[snuxoll]

And said logic determined your input was invalid, HTTP 400 Bad Request is a perfectly reasonable response.

If you want to use HTTP as nothing more than a transport layer and return 200 OK's all day then fine, but why are you even using HTTP at that point?

[grandalf]

Unless I'm doing some sort of pure REST API I'm simply using HTTP as a transport protocol for my application specific protocol.

If HTTP delivers my application protocol's message successfully, it's an HTTP 2xx, but my application's protocol may have a range of statuses that are not "transport-like" in the way HTTP status codes are, which are the main things that API consumers care about, assuming the basic transport is working.