---
title: "510 Not Extended — what it means and how to fix it"
description: "An obsolete code from an abandoned extension framework; the server wanted extra information the request did not carry. What causes an HTTP 510, what to try as a visitor, and what to check if it is your own site."
url: https://atoneplace.net/error/510/
site: "At One Place"
---

# HTTP 510 — Not Extended

**HTTP 510: Not Extended**

An obsolete code from an abandoned extension framework; the server wanted extra information the request did not carry.

## What 510 actually means

Defined by RFC 2774, an experimental HTTP extension framework that was moved to Historic status and never saw real adoption. IANA now marks the code as obsoleted. The original idea was that a server could declare "your request needs to declare which extensions it uses", and answer 510 when it did not. Nothing meaningful uses it today, and encountering one means something unusual is going on.

## Common causes

- A server implementing the abandoned RFC 2774 extension framework
- A bespoke API borrowing the code to mean "missing required extension"

## If you're just trying to view the page

1. Nothing — this is an obsolete code and effectively never appears in normal use
2. Report the URL to the site owner if you genuinely encountered it

## If it's your site

1. Do not use 510; it is obsoleted and no client implements sensible behaviour for it
2. Use 400 or 428 with a clear body if you need to demand something extra from the client

## Reference

- **Status code:** 510
- **Reason phrase:** Not Extended
- **Category:** 5xx — Server Error
- **Cacheable by default:** No
- **Defined in:** RFC 2774 §7 (obsoleted)

## Common questions

### What does HTTP 510 mean?

An obsolete code from an abandoned extension framework; the server wanted extra information the request did not carry. Defined by RFC 2774, an experimental HTTP extension framework that was moved to Historic status and never saw real adoption. IANA now marks the code as obsoleted.

### How do I fix a 510 error?

Nothing — this is an obsolete code and effectively never appears in normal use

### Is 510 a client error or a server error?

510 is in the 5xx range, which means server error. The request was acceptable but the server could not fulfil it, so retrying later can succeed.

## Related pages

- [500 Internal Server Error](https://atoneplace.net/error/500/)
- [502 Bad Gateway](https://atoneplace.net/error/502/)
- [503 Service Unavailable](https://atoneplace.net/error/503/)
- [504 Gateway Timeout](https://atoneplace.net/error/504/)
- [524 A Timeout Occurred](https://atoneplace.net/error/524/)
- [521 Web Server Is Down](https://atoneplace.net/error/521/)
- [522 Connection Timed Out](https://atoneplace.net/error/522/)
- [520 Web Server Returned an Unknown Error](https://atoneplace.net/error/520/)
- [525 SSL Handshake Failed](https://atoneplace.net/error/525/)
- [526 Invalid SSL Certificate](https://atoneplace.net/error/526/)
- [530 Origin DNS Error](https://atoneplace.net/error/530/)
- [501 Not Implemented](https://atoneplace.net/error/501/)

## Sources

- Hypertext Transfer Protocol (HTTP) Status Code Registry — IANA (https://www.iana.org/assignments/http-status-codes/)
- RFC 9110: HTTP Semantics — IETF (https://www.rfc-editor.org/rfc/rfc9110.html)