statuscodes

Status codes are standardized numeric indicators returned by a server to indicate the result of a client request. They communicate success, redirection, or error, and guide client behavior. While HTTP is the most widely used context, many protocols employ similar codes.

In HTTP, status codes are three-digit integers divided into five classes by the first digit: 1xx informational, 2xx success, 3xx redirection, 4xx client error, and 5xx server error. Informational responses include codes such as 100 Continue and 101 Switching Protocols, which manage request bodies and protocol changes. The 2xx class signals success, with common codes like 200 OK, 201 Created, and 204 No Content. Redirection codes include 301 Moved Permanently, 302 Found, and 304 Not Modified, the latter often used in caching. Client error codes in the 4xx range indicate problems with the request, such as 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, and 408 Request Timeout. Server error codes in the 5xx range reflect issues on the server side, including 500 Internal Server Error and 503 Service Unavailable.

Usage considerations: clients typically rely on the category to determine how to proceed—retrying, following a redirect, or presenting an error to the user. Some codes, such as 304 Not Modified, interact with caching mechanisms to avoid unnecessary data transfer. In HTTP, the precise definitions and recommended behavior are documented in standards such as RFC 7231.

Beyond HTTP, many protocols use status or response codes with their own meanings. Status codes provide a concise, machine-readable way to communicate outcomes across diverse systems.