All HTTP status codes

HTTP 400 Bad Request

Client Error

The server can't or won't process the request because the client sent something malformed — invalid JSON, bad query parameters, broken headers or framing.

Defined in RFC 9110

Common causes

  • Malformed JSON/XML body or wrong Content-Type
  • Invalid query string or header values
  • Request smuggling protections rejecting odd framing
  • Oversized or malformed cookies

How to fix it

  1. Validate the payload against the API schema before sending

  2. Set the correct Content-Type (e.g. application/json)

  3. Clear site cookies if a corrupted cookie triggers it

  4. Log the server-side validation error to pinpoint the exact field

Related status codes

422 Unprocessable Content

The request is syntactically valid but semantically wrong — well-formed JSON that fails business validation (missing fields, invalid values).

415 Unsupported Media Type

The server refuses the request because the body's media type (Content-Type/Content-Encoding) isn't supported by the endpoint.

413 Content Too Large

The request body exceeds what the server is willing to process. Formerly called 'Payload Too Large'.

Want to explore status codes interactively? Try the free HTTP Status Code Reference — search, categories and example responses, right in your browser.