title: REST API Framework. Recommended behavior for invalid querystring parameter
body: <p>I am implementing a REST API Framework, and I wonder what the recommendedbehavior is, when a client submits an invalid querystring parameter.</p>
  
  <p>I will illustrate what I mean with a specific example:
  Say, I have an API handler on the /api/contacts/ endpoint, and the handler provides a querystring filter named <code>id</code>, which enables clients to select certain contacts with the provided IDs.</p>
  
  <p>So, a GET or DELETE request could be <code>/api/contacts/?id=2&amp;id=4&amp;id=lalalala</code>.</p>
  
  <p>Clearly, there is no such thing as a Contact with <code>id=lalalala</code>. In this case, what should the server behave like?</p>
  
  <ul>
  <li><p>Ignore the invalid Contact with <code>id=lalalala</code>, and only filter the contacts on the valid ids, 2 and 4.</p></li>
  <li><p>Respond with an error code that indicates this error. If yes, which error code should be provided?</p></li>
  </ul>
  
  <p>Thanks in advance.</p>
  
  <p>Edit: To clarify; The main focus of the framework I develop, is having a predictable behavior and hence response codes. For this reason, I want the clients consuming an API built on this framework, to expect the least possible surprises. 
  So, the question basically is: Should the API return an error in this case(and if yes, which)? Or ignore invalid filter entries, and only filter on the correct querystring parameters?</p>