HTTP/REST API response status codes

1xx — Informational

Code	Meaning	Typical Use
100 Continue	Server received request headers; client can send body	Used in large file uploads
101 Switching Protocols	Protocol change (e.g., HTTP → WebSocket)	Rare in REST APIs

 2xx — Success

Code	Meaning	When Used
200 OK	Request succeeded	Standard success for GET, PUT, PATCH, or POST (no new resource)
201 Created	Resource created successfully	After a POST creating new resource
202 Accepted	Request accepted for async processing	Async or background jobs
204 No Content	Success, no response body	DELETE success or PUT with no content to return

3xx — Redirection

Code	Meaning	When Used
301 Moved Permanently	Resource URL changed permanently	Rare in APIs
302 Found	Temporary redirect	Sometimes used for login redirects
304 Not Modified	Cached content still valid	Used with caching headers (ETag, If-Modified-Since)

4xx — Client Errors

Code	Meaning	When Used
400 Bad Request	Invalid input or malformed JSON	Missing parameters, invalid field format
401 Unauthorized	Authentication required or token invalid	Missing/expired token
403 Forbidden	Authenticated but no permission	Token valid, but lacks required role/scope
404 Not Found	Resource doesn’t exist	Wrong URL or ID
405 Method Not Allowed	Wrong HTTP method	e.g., POST used instead of GET
409 Conflict	Resource conflict	e.g., Duplicate user or record
415 Unsupported Media Type	Wrong content type	e.g., Expecting application/json
422 Unprocessable Entity	Validation error	Input valid JSON but invalid data
429 Too Many Requests	Rate limiting	Gateway/API throttling

5xx — Server Errors

Code	Meaning	When Used
500 Internal Server Error	Server-side exception	Generic backend failure
501 Not Implemented	Endpoint not implemented	Placeholder APIs
502 Bad Gateway	Upstream service failure	In microservice/API Gateway setups
503 Service Unavailable	Server temporarily overloaded	Maintenance or scaling downtime
504 Gateway Timeout	Upstream did not respond	Slow backend call or timeout