HTTP 505 HTTP Version Not Supported

When is 505 used?

• Requesting HTTP/3 from server that only supports HTTP/1.1

How to handle a 505 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 505 from your server

Node.js (Express)

res.status(505).json({ message: 'HTTP Version Not Supported' });

Python (Flask)

return {"message": "HTTP Version Not Supported"}, 505

Go (net/http)

w.WriteHeader(505)