Return plain text 404 for non-HTML subresource requests to unknown paths

[gaearon]

Companion to vercel/next.js#95930, which uses Sec-Fetch-Dest to serve a plain text 404 for subresource requests (images, fonts, manifests, scripts, etc.) to unknown paths instead of rendering the app's not-found output.

That fix only covers next dev/next start/minimal mode — deployments on this adapter still returned the full prerendered not-found page for these requests, since the CDN routing layer serves it directly without invoking Next.js.

This closes that gap at the routing layer.

Adds a route in the error phase, right after { handle: 'error' } and before the existing not-found dispatch, matching the same Sec-Fetch-Dest values as is-non-html-sec-fetch-dest.ts in vercel/next.js, and serving the shared _next/static/not-found.txt asset with text/plain + 404 instead.

Stacked on #93, which fixes the same missing-cache-control bug on the pre-existing /_next/static/ 404 route and extracts a shared NOT_FOUND_TXT_HEADERS constant. This PR's new route reuses that constant instead of duplicating the header object.

No local unit test — this repo relies on vercel/next.js's e2e suite against real deployments (see #2); coverage comes from not-found-non-document.test.ts and not-found-non-document-dynamic.test.ts, which had isNextDeploy TODOs for exactly this gap.

Verification

The real CI deploy job (NEXT_ENABLE_ADAPTER=1) can't exercise an unpublished branch — it just selects a Vercel team whose build infra runs whatever adapter version is pinned platform-side.

So I ran the actual vercel/next.js Jest deploy suites locally against this branch: packed this branch into a tarball, wired it into not-found-non-document.test.ts and not-found-non-document-dynamic.test.ts via adapterPath + a file: dependency, and ran pnpm test-deploy-turbo for real against fresh Vercel deployments (not scratch curl checks).

The full matrix is static/dynamic fixture × cacheComponents on/off. All four cells go through the new routing rule — I initially wrote off the dynamic/cacheComponents: false cell as "already correct pre-fix," but that was wrong: it hits the same route as the rest, and before the fix the plain-text response's cache-control wasn't actually overridden, it just inherited the CDN's default for the static not-found.txt asset.

	cacheComponents: false	cacheComponents: true
static fixture	had a TODO, needs this fix — 6/6	builds to the same static output as the cell to the left — confirmed by deploying it separately and byte-diffing headers/body against the cacheComponents: false cell, including a path never previously requested
dynamic fixture	needs this fix — 3/3	had a TODO, needs this fix (PPR) — 3/3

Manual test plan

Live scratch deployments of this branch (rebuilt with the #93 fix included) for the two matrix rows:

• Static /_not-found — https://verify-todo-static-cc.vercel.app
• PPR /_not-found under cacheComponents: true — https://verify-todo-dynamic-manifest.vercel.app

Open either URL with browser dev tools open on the Network tab, then reload. The page loads a missing image, a missing @font-face, and a missing web app manifest, so the browser itself issues image/font/manifest Sec-Fetch-Dest requests to unknown paths — no header spoofing needed.

For each of missing-icon.png, missing-font.woff2, site.webmanifest: response status 404, content-type: text/plain, body Not Found, cache-control: private, no-cache, no-store, max-age=0, must-revalidate (matching the header Next.js itself sets for this response — see router-server.ts in vercel/next.js).

Then navigate to /does-not-exist directly (document request) — still renders the custom not-found page as text/html, unaffected.

[changeset-bot]

🦋 Changeset detected

Latest commit: 40807f6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@next-community/adapter-vercel	Patch

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Project	Deployment	Actions	Updated (UTC)
adapter-vercel-adapter	Ready	Preview	Jul 29, 2026 8:17pm