ERR_REQUIRE_ESM on Vercel runtime with "type": "module" monorepo — build succeeds, fails at runtime

Link to the code that reproduces this issue

https://github.com/ferc/next-16-2-esm-bug

To Reproduce

1. Clone the repo and deploy to Vercel with root directory set to apps/web and build command cd ../.. && turbo run build --filter={apps/web}... && cd apps/web
2. Visit the deployed URL (e.g. https://next-16-2-esm-bug.vercel.app/)
3. The page returns 500 with ERR_REQUIRE_ESM

The repo is a pnpm monorepo with:

• apps/web: Next.js 16.2.0 app with "type": "module" and turbopack.root set to monorepo root
• packages/utils: workspace package with "type": "module" that re-exports a file using fs/promises and path
• packages/storage, packages/ui, packages/ai-service: workspace packages with "type": "module" and dependencies (sentry, drizzle-orm, zod, tanstack, etc.)
• Two dynamic pages (/ and /login) importing from workspace packages

Current vs. Expected behavior

Current: The build compiles successfully on Vercel, but dynamic pages fail at runtime with:

ERR_REQUIRE_ESM: require() of ES Module .next/server/app/page.js
from ___next_launcher.cjs not supported.
page.js is treated as an ES module file as it is a .js file whose nearest parent
package.json contains "type": "module"

next build generates .next/package.json with {"type": "commonjs"} which should override the parent's "type": "module". The error indicates this file is either missing or not found in the serverless function bundle at runtime.

Locally, next build + next start works fine — the issue only occurs when deployed to Vercel.

Expected: Pages should render correctly on Vercel, as they do locally.

Notes:

• Works on 16.1.6, fails on 16.2.0
• Build succeeds on Vercel — only fails at runtime in serverless functions
• Works locally with next build + next start
• The issue is intermittent on the test repo but consistent on a larger monorepo with the same setup

Provide environment information

Operating System:
  Platfor