HTTP response for an unacceptable Accept header

Question

My RESTFul API can only respond JSON-encoded data (i.e. all my headers have Content-Type: application/json). What should I return, if the request has an Accept header that does not allow JSON (for example, Accept: text/html)? Should I just return a 400 Bad Request with an explanation in the body, or is there a more specific status code for this exception?

Note that this is different from unsupported request content-types.

Answer 1

If you want to be semantically correct:

If the request is HTTP/1.0:

a 406 Not Acceptable is the correct thing to return, as the client may not be able to handle a response that isn't of the type requested.

in HTTP/1.1, that's still the "right" thing to do, but there are exceptions,

From RFC 2616 Sec 10.4.7

 Note: HTTP/1.1 servers are allowed to return responses which are
      not acceptable according to the accept headers sent in the
      request. In some cases, this may even be preferable to sending a
      406 response. User agents are encouraged to inspect the headers of
      an incoming response to determine if it is acceptable.

In truth, the likelihood of it mattering is pretty low, as @Jack has mentioned. I'm only including this answer in the interests of completeness.