nextjs - 💡(How to fix) Fix @next/mdx: unhandled promise rejection when plugin path fails to resolve in mdx-js-loader [1 pull requests]

Link to the code that reproduces this issue

https://github.com/Jinoko01/mdx-catch-bug

To Reproduce

1. Clone the reproduction repo
2. Run npm install
3. Run npm run build (next build --webpack)
4. Observe the build hangs indefinitely or throws UnhandledPromiseRejection

Current vs. Expected behavior

Current: The build process hangs without any error message, or Node.js throws an UnhandledPromiseRejection without webpack properly reporting the error.

node:internal/process/promises:288 UnhandledPromiseRejection: Cannot find module 'remark-plugin-does-not-exist'

Expected: Webpack should receive the error via callback and report it clearly:

Error: Cannot find module 'remark-plugin-does-not-exist' Require stack: ...

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Enterprise
  Available memory (MB): 32488
  Available CPU cores: 16
Binaries:
  Node: 24.14.0
  npm: 11.9.0
  Yarn: N/A
  pnpm: 11.1.1
Relevant Packages:
  next: 16.3.0-canary.29 // Latest available version is detected (16.3.0-canary.29).
  eslint-config-next: N/A
  react: 19.2.6
  react-dom: 19.2.6
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Markdown (MDX), Webpack

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

next build (local)

Additional context

Root cause: In packages/next-mdx/mdx-js-loader.js, the Promise returned by getOptions() is missing a .catch() handler. When a plugin string path fails to resolve (e.g., module not found, ESM import failure), the rejection is never forwarded to webpack's callback, causing the build to stall.

// Current code (line 61) getOptions(options, this.context).then((userProvidedMdxOptions) => { mdxLoader.call(proxy, ...args) }) // Missing: .catch((error) => callback(error))