When the host app of a host/AUv3 extension pair tries to find the audio extension it fails. The system should have scanned the bundle for the extension, but this appears to fail. This basically happens in SimplePlayEngine.initComponent. I've seen this intermittent problem for years. There are many sometimes-effective workarounds that I've gathered and used, but the problem still exists. It is not a problem of validation -- the same AU can validate in some run cycles and then not be found via the host app in others. I've got a Feedback Assistant ticket for the latest iteration of the problem (FB22860885), which has affected beta testers. Usually I only see this when developing with Xcode myself. So even though the testers' app lives in ~/Applications it is still failing for some of them.

I've been generating new Audio Unit Extension apps with Xcode 16 (and newer), and although they generally work initially, it is easy (although I'm not sure how to do it reliably) to cause the app to no longer be able to instantiate the audiounit. Generally the call to AVAudioUnit.findComponent fails and SimplePlayEngine hits the fatalError("Failed to find component with type...") In the most recent project, merely adding files to the extension (without making any use of them) caused it to go off the rails. If I "Archive" the app+plugin, there is no audio unit extension in the bundle. If I switch to the audiounit extension and build it it's fine. If I look at the build folder in Library/Developer/Xcode/project_folder the extension_name.appex is there. Any ideas? If I can coax an unmodified audio unit extension project to exhibit this behavior I'll attach it here. Right now what I have has code I don't want to share.