Using pageExtensions + app route build fail `ENOENT: .next/server/app/api/.../route_client-reference-manifest.js`

Link to the code that reproduces this issue

https://github.com/matbour/next15.2.1-page-extensions-error/tree/9bd6f268d4d952fb6ac7b80391a04cf26dd6550c

To Reproduce

Considering this simple route in src/app/api/test/route.ts:

export async function GET() {
	return new Response("Hello, world!");
}

If I want to use the pageExtensions feature in the next.config.ts:

import type { NextConfig } from "next";

const nextConfig: NextConfig = {
	/* config options here */
	pageExtensions: ["p.ts", "p.tsx"],
};

export default nextConfig;

Meaning that I rename:

• src/app/layout.tsx => src/app/layout.p.tsx
• src/app/page.tsx => src/app/page.p.tsx
• src/app/test/route.ts => src/app/test/route.p.ts

Then the build crashes with the following log:

$ pnpm build

> next-v15@0.1.0 build /Users/mathieu/Code/FantiumAG/next-v15
> next build

   ▲ Next.js 15.2.1

   Creating an optimized production build ...
 ✓ Compiled successfully
 ✓ Linting and checking validity of types    
   Collecting page data  .[Error: ENOENT: no such file or directory, open '/Users/mathieu/Code/FantiumAG/next-v15/.next/server/app/api/test/route_client-reference-manifest.js'] {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: '/Users/mathieu/Code/FantiumAG/next-v15/.next/server/app/api/test/route_client-reference-manifest.js'
}

> Build error occurred
[Error: Failed to collect page data for /api/test] { type: 'Error' }

See https://github.com/matbour/next15.2.1-page-extensions-error/tree/9bd6f268d4d952fb6ac7b80391a04cf26dd6550c reproduction + associated GitHub workflow run

Current vs. Expected behavior

Expected behavior is that the build passes, like in https://github.com/matbour/next15.2.1-page-extensions-error/commit/a13fb6d0010073e9184624b7c48e2c3abb00235b where I remove the page extensions. You can also see that the build passes in Github Actions.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.3.0: Thu Jan  2 20:24:24 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T6030
  Available memory (MB): 18432
  Available CPU cores: 12
Binaries:
  Node: 22.13.0
  npm: 10.9.2
  Yarn: N/A
  pnpm: 9.13.2
Relevant Packages:
  next: 15.2.1 // Latest available version is detected (15.2.1).
  eslint-config-next: 15.2.1
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.8.2
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

No response

Clone the linked reproduction at commit 9bd6f268d4d952fb6ac7b80391a04cf26dd6550c and inspect src/app/api/test/route.p.ts alongside next.config.ts. Run pnpm build with pageExtensions set to p.ts and p.tsx, then compare the generated .next/server/app/api/test files with the failing lookup for route_client-reference-manifest.js. Done means the same renamed route builds successfully, as in commit a13fb6d0010073e9184624b7c48e2c3abb00235b.