Reference

HTTP status codes, explained

A status code is the first thing a client reads about a response. These entries explain the meaning defined by the HTTP standard, then the causes and fixes you actually meet in logs, APIs and browsers.

How to read a status code

The first digit names the class: 1xx interim, 2xx success, 3xx redirection, 4xx client error, 5xx server error. The last two digits identify the specific condition. A code is a promise about the response, not a description of your application: a 404 says the server did not find a representation for the target URI, whether or not a database row exists.

Two attributes matter when debugging: whether a response is cacheable by default, and whether the request method is idempotent. Both are defined per code in RFC 9110 and decide whether a retry is safe and whether a proxy may reuse an answer.

Where these definitions come from

Meanings follow RFC 9110 (HTTP Semantics) and the WebDAV extensions in RFC 4918. The code registry is maintained by IANA. Facts are stated as the standard defines them; the causes and fixes are practical notes from server logs, not part of the specification.

All entries

100 Continue100 Continue is an interim response that tells the client the request headers were accepted and the body can be sent. 101 Switching Protocols101 Switching Protocols confirms that the server is switching to a protocol requested in the Upgrade header. 200 OK200 OK means the request succeeded and the response body contains the representation the client asked for. 201 Created201 Created means the request succeeded and a new resource was created, usually identified by a Location header. 204 No Content204 No Content means the request succeeded but there is no body to return. 206 Partial Content206 Partial Content returns one requested byte range of a resource instead of the whole representation. 301 Moved Permanently301 Moved Permanently says the resource now lives at a new URL and future requests should use that URL. 302 Found302 Found says the resource is temporarily available at another URL, and the original URL should remain the address clients use. 303 See Other303 tells the client to fetch another URL with GET, usually after a form submission. 304 Not Modified304 Not Modified tells a client that its cached copy is still fresh, so no body needs to be transferred. 307 Temporary Redirect307 Temporary Redirect moves a request to another URL while preserving its method and body. 308 Permanent Redirect308 Permanent Redirect is a permanent move that preserves the request method and body. 400 Bad Request400 Bad Request means the server cannot process the request because the client sent something malformed. 401 Unauthorized401 Unauthorized means the request lacks valid authentication credentials for the target resource. 403 Forbidden403 Forbidden means the server understood the request but refuses to authorize it. 404 Not Found404 Not Found means the server cannot find a current representation for the target URL. 405 Method Not Allowed405 Method Not Allowed means the URL exists but does not support the HTTP method used in the request. 409 Conflict409 Conflict means the request conflicts with the current state of the target resource. 410 Gone410 Gone means the target resource is permanently unavailable and the server has no forwarding address. 412 Precondition FailedA conditional request failed because the resource no longer matches the client’s validator. 413 Content Too LargeThe request body exceeds the limit the server is willing to process. 416 Range Not SatisfiableA Range request asks for bytes outside the resource, or the server cannot satisfy the range. 422 Unprocessable Content422 Unprocessable Content means the request was well formed but its instructions could not be processed. 426 Upgrade RequiredThe server refuses the request until the client switches to a required protocol. 429 Too Many Requests429 Too Many Requests means the client has sent too many requests in a given amount of time. 500 Internal Server Error500 Internal Server Error means the server hit an unexpected condition and could not complete the request. 501 Not ImplementedThe server does not recognize or support the request method. 502 Bad Gateway502 Bad Gateway means a gateway or proxy received an invalid response from the upstream server. 503 Service Unavailable503 Service Unavailable means the server is temporarily unable to handle the request, often due to overload or maintenance. 504 Gateway Timeout504 Gateway Timeout means a gateway or proxy did not receive a timely response from an upstream server.

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