226 IM Used
The server fulfilled a GET using delta encoding — returning the difference from a version the client already holds rather than the full resource.
What it means
226 IM Used (RFC 3229) responds to an A-IM request by returning one or more instance manipulations — typically a delta against a cached version — instead of the whole resource. It is rare in practice but conceptually powerful for bandwidth savings.
Why it matters
- Delta encoding can dramatically cut transfer size for resources that change slightly between requests.
Common mistakes
- Assuming broad support — very few servers and clients implement instance manipulations.
How WebInspect checks this
- WebInspect reports the status and any A-IM/IM headers involved in the exchange.