4xx Client Errors
HTTP 426 Upgrade Required
The client should switch to a different protocol (e.g., TLS 1.3).
When is 426 used?
- Forcing TLS upgrade
- Protocol version upgrade required
How to handle a 426 response
Inspect the request — URL, method, headers, credentials, and body. Retrying the same request unchanged will usually fail again; 429 is the exception, where you should wait for the Retry-After period.
Check the status code a URL returns from the command line:
curl -s -o /dev/null -w "%{http_code}\n" https://example.comHow to return 426 from your server
Node.js (Express)
res.status(426).json({ message: 'Upgrade Required' });Python (Flask)
return {"message": "Upgrade Required"}, 426Go (net/http)
w.WriteHeader(426)Frequently Asked Questions
What does HTTP 426 mean?
426 Upgrade Required: The client should switch to a different protocol (e.g., TLS 1.3).
Is 426 an error?
Yes. 426 is in the 4xx Client Errors class. Client errors: the request is malformed, unauthorized, or targets something that does not exist.