HAL Plugin (AudioServerPlugin) — P…

HAL Plugin (AudioServerPlugin) — Plugin_StartIO never called from AUHAL input client

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?