4xx

4xx is the class of HTTP status codes in the 400 range. They signal a client error: the request cannot be processed due to something the client did or did not provide, rather than a problem with the server. These codes typically indicate issues with the request itself, such as invalid syntax, missing authentication, or insufficient permissions. They are distinct from 5xx server errors, which indicate the server failed to fulfill a valid request.

Common 4xx codes include 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 405 Method Not Allowed, 408 Request Timeout, 409 Conflict, 410 Gone, 411 Length Required, 429 Too Many Requests, 431 Request Header Fields Too Large, and 451 Unavailable For Legal Reasons. Each code has a specific meaning guiding client retry or correction.

Response bodies often include additional details such as an error message, error codes, and links to documentation. Some APIs return a machine-readable error object with a problem detail per RFC 7807 (problem details). Clients should adjust the request rather than retry without change, except in cases like 429 or 408 where the specification may permit retries after a delay.

Caching: GET responses with 4xx are generally not cached; certain proxies may cache based on explicit headers. Server operators use 4xx to indicate client misconfiguration or invalid requests, while servers usually return 4xx for authorization errors if credentials are missing or insufficient.