Imagine an authentication endpoint that accepts either a username/password or a SSH key. If you go the optional fields route, your parser will accept invalid c

Imagine an authentication endpoint that accepts either a username/password or a SSH key. If you go the optional fields route, your parser will accept invalid combinations, for example password+ssh key. If you go the proper alternatives way, then you need a parser with two branches that parses user/password in one way, or ssh key the other way. You tell me simple verification is enough, hence you either have a parser that accepts bad data, or deal only with simplistic data so that the parser can't have branches.