nextjs - 💡(How to fix) Fix Next.js 16 + turbopack + Cache Components: build emits a truncated prerendered HTML shell (no inlined RSC payload, no </html>) → blank page + "Connection closed", served with x-vercel-cache: HIT

Link to the code that reproduces this issue

Environmental / production-only — no isolated public reproduction (same situation as #93890). Filing with full production forensics from two coexisting Vercel deploys built from near-identical source — one corrupt, one healthy — because that is the strongest signal I can provide, plus a build-cache correlation the related issues don't mention.

To Reproduce

1. App Router on Next.js 16.2.6, React 19.2.x, next build with turbopack.
2. next.config.js: cacheComponents: true and experimental.turbopackFileSystemCacheForBuild: true.
3. The home route / is a PPR route: a server component rendering a client tree composed of several next/dynamic deferred children, with a loading.tsx streaming a skeleton shell.
4. Deploy to Vercel as Production. The build runs incrementally on a restored cross-deploy turbopack build cache (build log line: Restored build cache from previous deployment (<id>)).
5. The build reports success (◐ / Partial Prerender). At runtime, a hard load of / renders blank.

Current vs. Expected behavior

Current: the cached prerendered shell for / is truncated — ~64KB, containing the bootstrap <script>s but zero self.__next_f pushes (no inlined RSC payload) and no closing </html>; the body ends mid-document on a bare multipart boundary (--<hex>--). Response headers: HTTP 200, content-type: text/html, x-vercel-cache: HIT, x-nextjs-prerender: 1.

In the browser, the turbopack runtime evaluates chunks, the React Flight client reads the (empty) inlined RSC stream, and throws:

Uncaught (in promise) Error: Connection closed.
    at eo (<hash>-i.js:1)            ← react-server-dom Flight client
    ...
    module evaluation @ <chunk>.js
    registerChunk @ turbopack-<hash>.js

Hydration aborts and the page stays blank. A healthy build of the same route emits a ~306KB shell with 63 self.__next_f.push([1,"…"]) rows ending in …</script></body></html>.

Key forensic facts:

• Same source, built twice. Only non-web commits sat between a healthy earlier deploy and the corrupt one; both deploys coexist. Rolling back to the earlier deploy fixed it with no code change.
• All assets intact. Every _next/static/chunks/*.js referenced by the broken shell returns 200 — only the prerendered HTML shell is corrupt.
• Only the build-time static prerender is broken. The dynamic (soft-navigation) RSC for / is complete (~160KB); navigating to / client-side works. Only a hard load of the static shell fails.
• Restored build cache correlation. The broken deploy built on a restored turbopack build cache; a from-scratch (cache-cleared) rebuild of the same source produces a healthy shell. This points at the incremental turbopackFileSystemCacheForBuild path, which none of the related reports mention.

Expected: a build that cannot emit a complete prerender for a route should fail the build (or fall back to dynamic), never write a truncated shell that is then cached and served 200.

Related issues

#93889, #93890 (silent incomplete RSC payload on turbopack prerender), #86511, #92992 (cacheComponents blank page / postponed-state corruption). All open/unresolved; none mention the restored build cache.

Provide environment information

Next.js: 16.2.6  (also reproduced on prior 16.x per production logs)
Bundler: turbopack (next build)
React: 19.2.x
Node: 20
Deployment: Vercel (Production)
Config: cacheComponents: true, experimental.turbopackFileSystemCacheForBuild: true

Which area(s) are affected?

Partial Prerendering (PPR) / Cache Components, Turbopack, Output (build)

Which stage(s) are affected?

next build (prerender), Vercel (Deployed)