I'm building a virtual audio device using the HAL Plugin (AudioServerPlugin) API on macOS. The plugin loads correctly, WriteMix is called with non-zero audio data, but Plugin_StartIO is never called from my app's AUHAL input client. Environment: macOS 26.5 Xcode (latest) Bundle ID: com.private.SubON Plugin installed at: /Library/Audio/Plug-Ins/HAL/SubONHALPlugin.driver What works: Plugin loads and is visible as "SubON Virtual Device" Plugin_AddDeviceClient is called with bundleID=com.private.SubON WriteMix is called with non-zero maxSrc values (audio is reaching the plugin) AUHAL setup completes: EnableIO input=1 output=0, CurrentDevice set correctly, AudioUnitInitialize succeeds pre-start readback confirms: currentDevice matches, enableIOIn=1 What doesn't work: Plugin_StartIO is never called from the SubON.app input client WillDoIOOperation is never called ReadInput is never called maxAmplitude in the input tap is always 0.0 AUHAL setup code (Swift): AudioUnitUninitialize(auhal) AudioUnitSetProperty(auhal, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, 1, &one, size) AudioUnitSetProperty(auhal, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Output, 0, &zero, size) AudioUnitSetProperty(auhal, kAudioOutputUnitProperty_CurrentDevice, kAudioUnitScope_Global, 0, &deviceID, size) AudioUnitInitialize(auhal) HAL Plugin — WillDoIOOperation: outWillDo = true; outWillDoInPlace = true; Returns true for both ReadInput and WriteMix. Observation: Even when selecting "SubON Virtual Device" as the system input device in System Settings, Plugin_StartIO is still never called. Any ideas what could prevent Plugin_StartIO from being called despite AUHAL configuration appearing correct?