I'm designing an API to be as HTTP compliant as I can. This includes sending specific response codes back and using the Accept header to specify versions and response types.
I understand this may appear subjective, but I'm sure there's a conventional way of doing this. I have a set of response types that the API supports, along with a vendor-specific mime type to specify the type and version.
Currently, when the client specifies a non-existent version or type, I'm just returning a 400 Bad Request with an empty body, however, I want to return a useful error message. In the event that I don't know the response type, I feel a bit dirty responding with plain text (or defaulting to JSON). Is there a header I'm missing, or some convention that I should follow? I want to get this one right from the offset.
Try the status code of 406 Not Acceptable.