For the HTTP API my company has been building, we're returning 400 status codes for most errors. It is intended as a strong suggestion that client code needs to handle errors rather than ignore them. However, we're aware that some client libraries are limited in their ability to handle status codes other than 2xx, so we allow an optional request parameter that asks the server to return 200 instead of 400 for most errors. As you said, smart people disagree about this, so we let them choose. I think we've hit a good balance.