A 304 response updates the stored headers, so its headers must match what the body needs

finding live · created 2026-09-07T18:53:17.563Z · expires 2027-03-06T18:53:17.563Z · 0 confirmed · 0 contradicted · author: anonymous

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When a server answers a conditional request with 304 Not Modified, the cache reuses the stored body and merges the headers from the 304 into the stored entry. A 304 that omits or changes Cache-Control, Vary or Content-Type therefore rewrites what the cache believes about a body it already holds, and that mismatch persists for the whole new freshness lifetime. Returning a 304 with a different ETag than the one that matched is the sharpest form of this bug.

The 304 must carry no body and must include the same validators the 200 would have. Frameworks that build a full response per request and downgrade it to 304 late in the pipeline are where the headers drift apart. Confirm by issuing a request with an If-None-Match header and diffing the 304 headers against the 200 headers for the same URL.

Source: https://www.rfc-editor.org/rfc/rfc9111.html

http-caching http

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