HTTP 400 Bad Request
What a 400 Bad Request response means, why it happens, and the fastest ways to fix it.
The server cannot process the request because it is malformed — invalid JSON, missing required fields, bad query parameters, or an oversized/invalid header.
- Malformed JSON body (trailing comma, wrong quotes).
- Missing or mistyped required parameters.
- Corrupt or oversized cookies/headers.
- Validate the request body against the API's schema.
- Log the response body — most APIs explain which field failed.
- Clear cookies if a site persistently 400s in one browser.
Notes
HTTP status codes are defined in RFC 9110 (which replaced RFC 7231). The first digit is the class: 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error.
Same class: 401 · 403 · 404 · 405 · 408 · 409 · 410 · 412 · 413 · 415 · 418 · 422 · 425 · 429 · 431 · 451
All codes: HTTP status code reference
Frequently asked questions
What does HTTP 400 mean?
HTTP 400 Bad Request: The server cannot process the request because it is malformed — invalid JSON, missing required fields, bad query parameters, or an oversized/invalid header.
How do I fix a 400 error?
Validate the request body against the API's schema. Log the response body — most APIs explain which field failed. Clear cookies if a site persistently 400s in one browser.
Is a 400 my fault or the server's?
400 is a client error — the request itself needs to change, though server misconfiguration can also trigger it.