[linyiru]

What?

With cacheComponents: true, a Link header set in next.config.js headers() (or by middleware) is duplicated on ISR pages that also emit a React preload Link (e.g. an <img fetchPriority="high">), and gains one more copy on every background revalidation:

req 3  Link: </llms.txt>; rel="llms-txt"
       Link: </llms.txt>; rel="llms-txt",</llms.txt>; rel="llms-txt",</hero.png>; rel=preload; as="image"; fetchpriority="high"
req 4  Link: </llms.txt>; rel="llms-txt"
       Link: </llms.txt>; rel="llms-txt",</llms.txt>; rel="llms-txt",</llms.txt>; rel="llms-txt",</hero.png>; rel=preload; ...
...

An on-demand render (cache MISS) of a page not prerendered at build time also returns both the config Link and the preload Link twice.

Why?

prerenderToStream handles React's onHeaders by appending each value to the live response and then storing res.getHeader(name) in metadata.headers, which becomes the APP_PAGE cache entry's headers. That snapshot includes values the render did not produce:

• the Link from headers() in next.config.js, already on the response before rendering;
• during a background revalidation, the Link values the serving path replayed from the previous cache entry onto the same response.

app-page-runtime appends the cache entry's headers to every response after the config headers are set, so each regeneration stores, and later serves, one more copy. On a MISS the render has already written the headers to the response, and the same values are appended again from metadata.headers.

How?

prerenderToStream now records only the values it produces (React's onHeaders output and the redirect location) in metadata.headers, and no longer writes them to the response. Every consumer of a prerender result already applies metadata.headers to the response (app-page-runtime for fresh renders and cache hits, the export worker at build time), so the headers reach the client exactly once. A Link from middleware or next.config.js is still merged with React's preload Link (#73431), because the cached value is appended to the response, not set over it.

renderToStream (the dynamic path) is unchanged. It writes onHeaders output to the response only and never stores it in metadata.headers, so it has no cache entry to accumulate into.

Added test/production/app-dir/cache-components-link-header, which configures a Link in headers(), renders a 'use cache' page with a 1s revalidate and a high-priority image, and checks the config Link and the preload Link appear exactly once on every response across four revalidations, for a build-time prerendered page, a prerendered dynamic param, and an on-demand dynamic param. Without the change it fails on webpack (the config Link count climbs 1, 2, 3, 4, 5, and the MISS response has 2 of each). With the change it passes on both webpack and Turbopack.

One behavioural difference: the segment-prefetch cache branch returns before the cached headers are replayed, so an RSC segment-prefetch response that happens to trigger a fresh render no longer carries the preload Link. Such responses have no use for a preload.

Standalone reproduction: https://github.com/linyiru/next-cache-components-link-header-repro

Fixes #100959

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Commit: 52e185c
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