HTTP Status Code Reference

HTTP status codes are three-digit numbers returned by a server in response to a client's request. They indicate whether the request was successful, redirected, or encountered an error. Status codes are grouped into five categories defined by the first digit: 1xx (Informational), 2xx (Success), 3xx (Redirection), 4xx (Client Error), and 5xx (Server Error).

Each category starts with a specific digit: 1xx for informational responses (request received and being processed), 2xx for successful responses (request understood and accepted), 3xx for redirection (further action needed), 4xx for client errors (bad request, unauthorized, etc.), and 5xx for server errors (server failed to fulfill a valid request).

200 OK is the most common success status code. 404 Not Found is the most common error code. On the server side, 500 Internal Server Error is the most common server error. However, the frequency depends on the application — APIs returning 201 Created or 204 No Content are also very common.

Use 401 Unauthorized when the client has not provided valid credentials (authentication required). Use 403 Forbidden when the client is authenticated but does not have permission to access the resource. For example, a logged-in user trying to access an admin endpoint should get 403, not 401.

301 Moved Permanently means the resource has a new permanent URL and all future requests should use it. 302 Found (temporary redirect) means the resource is temporarily at a different URL and the original URL should be used for future requests. 301 redirects are cached and affect SEO rankings, while 302 redirects do not.

Use 201 Created when a POST request successfully creates a new resource. Include a Location header pointing to the new resource. Use 200 OK when the POST request processes data without creating a new resource (e.g., a search or calculation endpoint).

418 I'm a Teapot is an April Fools' joke defined in RFC 2324 (Hyper Text Coffee Pot Control Protocol). It indicates that the server is a teapot and cannot brew coffee. While not intended for production use, many frameworks and libraries include it as an Easter egg.

A 502 Bad Gateway error means a gateway or proxy server (like Nginx, HAProxy, or Cloudflare) received an invalid response from the upstream server. Common causes include: the application server crashed or is overloaded, the database is unavailable, the application has a bug causing a crash loop, or there is a network issue between the proxy and the application server.