fix(headers): preserve config Link headers alongside React preload links

[MaxtuneLee]

Summary

Fixes #2788.

• treat Link as an appendable response header when applying matching next.config.js headers() rules in the App Router
• preserve framework-generated React and next/font preload links when a config rule contributes another Link relation
• add regression coverage at the response-finalizer, development-server, and production-server levels

Problem

App Router response finalization applies matching config headers after React has emitted preload headers. The config-header merger only appended Vary and Set-Cookie; for every other existing header name it skipped the config value. As a result, a React or next/font preload Link caused an unrelated config Link, such as rel="describedby", to disappear from the final response.

Link is a list-valued field, so the configured relation and framework preload can coexist in one comma-combined field or separate fields. Next.js preserves both values.

Fix

Include link in the set of response headers that use Headers.append() during App Router config-header application. The existing precedence rules for singular response headers are unchanged.

The fixture reproduces the reported behavior with:

• ReactDOM.preload("/agent-test.woff2", ...)
• next.config.ts setting Link: </llms.txt>; rel="describedby"; type="text/plain"

Both relations are now present in development and production responses.

This change is limited to config-header application. The separate ISR cache provenance issue reported in #2782 is not changed here.

Test plans

Add new finalizer regression test:

Expected: </llms.txt>; rel="describedby"; type="text/plain"
Received: </agent-test.woff2>; rel=preload; as="font"; crossorigin=""; type="font/woff2"

After the fix, the following checks pass:

• vp test run tests/app-rsc-response-finalizer.test.ts tests/request-pipeline.test.ts
• vp test run tests/app-router-next-config-dev.test.ts
• vp test run tests/app-router-production-server.test.ts -t "preserves config Link headers alongside React preload links"
• vp test run tests/routing.test.ts tests/route-sorting.test.ts
• vp check

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@vinext/cloudflare@2791

npm i https://pkg.pr.new/create-vinext-app@2791

npm i https://pkg.pr.new/@vinext/types@2791

npm i https://pkg.pr.new/vinext@2791

commit: c740a4c

[github-actions]

Performance benchmarks

Compared c740a4c against base 3c5cea4 using alternating same-runner rounds. Next.js was unchanged and skipped.

1 improved · 0 regressed · 5 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	134.6 KB	134.6 KB	⚫ +0.0%
Client entry size (gzip)	vinext	122.1 KB	122.2 KB	⚫ +0.0%
Dev server cold start	vinext	3.00 s	2.94 s	🟢 -2.1%
Production build time	vinext	3.19 s	3.17 s	⚫ -0.5%
RSC entry closure size (gzip)	vinext	113.8 KB	113.9 KB	⚫ +0.0%
Server bundle size (gzip)	vinext	191.4 KB	191.4 KB	⚫ +0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}