REST API Framework. Recommended behavior for invalid querystring parameter

Question

I am implementing a REST API Framework, and I wonder what the recommendedbehavior is, when a client submits an invalid querystring parameter.

I will illustrate what

Answer 1

Since this is a REST call, we are talking about resources. And whenever we have a wrong filter, we should return a proper error code.

In this case i would go for 400 - bad request as the resource was found and correctly mapped (/api/contacts), but there was a problem with the query string part. Therefore a 400 and not a 404.

Would return a 404 if someone requested /api/contacts-all or some non-existant resource.

EDIT based on comments below

Agree to your comment. Ideally a 400 is a problem with the request. Going by that, you could use a 422 Unprocessable Entity. Please look at the stackoverflow link below and it talks about the same thing.

I would guess that developers around the world would be more comfortable seeing a 400 than 422 for such logical errors due to the fact that bigger companies are using 400 and not 422.

References: Http status codes and 400 for logical error vs malformed request

Answer 2

Following the letter of the law, the response should be a 404 Not found. However, nobody is going to get too upset with you if you prefer to return 400 - bad request.

I would definitely return a 4XX status code though. You want the client to know that they made an error.