/error from Pages Router is being bundled with App Router

[AnYiEE]

Link to the code that reproduces this issue

https://github.com/AnYiEE/touhou-mystia-izakaya-assistant

To Reproduce

1. Clone the repository
2. Execute pnpm add @next/bundle-analyzer
3. Modify next.config.ts to import Analyzer
4. Create a .env.local file and write SELF_HOSTED=true in it
5. Execute pnpm build
6. See output .next/analyze/client.html

Current vs. Expected behavior

From the image, it can be seen that react-dom-client.production.js and react-dom.production.js are bundled, with the former being version 19.1.0-rc, while my app uses 18.3.1, which is the latter.

Is this a bug? Or is Next.js 15 not fully compatible with React 18? (I remember Next.js 14 didn't have this issue).

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 65410
  Available CPU cores: 12
Binaries:
  Node: 22.14.0
  npm: 11.1.0
  Yarn: N/A
  pnpm: 10.5.2
Relevant Packages:
  next: 15.2.0
  eslint-config-next: 15.2.0
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.7.3
Next.js Config:
  output: N/A

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

Output, Performance

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

next build (local)

Additional context

File generated by the Analyzer (extension needs to be changed back)

client.html.txt

[AnYiEE]

And the output bundled with v15.2.0 is approximately 12kb larger than that of v15.1.7. (form cli log massage, shared js, 106kb before, 118kb after)

[samcx]

@AnYiEE Thank you for submitting an issue!

We are taking a look—

[samcx]

@AnYiEE I've updated the title to reflect what's going on here!