# 510 Not Extended

> Further extensions to the request are required for the server to fulfill it.

Source: https://apiguide.dev/status-codes/510
Class: server-error
Defined in: RFC 2774 <https://www.rfc-editor.org/rfc/rfc2774.html>
Updated: 2026-10-14

## What does HTTP status code 510 mean?

The 510 Not Extended status code (RFC 2774) indicates that the server requires further protocol extensions to complete the request. RFC 2774, the HTTP Extension Framework, has since been moved to Historic, and the IANA registry marks 510 as obsoleted.

## When to use it in APIs

Do not use it. The extension framework it belongs to is obsolete, so clients have no defined way to respond to it.

## How it compares to other codes

* **500 Internal Server Error**: Represents standard application failures.