400 Bad Request

What does 400 Bad Request mean?

The server could not understand the request because it is malformed. It belongs to the 4xx Client Error class, which means the request contains bad syntax or cannot be fulfilled. Something about the request needs to change.

When do APIs return 400?

Invalid JSON syntax, a missing required field, a wrongly typed value, or a malformed query parameter.

How to fix a 400 Bad Request

Validate your request body as JSON first — a trailing comma or a single quote is the usual culprit. Confirm Content-Type: application/json is set, and read the response body: most APIs explain exactly which field failed.

Reproduce a 400 response

The fastest way to understand a status code is to trigger one and read the full response. Open the API tester and send a request to https://httpbin.org/status/400 — you will see the 400 status along with every response header the server sent. Comparing those headers against a working request is often what reveals the cause.

When you are debugging a real endpoint, check the response headers as carefully as the body, and confirm you are using the right HTTP method for the operation.

Related 4xx status codes