HTTP Status Codes
HTTP 226 IM Used
IM Used — the server fulfilled a delta-encoding request for the resource
What 226 Means
The 226 error on the HTTP Status Codes indicates im used — the server fulfilled a delta-encoding request for the resource. This typically occurs due to client requested delta encoding and the server supports instance manipulations.
A 226 IM Used response indicates the server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations applied to the current instance.
Technical Background
The 226 IM Used status code is part of the HTTP delta encoding extension. It tells the client that the server performed one or more 'instance manipulations' to produce the response, rather than serving the full resource.
Delta encoding reduces bandwidth by transmitting only the differences between the current and previous version of a resource. While this concept is powerful, 226 has not seen widespread adoption because modern CDNs and compression algorithms have largely solved the bandwidth problem it was designed to address.
Common Causes
- Client requested delta encoding and the server supports instance manipulations
- A feed reader requested only the changes since its last fetch
- The server applied one or more instance manipulations to produce the response
Typical Scenarios
- An RSS feed aggregator uses delta encoding to fetch only new articles since its last request
- A bandwidth-constrained client requests incremental updates to a large resource instead of re-downloading it entirely
What to Know
The 226 status code represents a niche optimization. Its presence confirms successful delta encoding between the server and client rather than indicating any error condition.
Frequently Asked Questions
Common questions about HTTP 226 error
Virtually never. This status code is defined for specialized delta-encoding scenarios and is extremely rare in standard web browsing.