Update: I checked MusicKit.swiftmodule's macOS interface in the 26.5 SDK, and questions 1 and 2 appear to be answered there. Every write method on MusicLibrary is marked @available(macOS, unavailable): add, add(_:to:), createPlaylist and edit. MusicLibraryAddable and MusicPlaylistAddable are macOS-unavailable as well. So adding a catalog item from a Mac app is a compile-time no, and reading via MusicLibraryRequest is the only half available. That leaves question 3 as the one I still cannot answer: is there any supported way to hand a catalog item to the Music app for playback on macOS? And a follow-up: is the macOS unavailability of the MusicLibrary write methods intentional and expected to remain, or a gap that may close?