Minimal Next.js Type Pollution Reproduction

[elitebot47]

Link to the code that reproduces this issue

https://github.com/elitebot47/social-media-app/blob/main/src/app/(main)/%5Busername%5D/page.tsx

To Reproduce

1. Clone this repo.
2. Run npm install.
3. Run npm run build.
4. Observe the type error above, even though all dynamic API routes use the correct { params: { username: string } } signature and there is no params: Promise<...> anywhere in the codebase.

Current vs. Expected behavior

Bug: Type Pollution in Dynamic API Route Params

Error

Type '{ params: { username: string; }; }' is missing the following properties from type 'Promise<any>': then, catch, finally, [Symbol.toStringTag]

Environment

• Next.js: 15.3.2

Reproduction Steps

1. Clone this repo.
2. Run npm install.
3. Run npm run build.
4. Observe the type error above, even though all dynamic API routes use the correct { params: { username: string } } signature and there is no params: Promise<...> anywhere in the codebase.

Example Dynamic API Route

// app/api/follow/[username]/route.ts
import { NextRequest, NextResponse } from "next/server";
export async function POST(
  req: NextRequest,
  { params }: { params: { username: string } }
) {
  return NextResponse.json({ message: `POST for ${params.username}` });
}

Notes

• This error persists after deleting .next, node_modules, and package-lock.json and reinstalling.
• No type pollution or incorrect params: Promise<...> usage is present in the codebase.
• This appears to be a Next.js type generation bug.

---

Please advise on a fix or workaround. Thank you!

Provide environment information

{
  
  "version": "0.1.0",
  "private": true,
  "scripts": {
    "dev": "next dev",
    "build": "next build",
    "start": "next start",
    "lint": "next lint"
  },
  "dependencies": {
    "react": "^19.0.0",
    "react-dom": "^19.0.0",
    "next": "15.3.2"
  },
  "devDependencies": {
    "typescript": "^5",
    "@types/node": "^20",
    "@types/react": "^19",
    "@types/react-dom": "^19"
  }
}

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

Not sure

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

next build (local)

Additional context

No response

[icyJoseph]

Hi,

This is rather, Next.js telling you that your function is not correct, and will cause issues.

Next.js 15 introduced a breaking change where a few APIs became asynchrounous: https://nextjs.org/docs/app/guides/upgrading/version-15#asynchronous-page

For the case you describe, params are now a Promise. Same would go with searchParams and the same applies for Route Handlers.

Update your code to:

export default async function ProfilePage({ params }: { params: Promise<{ username: string }> }) {
  const session = await auth();
  if (!session) {
    return <div>Not authenticated , go to Signin page</div>;
  }
  const { username } = await params; // Don't forget this part!

 // rest of implementation

EDIT: I'll move this to a discussion thread instead

[icyJoseph]

@elitebot47 did you sort this out?