# 226 IM Used: HTTP Delta Encoding

> 226 IM Used is the response to a delta-encoded request under RFC 3229. See the A-IM and IM headers, how it works, and why almost nothing implements it.

Source: https://howhttpworks.com/status-codes/226
Last reviewed: 2026-10-04

> **TL;DR:** 226 IM Used is the success status for RFC 3229 delta encoding: the client says which cached version it holds, and the server replies with just the difference. It is registered with IANA but not implemented by browsers or mainstream servers, so you are unlikely to meet it.

## How it works

The client sends the ETag of the version it already has in `If-None-Match`, and lists the delta formats it can apply in `A-IM`. If the server can produce a delta against that version, it replies 226 with `IM` naming the format used.

```http
GET /feed.xml HTTP/1.1
Host: example.com
If-None-Match: "v41"
A-IM: vcdiff, gzip

HTTP/1.1 226 IM Used
ETag: "v42"
IM: vcdiff
Delta-Base: "v41"
Content-Length: 812
Cache-Control: no-cache

...vcdiff delta bytes...
```

The client applies the delta to its stored copy `"v41"` to reconstruct `"v42"`. If the base is not available, the server can send the full response with 200 as usual.

The original use case was RSS and Atom polling, where each fetch re-downloaded a feed that had changed by a single item. Servers had to keep or compute deltas per base version, caches in the middle needed to understand `Delta-Base`, and clients needed patch code. Gzip and `304 Not Modified` turned out to get most of the benefit.

## Why it did not catch on

Three things worked against it. Delta generation needs the server to retain old versions or compute a diff per client base, which does not scale on a CDN where each edge node sees different clients. Intermediaries that do not know `A-IM` cannot cache the delta response safely, because the same URL and the same request headers (apart from `If-None-Match`) produce different bodies. And clients need a diff library for every format they advertise, plus a way to fall back when the base copy has been evicted. A plain 304 plus gzip wins on simplicity.

The IANA registry still lists 226 under RFC 3229, so tools such as status code enumerations and the Python `http.HTTPStatus` enum include it. That is the most likely place you will meet the name.

## Practical notes

- 226 is not in RFC 9110's list of status codes that are cacheable by default, so send explicit freshness headers if you ever implement it.
- Clients must not send `A-IM` to arbitrary servers expecting the feature to work. A normal server ignores it and returns 200, which is the safe fallback.
- If you want incremental sync today, design it into the API (a `since` cursor, `If-None-Match` with 304, or a change stream), not into the protocol layer.

## Try it with curl

`226 IM Used` comes from delta encoding (RFC 3229), which almost no server or CDN implements. If a server does, you ask for it with `A-IM` plus the ETag of the version you already hold, and the reply carries an `IM` header naming the delta format. curl does not apply deltas, it only shows the response.

```bash
curl -i -H 'If-None-Match: "v41"' -H 'A-IM: vcdiff, gzip' https://example.com/feed.xml
```

## Related

- [206 Partial Content](https://howhttpworks.com/status-codes/206)
- [304 Not Modified](https://howhttpworks.com/status-codes/304)
- [200 OK](https://howhttpworks.com/status-codes/200)
- [ETag](https://howhttpworks.com/headers/etag)