I don't get it - what does this look like in JSON?
(looks to me like it'll simply emit {'room_key': NNN} but that would require clients to deduce from the absence of some other key/value pair that this is a room-full error, which indeed would be unfortunate; if that's what you meant by your q then yes I agree it would be better to pass an 'error': 'full' key/val pair)