MDX Plugin String Format resolution doesn't work with ESM plugins with multiple exports

[wesbos]

Link to the code that reproduces this issue

https://github.com/wesbos/next-mdx-plugin-string-issue

To Reproduce

When using strings to import a MDX Rehype plugin, the Next.js importer fails.

I believe this is only when the package has multiple ESM exports - like this package: https://github.com/stefanprobst/rehype-extract-toc/blob/main/package.json

Thansk to @karlhorky for linking me to the possible commit / code. CC @timneutkens

https://github.com/vercel/next.js/pull/72802/files#diff-d5904dff78d88856dc003d263e6f70f0a607166230fba8e4b947a3bae5e4e87cR10

import createMDX from "@next/mdx";

const withMDX = createMDX({
  options: {
    rehypePlugins: [
      ["@stefanprobst/rehype-extract-toc"],
      ["@stefanprobst/rehype-extract-toc/mdx"],
    ],
  },
});

We get the Error:

Error: No "exports" main defined in /Users/wesbos/Sites/delete-me/mdx-plugin-issue/node_modules/@stefanprobst/rehype-extract-toc/package.json
    at Array.map (<anonymous>) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}
 ⨯ unhandledRejection: Error: No "exports" main defined in /Users/wesbos/Sites/delete-me/mdx-plugin-issue/node_modules/@stefanprobst/rehype-extract-toc/package.json
    at Array.map (<anonymous>) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}
 ⨯ unhandledRejection:  Error: No "exports" main defined in /Users/wesbos/Sites/delete-me/mdx-plugin-issue/node_modules/@stefanprobst/rehype-extract-toc/package.json
    at Array.map (<anonymous>) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}

Or

Error: Package subpath './mdx' is not defined by "exports" in /Users/wesbos/Sites/delete-me/mdx-plugin-issue/node_modules/@stefanprobst/rehype-extract-toc/package.json
    at Array.map (<anonymous>) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}

This error does not happen if the plugin is imported inside next.config.mjs and passed as a javascript function, but since Turborepo must pass as a string, I cannot do this.

The error exists both with and without --turbo.

Current vs. Expected behavior

_

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:03:15 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6000
  Available memory (MB): 65536
  Available CPU cores: 10
Binaries:
  Node: 23.1.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: 9.10.0
Relevant Packages:
  next: 15.0.4-canary.51 // Latest available version is detected (15.0.4-canary.51).
  eslint-config-next: N/A
  react: 19.0.0-beta-04b058868c-20240508
  react-dom: 19.0.0-beta-04b058868c-20240508
  typescript: 5.1.3
Next.js Config:
  output: N/A

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

Markdown (MDX)

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

next dev (local)

Additional context

No response

[wesbos]

It looks like the issue here is when the package doesn't have a "default" export - see the above PR that fixed it for me. I still think this is a bug though because I could import the package without issue, it only happened when passing a string