HTTP status codes / 5xx — Server errors
501 Not Implemented
The server does not support this method/functionality at all.
What it means
The server does not recognize or cannot fulfill the request method for any resource — distinct from 405, which means "not for this URL". Also used by proxies for unsupported features.
Common causes
- Method not recognized by the server at all (distinct from 405: wrong method for this URL)
- Proxy faced a feature it does not implement
How to debug it
Check for typos in custom methods and confirm the API version actually ships the feature you are calling.
Server-side note: Check for typos in custom methods and confirm the deployed API version actually ships the feature.
The fastest way to pin down a 501 is to reproduce the exact request and inspect what actually went over the wire — status, headers, timing and body, without your app code in the way. That is what an API client is for; ReqPad does it from your phone, with every request saved to history.
Related codes
500 Internal Server Error · 502 Bad Gateway · 503 Service Unavailable · 504 Gateway Timeout · 505 HTTP Version Not Supported · 506 Variant Also Negotiates — or the full reference.
Reproduce that 501 in 10 seconds.
Build the request, send it, read status + headers + timing — on your iPhone. Free to start.