226 IM Used

The server applied instance manipulations (delta encoding) to the resource, as requested with A-IM.

226 at a glance

Code: 226
Name: IM Used
Class: 2xx Successful
Status: IANA-registered standard code
Defined in: RFC 3229, Section 10.4.1
Cacheable by default: No by default. Caches store it only when the response carries explicit freshness (Cache-Control max-age, s-maxage or Expires).

What 226 means

226 IM Used (RFC 3229) means the server fulfilled a GET with a delta or other instance manipulation instead of the full resource, because the client asked for it with A-IM. The client applies the delta to its cached copy. Very little software implements RFC 3229.

What typically causes a 226

RSS/Atom feed servers supporting A-IM: feed to return only new entries (some older blog platforms).
Experimental delta-encoding setups.

Fix it on the server

Only send 226 if the request carried A-IM with a manipulation you support; otherwise return 200.

Fix it as a client or visitor

If you did not send A-IM, a 226 is a server bug; retry without caching.

Headers that go with 226

Header	Role
`IM`	The manipulations applied.
`A-IM`	Request header listing acceptable manipulations.

Example response

HTTP/1.1 226 IM Used
IM: feed
ETag: "a1b2"
Content-Type: application/atom+xml