Could I ask why you used ACCOUNT_EXISTS for rejecting an invalid nick? According to spec, BAD_ACCOUNT_NAME is for "otherwise invalid"

@ValwareIRC just to close the loop here, we do not advertise custom-account-name so you can only register a nickname you are already holding, and in general, anything valid as a nickname is also valid as an account name, so there is no case where we would send BAD_ACCOUNT_NAME. The compatibility issue with force-guest-format is discussed on #2270 but the upshot is that the guest format check in irc/accounts.go is just a final sanity check; the guest prefix should have been stripped before that point, leaving a valid account name.