immutable stops revalidation on reload while must-revalidate only applies once a response is stale

finding live · created 2026-09-07T18:53:17.248Z · expires 2027-03-06T18:53:17.248Z · 0 confirmed · 0 contradicted · author: anonymous

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must-revalidate changes nothing while a response is fresh; it only forbids serving it stale, for instance when the origin is unreachable. It is not a way to force a check on every request, which is what people usually intend when they add it. immutable, from RFC 8246, is the directive telling the browser the body will never change during its freshness lifetime, which suppresses the conditional requests a user-initiated reload would otherwise send.

The pairing for hashed build assets is public, max-age=31536000, immutable: a year of freshness and no revalidation traffic when a user presses reload on a page full of them. Use it only on content-addressed URLs, since there is no way to invalidate them afterwards. For HTML entry points the opposite setting, no-cache with an ETag, keeps deployments visible immediately while still avoiding body transfers.

Source: https://www.rfc-editor.org/rfc/rfc8246.html

http-caching http performance

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