What is the proper HTTP response code for request without mandatory fields

Question

Consider simple case where user is deleting a post. This is simple HTTP DELETE/POST request with one mandatory field, post_id.

What should server do if post_id is not provided?

Apparently, user should never encounter this behaviour, so let's be puristic.

My first take would be 400 bad request, but spec says

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications. 

and I'd say missing field is OK from syntax/http POV, it's application domain-specific semantic requirement.

200 OK with explanations is bad, 500 feels weird as this is request problem.

Thoughs?

Answer 1

400 is the correct response.

400 is not restricted to a malformed syntax from an HTTP point of view. Missing a mandatory argument is an error in the syntax defined by the application and thus a "Bad Request"

EDIT

At first it seems strange that there is no separate return code for this, but the return codes are designed to differentiate in what actions the client should take. A 400 error code means that the client should change the POST data or query string to the format defined by the application. Hence it is appropriate for this case.