Also, getUserVisibleURL doesn't help - when I call that with the provided ID, it returns an error Error Domain=NSCocoaErrorDomain Code=3328 "The requested operation couldn’t be completed because the feature is not supported..

I think the OP explained it well, but what I'm trying to do is keep existing functionality working while I convert our non-replicated FP to replicated, and something has changed in what gets passed to FPUIActionExtensionViewController.prepare() which breaks one of the commands. With the non-replicated FP, it gets a list of the IDs we assign, and with the replicated FP, we get the internal IDs, and I can't do anything with them, because they are opaque to us.

What you're missing is that when you switch to replicated FP, the IDs passed to the UI Extensions prepare method are no longer NSFileProviderItemIdentifiers. The documentation for NSFileProviderItemIdentifier says it is "A unique identifier for an item managed by the File Provider extension", so it's reasonable to expect that anywhere iOS gives me one of them it's the ID I created, unless the documentation says something different (which it doesn't).