HTTP 501 Not Implemented

When is 501 used?

• Unsupported HTTP methods
• Feature not yet implemented

How to handle a 501 response

The problem is on the server side. Check server and proxy logs, retry idempotent requests with exponential backoff, and alert on sustained 5xx rates.

Check the status code a URL returns from the command line:

curl -s -o /dev/null -w "%{http_code}\n" https://example.com

How to return 501 from your server

Node.js (Express)

res.status(501).json({ message: 'Not Implemented' });

Python (Flask)

return {"message": "Not Implemented"}, 501

Go (net/http)

w.WriteHeader(501)