226 IM Used is part of delta encoding in HTTP, RFC 3229. A client that has a cached version of a resource sends an A-IM header, for example A-IM: vcdiff, together with If-None-Match naming that version. If the server supports it, it replies with 226 and a body that contains only the changes. The IM header names the "instance manipulations" that were applied, and Delta-Base often names the base version.
Clients that did not ask for delta encoding never receive 226. The idea saves bandwidth for resources that change often, such as feeds, but it has seen very little use.
Example response
HTTP
HTTP/1.1 226 IM Used
Date: Fri, 06 Nov 2026 10:15:32 GMT
ETag: "3f80f-1b6-3e1cb03b"
IM: vcdiff
Delta-Base: "3f80f-1a1-3e1cb000"
Content-Length: 212
[212 bytes of vcdiff-encoded delta]SEO and redirect checking
No SEO impact. Crawlers do not request delta encoding, and normal page requests never produce 226.
Common causes and use cases
- A feed reader requested an RSS or Atom feed with
A-IM: feedand received only the new entries. - A custom client and server use vcdiff or other deltas to sync large documents.
- It almost never appears on the public web.