Should a RESTful API return 400 or 404 when passed an invalid id

Question

When building a RESTful API and a user provides an id of resource that does not exist, should you return 404 Not Found or 400 Bad Request

Answer 1

Should be 404 ( Not Found ). 400 is used if you can't fulfill the request due to bad syntax, however for your case, the syntax is correct, however there is no resource foobar.

You can use 400 if user uses non-existent API like below :

https://api.domain.com/v1/nonexistAPI/xyz/xyz

You can also refer to this REST API Design Blog which tell you how to design your REST error codes.