100	Continue — server received headers, client should send body
101	Switching Protocols — upgrading to WebSocket or HTTP/2
102	Processing — server received request, still working (WebDAV)
103	Early Hints — preload resources before final response

200	OK — standard success response
201	Created — resource successfully created (POST/PUT)
202	Accepted — request received, processing async
203	Non-Authoritative Info — transformed by proxy
204	No Content — success with no response body (DELETE)
205	Reset Content — success, client should reset form
206	Partial Content — range request fulfilled
207	Multi-Status — multiple status codes (WebDAV)

GET → 200	Return resource with body
POST → 201	Resource created, include Location header
PUT → 200/204	Updated resource (with/without body)
DELETE → 204	Deleted, no body returned
PATCH → 200	Partial update, return modified resource

300	Multiple Choices — multiple representations available
301	Moved Permanently — resource moved, update bookmarks
302	Found — temporary redirect (often misused as 303)
303	See Other — redirect with GET after POST
304	Not Modified — use cached version (ETag/If-Modified)
307	Temporary Redirect — same method, temporary location
308	Permanent Redirect — same method, permanent location

301/308	Permanent — search engines update index
302/307	Temporary — original URL stays canonical
301/302	May change method to GET on redirect
307/308	Must preserve original HTTP method

400	Bad Request — malformed syntax or invalid parameters
401	Unauthorized — authentication required or failed
403	Forbidden — authenticated but not permitted
404	Not Found — resource does not exist
405	Method Not Allowed — HTTP method not supported
406	Not Acceptable — can't satisfy Accept header
408	Request Timeout — client too slow to send request
409	Conflict — request conflicts with current state

410	Gone — resource permanently deleted (not just missing)
411	Length Required — Content-Length header missing
412	Precondition Failed — If-Match/If-Unmodified failed
413	Content Too Large — request body exceeds limit
414	URI Too Long — URL exceeds server limit
415	Unsupported Media Type — Content-Type not accepted
422	Unprocessable Content — valid syntax, semantic errors
429	Too Many Requests — rate limit exceeded

500	Internal Server Error — unhandled exception on server
501	Not Implemented — server doesn't support the method
502	Bad Gateway — upstream server sent invalid response
503	Service Unavailable — overloaded or in maintenance
504	Gateway Timeout — upstream server didn't respond in time
505	HTTP Version Not Supported — version not handled
507	Insufficient Storage — server can't store request (WebDAV)
511	Network Auth Required — captive portal login needed

500	Retry with backoff; may be transient
502/504	Retry — upstream issue may resolve
503	Check Retry-After header before retrying
501/505	Do not retry — fix client request

200	OK — everything worked
201	Created — new resource made
204	No Content — success, empty body
301	Moved Permanently — update URL
304	Not Modified — use cache
400	Bad Request — fix your request
401	Unauthorized — log in first
403	Forbidden — insufficient permissions
404	Not Found — wrong URL or deleted
422	Unprocessable — validation errors
429	Too Many Requests — slow down
500	Server Error — not your fault
502	Bad Gateway — proxy/upstream failure
503	Unavailable — try again later

Accept	Desired response media types (e.g. application/json)
Authorization	Credentials (Bearer token, Basic base64)
Content-Type	Media type of request body
If-None-Match	Conditional: ETag for cache validation
If-Modified-Since	Conditional: date for cache validation
Cache-Control	Caching directives (no-cache, max-age)
User-Agent	Client identification string

Content-Type	Media type of response body
Location	Redirect target or created resource URL
ETag	Entity tag for cache validation
Cache-Control	Caching directives (max-age, no-store)
Retry-After	Wait time before retrying (429/503)
WWW-Authenticate	Auth scheme required (sent with 401)
Set-Cookie	Set cookie on client