Link to the code that reproduces this issue

https://github.com/pieterjanscheir/my-minimal-nextjs-issue-reproduction

To Reproduce

1. Clone the reproduction repository:

   git clone https://github.com/pieterjanscheir/my-minimal-nextjs-issue-reproduction.git
   cd my-minimal-nextjs-issue-reproduction
   pnpm install

2. Ensure the project has MDX configured via @next/mdx (as in the reproduction), and at least one MDX page in the App Router (for example, mirroring your real setup with app/berichten/adreswijziging/page.mdx).

3. Run a production build:

   pnpm build
   # or
   pnpm next build --webpack

4. Observe the build output. The build succeeds, but the following warning appears:

   <w> [webpack.cache.PackFileCacheStrategy/webpack.FileSystemInfo] Parsing of /.../node_modules/@next/mdx/mdx-js-loader.js for build dependencies failed at 'import(process.platform === 'win32' ? pathToFileURL(path) : path)'.
   <w> Build dependencies behind this expression are ignored and might cause incorrect cache invalidation.

Current vs. Expected behavior

Current behavior

• next build completes successfully, but always prints this webpack cache warning originating from @next/mdx:

  [webpack.cache.PackFileCacheStrategy/webpack.FileSystemInfo] Parsing of /.../node_modules/@next/mdx/mdx-js-loader.js for build dependencies failed at 'import(process.platform === 'win32' ? pathToFileURL(path) : path)'.
  Build dependencies behind this expression are ignored and might cause incorrect cache invalidation.

• This suggests that webpack cannot statically analyze the dynamic import(...) in mdx-js-loader.js and therefore might not track all dependencies for its filesystem cache.

Expected behavior

• next build should either:
  • Not emit this warning at all for the official @next/mdx loader, or
  • Handle this case internally such that cache dependencies are correctly tracked without a warning.
• From a user perspective, using the officially supported MDX integration in a standard App Router project should not produce persistent webpack cache warnings in a clean production build.

Provide environment information

Output of `npx --no-install next info` in the project:


Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.2.0: Tue Nov 18 21:03:25 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T8122
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 25.2.1
  npm: 11.6.2
  Yarn: N/A
  pnpm: 10.28.2
Relevant Packages:
  next: 16.1.6 // Latest available version is detected (16.1.6).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Webpack

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

next build (local)

Additional context

• The warning appears even though the build completes successfully and all pages render as expected.
• The warning comes from the official @next/mdx integration (mdx-js-loader.js), not from custom loaders or custom webpack configuration in the app.
• It seems related to the dynamic import(process.platform === 'win32' ? pathToFileURL(path) : path) used inside the loader, which webpack’s cache analyzer cannot fully understand for dependency tracking.
• The concern is:
  • Noise in production builds caused by a warning coming from official Next.js MDX tooling, and
  • The note that “build dependencies behind this expression are ignored and might cause incorrect cache invalidation,” which suggests that cache correctness may rely on implementation details in this loader.
• It would be helpful to know whether this warning can be safely ignored, or if @next/mdx can be adjusted so that webpack can analyze its dependencies without emitting this warning.