HTTP Status Code Reference

HTTP response status codes indicate whether a specific HTTP request has been successfully completed. Responses are grouped in five classes: informational responses (1xx), successful responses (2xx), redirection messages (3xx), client error responses (4xx), and server error responses (5xx).

1xx: Informational

Provisional responses. The server has received the request and is continuing to process it. Most clients never see these in practice. Key codes:

100 Continue: the server has received request headers and the client should proceed to send the body (used when the client sends an Expect: 100-continue header for large uploads).
101 Switching Protocols: the server is switching to the protocol specified in the request's Upgrade header - most commonly seen during the WebSocket handshake and HTTP/2 upgrade negotiation.

2xx: Success

The request was received, understood, and accepted. 200 OK is the most common; 201 Created is returned when a new resource is created; 204 No Content is common for successful DELETE requests.

3xx: Redirection

Further action is needed to complete the request. 301 Moved Permanently tells clients (and search engines) to update their links; 302 Found is a temporary redirect; 304 Not Modified avoids re-downloading an unchanged resource.

4xx: Client Error

The client sent a request that the server cannot fulfill. 400 Bad Request means malformed syntax; 401 Unauthorized means authentication is needed; 403 Forbidden means the server refuses regardless of credentials; 404 Not Found means the resource doesn't exist.

5xx: Server Error

The server failed to fulfill a valid request. 500 Internal Server Error is a catch-all for unexpected server conditions; 503 Service Unavailable is returned during maintenance or overload; 504 Gateway Timeout means a proxy didn't get a timely upstream response.

Status code decision tree for API designers

Situation	Use this code	Why
Malformed request syntax	400 Bad Request	Client sent unparseable data
Validation error (valid syntax, invalid data)	422 Unprocessable Entity	Syntax OK but business rules violated
No credentials provided	401 Unauthorized	Authentication required
Credentials valid but access denied	403 Forbidden	Authorization failed
Resource doesn't exist (temporarily)	404 Not Found	May exist again later
Resource permanently removed	410 Gone	Caches and crawlers should deindex
Successful creation	201 Created	Include Location header with new resource URL
Async operation accepted	202 Accepted	Processing will happen later