The server cannot or will not process the request due to an apparent client error.
What does HTTP 400 mean?
The 400 Bad Request code is part of the 4XX class. This is a client error code — the request was faulty, unauthorized, or cannot be fulfilled. The problem is on the request side.
When is it used?
A server returns 400 Bad Request in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.