EC
226
HTTPSUCCESSCommon2xx SuccessHIGH confidence

IM Used

Production Risk

Low. This is a rare and optional optimization ('A-IM' stands for 'Applied Instance Manipulation'). Most modern compression is handled via Content-Encoding.

What this means

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. The server must generate a 226 response if it has fulfilled a GET request and the response represents the result of one or more instance manipulations.

Why it happens
  1. 1A client requests a resource and indicates it can accept a delta-encoded response using the 'A-IM' header.
  2. 2The server determines it can send a more efficient response by describing changes from a version the client already has cached.
  3. 3This reduces bandwidth by only sending the difference (delta) between documents.
How to reproduce

A client that has a cached version of a large document requests it again, and the server sends only the changes made since the client's version.

trigger — this will error
trigger — this will error
GET /large-doc.html HTTP/1.1
Host: example.com
A-IM: gdiff

expected output

HTTP/1.1 226 IM Used

Fix

Signal 226 IM Used when returning a delta-encoded response

Signal 226 IM Used when returning a delta-encoded response
// Server-side concept: returning a delta response (RFC 3229)
// Most applications use Content-Encoding: gzip/br instead — 226 is rare.

// If you implement delta encoding, the server must:
// 1. Check the client's A-IM and If-None-Match headers
// 2. Compute the delta between the cached and current version
// 3. Respond with 226 and the correct headers

HTTP/1.1 226 IM Used
ETag: "v3"
IM: gdiff
Content-Type: application/gdiff

// Client must apply the delta to its cached copy to reconstruct the full document.

// For most use cases, standard HTTP caching with ETags achieves similar
// bandwidth savings without the complexity of delta encoding:
app.get('/large-doc', (req, res) => {
  const etag = computeEtag(document);
  if (req.headers['if-none-match'] === etag) {
    return res.status(304).end(); // Not Modified — client cache is current
  }
  res.setHeader('ETag', etag);
  res.send(document);
});

Why this works

226 IM Used is the response code for RFC 3229 instance manipulation (delta encoding). The server computes a diff between the client's cached version (identified by If-None-Match) and the current version, then sends only the diff with the applied manipulation identified in the IM header. This is rarely implemented in practice — standard 304 Not Modified with ETag caching achieves most of the same bandwidth savings with far less complexity.

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