Link to the code that reproduces this issue

https://github.com/mausferd/turbopack-pnpm-app-router-repro

To Reproduce

1. git clone https://github.com/mausferd/turbopack-pnpm-app-router-repro && cd turbopack-pnpm-app-router-repro
2. pnpm install
3. cd apps/web && pnpm dev (uses Turbopack — the default)

Current vs. Expected behavior

Expected: Next 16.2.3 starts the dev server. turbopack.root is set in apps/web/next.config.mjs to an absolute path pointing at apps/web, so Turbopack should use apps/web as the project root, find apps/web/node_modules/next/package.json via the standard pnpm symlink, and serve app/page.tsx.

Actual: dev server reports Ready in ~170ms, then immediately crashes with:

Error: Turbopack build failed with 1 errors:
./app
Error: Next.js inferred your workspace root, but it may not be correct.
    We couldn't find the Next.js package (next/package.json) from the project directory: /private/tmp/turbopack-pnpm-app-router-repro/apps/web/app
     To fix this, set turbopack.root in your Next.js config, or ensure the Next.js package is resolvable from this directory.

Two things to note in the error message:

1. The "project directory" is apps/web/app, not apps/web. Turbopack is treating the App Router app/ folder as the project root. app/ is the Next.js App Router routes directory — it should never be inferred as a project root.
2. turbopack.root IS set in apps/web/next.config.mjs:

   import { fileURLToPath } from 'node:url'
   import { dirname } from 'node:path'
   const __dirname = dirname(fileURLToPath(import.meta.url))
   
   export default {
     turbopack: { root: __dirname },          // /…/apps/web (absolute)
     outputFileTracingRoot: __dirname,
   }

   Both turbopack.root and outputFileTracingRoot are pinned to absolute apps/web, but Turbopack ignores them and still reports apps/web/app as the project directory. The error message advises setting turbopack.root — which is already set.

Workaround

next dev --webpack works perfectly with the same structure:

cd apps/web && pnpm dev:webpack
✓ Ready in 263ms

So the bug is Turbopack-specific. Webpack mode honours the workspace correctly.

Provide environment information

- OS: macOS Darwin 25.3.0 (arm64)
- Node.js: v22.x
- pnpm: 9.15.0
- next: 16.2.3
- react: 19.2.4
- react-dom: 19.2.4
- Router: App Router

Which area(s) are affected? (Select all that apply)

Turbopack

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

• Minimal repro: 7 source files, no extra deps. Plain pnpm workspace + one Next App Router app. No nested workspaces, no file: deps, no custom resolver.
• The repo's apps/web/node_modules/next resolves correctly via the standard pnpm symlink (../../../node_modules/.pnpm/[email protected].../next).
• This was a regression for us when upgrading from [email protected] (no such issue) to 16.2.3.
• Possibly related but distinct: #92534 (Windows pnpm "Next.js package not found" — runtime panic, not workspace inference) and #85057 (closed; sibling file: package, not pnpm workspace).