So I tried changing the bmNetworkCapabilities to 0xD0 as suggested, but that means that bit 7 is set ("Function supports extended NCM 1.1 capabilities") which my device doesn't - it is an NCM 1.0 device. So the host then tries to send a load of extended capabilities commands which my device doesn't understand. Could you confirm whether macOS is compatible with NCM 1.0 devices? My device works just fine on Linux and Windows with their native NCM drivers. Alternatively, is there a way I can get more debug information out of the NCM kernel extension to figure out what it's doing? I've tried: sudo log config --mode "level:debug" --subsystem com.apple.driver.usb.cdc.ncm but it made no difference.

Sure - just done this.

Thanks - I have raised FB22704546 with a sysdiagnose as requested

Thanks for the reply. I can't post too much about the use case, other than to say it is something very bespoke and wouldn't be distributed publicly. However, I'd expect that mounting a user's entire home directory via an FSKit module is something that many people might want to do (e.g. home directory on a network file share). I can raise an enhancement request. In the meantime, any chance you could give some details on how FSKit "knows" that a particular FSKit module is enabled for a given user? I have noticed that, when I enable the module in "Login Items & Extensions", a file at "/Users//Library/Group Containers/group.com.apple.fskit.settings/enabledModules.plist" is getting updated. If I were to somehow create that file programmatically and add my module's ID to it, would that do the trick and enable my FSKit module to run before the user has logged in?