Do I need to add my own unique id?

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I suspect that the persistent identifier is a wrapper for the underlying Core Data objectID: NSManagedObjectID. The caveat with using the core data NSManagedObjectID is that it is unique, and permanent, EXCEPT when the object is first created and before it is persisted. There is an initial temporary objectID assigned that is later replaced with the permanent id.

If you want to use the SwiftData PersistentIdentifier, I would recommend checking that this behavior does or does not transfer over from Core Data.

Personally, I've been burned enough by the complexity of dealing with that Core Data NSManagedObjectID that I just create an id: UUID on my objects rather than rely on the persistence layer id.

I think you probably should use your own ID. I tried to rely on it, but I've watched it with the debugger (probably not in the latest beta) and as rkhamilton said it changes value early in the lifecycle. Unless it's documented somewhere as safe for this purpose, I wouldn't trust it (and possibly not even then).

As others have said, it's probably a good idea to have your own unique id (UUID is probably the easiest way). I believe the core data PersistentIdentifier.ID may only be unique to the record on the current install of the current device. If you're doing syncing and want to reference a record on another device, it might not return the same PersistentIdentifier.ID.