All HTTP status codes
5xx Server Error
RFC 2774 (obsolete)
510 Not Extended
Further extensions to the request are required for the server to fulfill it.
What it means
HTTP 510 Not Extended indicates that further extensions to the request are required for the server to fulfill it. Used by HTTP extension framework (RFC 2774) which is now obsolete.
When it's used
Legacy HTTP extension framework — essentially obsolete in modern systems.
Common causes
- Server using deprecated HTTP extension framework
- Custom protocol extension requirements
How to fix it
- Migrate away from RFC 2774 extensions — they are obsolete
- Use modern API design patterns instead
- Treat any 510 as a server-side bug to investigate
Example response
HTTP/1.1 510 Not Extended
SEO impact
Treated as 5xx — page will not be indexed.