macOS

Loopback capture is implemented — the standalone taps system audio through ScreenCaptureKit, so it needs macOS 13+ and the Screen Recording permission (that API will not run an audio-only stream, so the capture carries a throwaway 2x2 px stub video alongside the audio). Grant it, then relaunch — the app does not pick the permission up mid-run.

The caveat is that this path has never run on Apple hardware: CI compiles it on every push, but no runner can play audio or drive a real Metal adapter, so the first live run is also its validation. A window with visuals but no reaction to music means capture did not start, not a crash — launch from Terminal to see the reason.