Link to the code that reproduces this issue

https://stackblitz.com/edit/nextjs-ru8yniyr?file=app%2Flayout.tsx

To Reproduce

1. run npm install && npx next dev --webpack
2. notice error

Current vs. Expected behavior

currently it outputs an error:

Type error: Route "app/api/route.ts" does not match the required types of a Next.js Route.
  "SOMETHING" is not a valid Route export field.

but it should not be raising a type error when I add EXTRA parameters.

Provide environment information

❯ next info

Operating System:
  Platform: linux
  Arch: x64
  Version: Ubuntu 20.04.0 LTS Sat Dec 20 2025 23:30:29 GMT+0900 (Japan Standard Time)
  Available memory (MB): NaN
  Available CPU cores: 8
Binaries:
  Node: 20.19.1
  npm: 10.8.2
  Yarn: 1.22.19
  pnpm: 8.15.6
Relevant Packages:
  next: 16.1.0 // Latest available version is detected (16.1.0).
  eslint-config-next: 13.5.1
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.2.2
Next.js Config:
  output: N/A

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

Route Handlers

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

next build (local)

Additional context

I have a use-case where I want to export an extra argument from the route parameter for typing purposes. Please let me do this without raising an error.