1xx Informational

Code	Name	When to use
100	Continue	Client should continue sending request body
101	Switching Protocols	Server agrees to upgrade (e.g., WebSocket)
103	Early Hints	Preload headers before final response

2xx Success

Code	Name	When to use
200	OK	Standard success response
201	Created	Resource created (POST to collection)
202	Accepted	Request accepted, processing async
204	No Content	Success, no body (DELETE, PUT)
206	Partial Content	Range request fulfilled

3xx Redirection

Code	Name	When to use
301	Moved Permanently	URL changed forever, update bookmarks
302	Found	Temporary redirect, method may change
303	See Other	Redirect to GET after POST (PRG pattern)
304	Not Modified	Cache still valid, use cached version
307	Temporary Redirect	Temporary, preserve HTTP method
308	Permanent Redirect	Permanent, preserve HTTP method

4xx Client Errors

Code	Name	When to use
400	Bad Request	Malformed syntax, invalid params
401	Unauthorized	Not authenticated (missing/invalid credentials)
403	Forbidden	Authenticated but not authorized
404	Not Found	Resource doesn't exist
405	Method Not Allowed	HTTP verb not supported for this endpoint
409	Conflict	State conflict (e.g., duplicate resource)
410	Gone	Resource permanently removed
422	Unprocessable Entity	Validation failed (REST APIs)
429	Too Many Requests	Rate limit exceeded

5xx Server Errors

Code	Name	When to use
500	Internal Server Error	Generic unhandled server error
501	Not Implemented	Feature not implemented
502	Bad Gateway	Upstream server returned invalid response
503	Service Unavailable	Server overloaded or in maintenance
504	Gateway Timeout	Upstream server timed out