[jaywalk]

I think a better solution would be to use the 404 code but include a body with the detailed error. That way the response to an invalid URL looks different from an actual item not being found.

[hn_throwaway_99]

But many clients don't even look at the body if the response is a 404.

[jaywalk]

True, but we're talking about fully internal systems here, where one party is in control of both the client and server.