safari sees it (according to pluginkit) but doesn't see it in the UI. If you want to package an extension to run outside of the AppStore you will need to use a Safari App Extension. When I created this new project in XCode, I selected "Safari Extension App". Apologies for the dumb question, but is this the same thing?