What HTTP status response code should I use if the request is missing a required parameter

httphttp-status-codes

I am thinking 412 (Precondition Failed) but there may be a better standard?

Best Solution

Status 422 seems most appropiate based on the spec.

The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions.

They state that malformed xml is an example of bad syntax (calling for a 400). A malformed query string seems analogous to this, so 400 doesn't seem appropriate for a well-formed query-string which is missing a param.

UPDATE @DavidV correctly points out that this spec is for WebDAV, not core HTTP. But some popular non-WebDAV APIs are using 422 anyway, for lack of a better status code (see this).