HTTP 226 IM Used
HTTP 226 IM Used is a rare success status from RFC 3229 (Delta encoding in HTTP). The server has fulfilled a GET request and the response is one or more instance-manipulations applied to the current instance — most commonly a delta against a version the client already holds. It is almost never seen in ordinary crawling and signals a specialised content-negotiation feature is in play.
What this means
226 IM Used is defined by RFC 3229, the specification for delta encoding in HTTP. When a client signals support via the A-IM (Accept-Instance-Manipulation) request header and the server applies one or more instance manipulations, the server may respond 226 instead of 200. The IM response header lists which manipulations were applied.
Delta encoding lets a server send only the difference between a cached version a client already has and the current version, saving bandwidth. The 226 status tells the client the body is that transformed result, not the complete resource.
Why crawlers rarely see it
Search and AI crawlers generally fetch full resources and do not negotiate delta encoding, so a 226 in your logs is uncommon and usually tied to a specialised client or proxy.
If you do see 226 responses, confirm they originate from a system that genuinely understands the IM header it received. A misconfigured intermediary returning 226 to a client that did not request instance manipulation would break that client.
- RFC 3229 status; triggered by the A-IM request header
- Body is a transformation (often a delta), not the full resource
- The IM response header names the manipulations applied
How it appears in analytics and logs
A 226 IM Used response means the server applied instance manipulations (such as a delta) and returned that result instead of the full entity. For crawlers it is unusual; most search and AI crawlers do not request delta encoding, so a 226 usually indicates a custom client or a very specific server configuration.
Diagnostic use case
Recognise a 226 in logs as a delta-encoding response rather than an error, and understand that the body is not the full resource but a transformation of it.
What WebmasterID can help detect
WebmasterID records the HTTP status returned to crawler fetches, so a 226 IM Used appears in bot-intelligence as a non-200 success rather than being mistaken for an error or a human page view.
Common mistakes
- Treating a 226 as an error because it is not 200 — it is a success code.
- Assuming the 226 body is the full resource; it is an instance-manipulation result.
- Returning 226 to clients that never sent an A-IM header.
Privacy and accuracy notes
A 226 is a protocol-level response code with no visitor identity attached. WebmasterID records the status of crawler fetches without linking them to any person.
Related pages
- HTTP 206 Partial Content and range requests
206 Partial Content is the response to a range request: the client asked for a byte range of a resource and the server returned just that portion. It underpins resumable downloads and media streaming, where players fetch a file in chunks. In crawler logs it usually reflects media or large-file fetching rather than page crawling.
- HTTP 304 Not Modified and crawl efficiency
304 Not Modified is the response to a conditional request when the resource has not changed since the client last fetched it. The server returns no body, so the crawler reuses its cached copy. Correct conditional-request support with ETag or Last-Modified saves bandwidth and crawl budget.
- HTTP status code cheat sheet for crawlers
This cheat sheet maps the five HTTP status classes to what they mean for crawlers and indexing. It is a quick reference for reading server logs and Search Console crawl data: which codes index normally, which redirect, which signal client errors, and which are server failures crawlers will retry. The aim is to interpret status codes through a crawl-and-index lens rather than a generic one.
- Website observability
See the HTTP status codes your server returns to crawlers, recorded server-side.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.