Turbopack: Path doubling with outputFileTracingRoot in monorepo causes vercel build failure

[domhede]

Link to the code that reproduces this issue

https://github.com/domhede/turbopack-monorepo-repro

To Reproduce

1. Clone the repo: git clone https://github.com/domhede/turbopack-monorepo-repro
2. Install dependencies: yarn install
3. Navigate to frontend: cd apps/frontend
4. Run Vercel build: npx vercel build

Current vs. Expected behavior

Expected: Build succeeds and produces a working standalone output.

Actual: Build fails with path doubling error:

Error: ENOENT: no such file or directory, lstat
'/path/to/apps/frontend/apps/frontend/.next/routes-manifest.json'

The path apps/frontend/ is doubled because outputFileTracingRoot causes the build to prepend the relative path from monorepo root, but we're already in that directory.

Provide environment information

• Node.js: 24.x
• Next.js: 15.3.3
• Vercel CLI: 50.4.0

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

Turbopack, Output (standalone)

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

vercel build

Additional context

next.config.js:

const path = require("path");

module.exports = {
  turbopack: {
    root: path.resolve(__dirname, "../.."),
  },
  outputFileTracingRoot: path.resolve(__dirname, "../.."),
  output: "standalone",
};

Notes:

• Without outputFileTracingRoot: turbopack.root is ignored and module resolution fails for workspace packages
• With outputFileTracingRoot: Path doubling occurs during the build
• There is currently no working configuration for Turbopack production builds in monorepos with shared packages
• Standard next build works fine; the issue only occurs with vercel build