What it means
The server fulfilled a GET with one or more instance manipulations (delta encoding) applied, so the body is a diff, not the full resource.
Common causes
- A client sent A-IM: vcdiff and the server supports delta encoding
How to fix it
As a client or visitor
Apply the delta to your cached copy using the method named in the IM header.
As the site or API owner
Only send 226 when the client explicitly requested a delta encoding it supports.
Example response
HTTP/1.1
HTTP/1.1 226 IM Used
Date: Tue, 07 Oct 2025 09:30:00 GMT
Server: nginx
IM: vcdiff
Content-Length: 0Specification
FAQ
› Is 226 widely used?
No. RFC 3229 delta encoding saw little adoption; you are unlikely to meet it outside niche systems.
› Which header requests a delta?
A-IM, for example A-IM: vcdiff, together with an If-None-Match for the cached version.