Got it. Under the target settings, Signing & Capabilities tab, at the end of the "hardened Runtime" section, there is now a checkbox for "Apple Events". This needed to be checked in addition to the other entries in the entitlements file in order to get things working again. Once I checked that box, and confirmed that yes, I really do want my app to do what I told it to do, it started working properly again.

Potentially interesting update: Events do seem to be working - that is, when Music changes tracks, for example, my application does get the event correctly, with all the relevant data. So Music.app->My app seems to be working fine, it's only *requesting* data from Music.app that seems to not be working correctly.

Nope. Even a completely brand new project, created just with the Music.h header file, the default .h,/m, and .xib files, three lines of code, a completly empty entitelements file, and that key in the info.plist file and I get the same behavior.I can't help but feel that one of the many idiodic "security" measures added in Catalina is preventing this from working, but since there are no errors being logged, I can't tell which one, or how to work around it...

Oh, so you aren't sandboxing your app? Interesting, though that didn't seem to make a difference when I removed that.Yeah, I'm working with XCode. Not sure about doing things straight in the terminal - with the frameworks, xib files, etc, I'm not sure of all the arguments I'd need.That said, maybe it would be worth starting over from scratch. Try the simplest possible sample, and grow it back out from there. See what happens.

Unfortunately that didn't seem to do it for me. What do you have in your entitlements file? Maybe I'm missing something there as well...