---
title: "226 IM Used — what it means and how to fix it"
description: "The server is returning a delta against a copy the client already has, rather than the whole resource. What causes an HTTP 226, what to try as a visitor, and what to check if it is your own site."
url: https://atoneplace.net/error/226/
site: "At One Place"
---

# HTTP 226 — IM Used

**HTTP 226: IM Used**

The server is returning a delta against a copy the client already has, rather than the whole resource.

## What 226 actually means

From the "delta encoding in HTTP" extension: rather than resending an entire document that changed slightly, the server sends only the difference, and answers 226 to say the body is one or more instance manipulations applied to the client's cached copy. It was a clever idea that almost nothing implements. If you have met it at all, it was probably in an academic paper or a protocol test suite.

## Common causes

- A client sent `A-IM:` requesting delta encoding and the server supports it
- A specialised sync or replication system built on RFC 3229

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

1. Nothing — effectively never seen in normal browsing
2. If a tool reports one, the request succeeded; the body is a patch rather than the whole document

## If it's your site

1. Do not build on this; support is essentially nonexistent in browsers and CDNs
2. For efficient updates, use conditional requests with ETags, or a purpose-built sync protocol

## Reference

- **Status code:** 226
- **Reason phrase:** IM Used
- **Category:** 2xx — Success
- **Defined in:** RFC 3229 §10.4.1

## Common questions

### What does HTTP 226 mean?

The server is returning a delta against a copy the client already has, rather than the whole resource. From the "delta encoding in HTTP" extension: rather than resending an entire document that changed slightly, the server sends only the difference, and answers 226 to say the body is one or more instance manipulations applied to the client's cached copy. It was a clever idea that almost nothing implements.

### How do I fix a 226 error?

Nothing — effectively never seen in normal browsing

### Is 226 a client error or a server error?

226 is in the 2xx range, which means success. It is not an error at all.

## Related pages

- [200 OK](https://atoneplace.net/error/200/)
- [204 No Content](https://atoneplace.net/error/204/)
- [201 Created](https://atoneplace.net/error/201/)
- [206 Partial Content](https://atoneplace.net/error/206/)
- [202 Accepted](https://atoneplace.net/error/202/)
- [207 Multi-Status](https://atoneplace.net/error/207/)
- [203 Non-Authoritative Information](https://atoneplace.net/error/203/)
- [205 Reset Content](https://atoneplace.net/error/205/)
- [208 Already Reported](https://atoneplace.net/error/208/)

## 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)