HTTP status codes

Every status code, what triggers it, and what to actually do about it — not just the one-line definition from the spec.

1xx — Informational

The request was received and the process is continuing. These are rarely seen by application code.

Code	Name	Meaning
100	Continue	The client sent an Expect: 100-continue header and the server is telling it to go ahead and send the request body.
101	Switching Protocols	The server is switching to the protocol the client asked for in the Upgrade header — almost always a WebSocket handshake.
103	Early Hints	The server is sending Link headers so the browser can start preloading critical resources before the real response is ready.

2xx — Success

The request was received, understood and accepted.

Code	Name	Meaning
200	OK	The request succeeded.
201	Created	The request succeeded and a new resource was created, usually by a POST or PUT.
202	Accepted	The request was accepted for processing, but the work has not finished.
204	No Content	The request succeeded and there is deliberately no body to return.
206	Partial Content	The server is returning only part of the resource because the client sent a Range header.

3xx — Redirection

Further action is needed to complete the request — usually following a redirect.

Code	Name	Meaning
301	Moved Permanently	The resource has a new permanent URL.
302	Found	A temporary redirect.
303	See Other	Redirects the client to fetch the result with GET, regardless of the original method.
304	Not Modified	The cached copy the client already has is still current, so no body is sent.
307	Temporary Redirect	Like 302, but guarantees the method and body are preserved.
308	Permanent Redirect	Like 301, but the method and body are preserved.

4xx — Client Error

The request contains something the server will not or cannot process. The fix is normally on the client side.

Code	Name	Meaning
400	Bad Request	The server could not understand the request: malformed JSON, an invalid query parameter, a header that does not parse.
401	Unauthorized	Authentication is required and either missing or invalid.
403	Forbidden	The server understood the request and knows who you are, but you are not allowed to do this.
404	Not Found	The server has no resource at this URL.
405	Method Not Allowed	The URL exists but does not accept this HTTP method — a POST to a GET-only endpoint, for example.
406	Not Acceptable	The server cannot produce a response matching the client's Accept header.
408	Request Timeout	The client took too long to send the complete request and the server gave up waiting.
409	Conflict	The request conflicts with the current state — a duplicate unique key, or an edit based on a stale version.
410	Gone	The resource existed but has been deliberately and permanently removed.
413	Payload Too Large	The request body exceeds a limit the server imposes.
415	Unsupported Media Type	The server does not accept the Content-Type the client sent.
418	I'm a Teapot	Defined in a 1998 April Fools' RFC for the Hyper Text Coffee Pot Control Protocol.
422	Unprocessable Content	The request is syntactically valid but semantically wrong — well-formed JSON where a field fails validation.
429	Too Many Requests	The client has been rate limited.
431	Request Header Fields Too Large	The headers exceed the server's limit, usually because of oversized cookies.
451	Unavailable For Legal Reasons	The content is blocked for legal reasons — a court order, a takedown, a regional restriction.

5xx — Server Error

The server failed to fulfil an apparently valid request. The fix is on the server side.

Code	Name	Meaning
500	Internal Server Error	The server hit an unhandled error.
501	Not Implemented	The server does not support the functionality required to fulfil the request.
502	Bad Gateway	A server acting as a proxy got an invalid response from the upstream server.
503	Service Unavailable	The server is temporarily unable to handle the request — overloaded, or down for maintenance.
504	Gateway Timeout	A proxy did not get a response from the upstream server in time.
505	HTTP Version Not Supported	The server does not support the HTTP protocol version the client used.
507	Insufficient Storage	The server cannot store the representation needed to complete the request.
511	Network Authentication Required	The client must authenticate to get network access — a captive portal on hotel or airport Wi-Fi.

The three you will actually debug

404 vs 410. 404 means "not here"; 410 means "gone deliberately". Search engines drop 410s from the index much faster.
401 vs 403. 401 is "I do not know who you are"; 403 is "I know who you are and the answer is no". Sending 401 when you mean 403 sends clients into a pointless re-authentication loop.
502 vs 504. 502 means the upstream answered with garbage or refused the connection; 504 means it never answered at all. The first points at a crashed process, the second at a slow one.