HTTP status code · Success (2xx)
226 IM Used
226 IM Used means the server fulfilled a GET by sending the result of one or more instance manipulations, typically a delta against a version the client already has, instead of the full resource. It belongs to RFC 3229 delta encoding, which never gained wide adoption.
| Class | 2xx, Success |
|---|---|
| Defined in | RFC 3229 §10.4.1 |
| Cacheable by default | Only with explicit Cache-Control or Expires; a cache may combine it with a stored base instance to build the current one |
| Safe to retry | If the delta cannot be applied, request the full resource without A-IM |
| Relevant headers |
|
What 226 means
RFC 3229, section 10.4.1, requires the request to carry an A-IM header listing at least one manipulation the client accepts, such as vcdiff, and the response to carry an ETag for the current instance. The client sends If-None-Match with the ETag of its copy, and the server returns only the difference.
The idea was to save bandwidth on feeds and pages that change a little at a time. In practice compression, CDNs and short cache lifetimes solved the same problem with less complexity, so 226 is mostly a registry entry today.
How to send 226
No mainstream framework implements RFC 3229. Sending 226 means generating deltas yourself and only for clients that sent A-IM; for most sites, compression and 304 revalidation are the practical answer.
Commonly confused with
- 226 vs 304
- 304 Not Modified says the client's copy is still current; 226 says it changed and sends only what changed.
- 226 vs 206
- 206 returns a byte range of the current file; 226 returns a computed transformation, such as a diff, that must be applied to an older copy.
Frequently asked questions
- Do browsers support 226 IM Used?
- No mainstream browser sends A-IM, so they never receive 226. It is used only by specialized clients and servers built around RFC 3229.
- What does IM stand for in 226 IM Used?
- Instance Manipulation: an operation applied to the current instance of a resource, such as a vcdiff delta or gzip compression, listed in the IM response header.
- Is a 226 response cacheable?
- RFC 3229 allows a cache to store it subject to normal expiration and Cache-Control, and to combine it with a cached base instance to build the current one.
Last reviewed by Arielton Oberek.