226 IM Used

HTTP 226 IM Used is returned when a GET request includes an A-IM header requesting an instance manipulation, most commonly a delta encoding, and the server actually applies that manipulation before responding. Instead of sending the full current representation, the server sends only the difference between the version the client already has and the current version, dramatically reducing transfer size for clients that are re-fetching a resource they have mostly already downloaded, such as a large document, feed, or dataset that changes incrementally. The response includes an IM header describing which instance manipulations were applied.

The delta encoding mechanism this status enables predates several more widely adopted alternatives, such as HTTP compression and diff-based sync protocols built at the application layer, and it never saw broad adoption across mainstream browsers or servers. Its use today is largely confined to specialized synchronization tools, some feed readers, and niche protocols that specifically implement RFC 3229's instance manipulation and delta encoding scheme, rather than general web APIs or typical CDN-served content.

Because so few clients send an A-IM header in the first place, and even fewer servers implement delta computation in response, 226 shows up almost nowhere in ordinary web traffic. When it is relevant, it matters mainly for bandwidth-constrained synchronization scenarios, where computing and transferring only a diff meaningfully beats re-downloading an entire large resource, and where both client and server have deliberately implemented the RFC 3229 delta mechanism together.