Link to the code that reproduces this issue

https://github.com/Git-I985/next-custom-extensions-breaks-instrumentation

To Reproduce

Baseline (works correctly)

next.config.ts — no pageExtensions:

const nextConfig: NextConfig = {
  output: 'standalone',
};

src/instrumentation.ts:

export function register() {
  // @ts-ignore
  const runtime = typeof EdgeRuntime === 'string' ? 'edge' : 'nodejs';
  console.log(`[instrumentation] register() called in ${runtime} (NEXT_RUNTIME=${process.env.NEXT_RUNTIME})`);
}

npm i && rm -rf .next && docker compose up --build --force-recreate

Result: register() is called, logs NEXT_RUNTIME=nodejs. ✅

Reproducing the bug

next.config.ts — add custom pageExtensions:

const nextConfig: NextConfig = {
  output: 'standalone',
  pageExtensions: ['tsx', 'ts', 'universal.ts', 'universal.tsx'],
};

Rename src/instrumentation.ts → src/instrumentation.universal.ts (same content).

npm i && rm -rf .next && docker compose up --build --force-recreate

Result: register() is never called. ❌

Now add proxy.universal.ts at the project root or src folder

npm i && rm -rf .next && docker compose up --build --force-recreate

Result: register() is called, but logs NEXT_RUNTIME=edge (why not node?). ❌

Current vs. Expected behavior

Expected: The file extension used for instrumentation should have no effect on:

1. Whether the file is executed
2. Which runtime (NEXT_RUNTIME) it executes in

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 12
Binaries:
  Node: 24.2.0
  npm: 11.3.0
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.0.10 // There is a newer version (16.2.2) available, upgrade recommended! 
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: 5.9.3
Next.js Config:
  output: standalone
 ⚠ There is a newer version (16.2.2) available, upgrade recommended! 
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue
Deployment: output: 'standalone', Docker

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

Instrumentation, Middleware, Runtime, Not sure

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

next build (local), Other (Deployed)

Additional context

Kinda two distinct but related bugs are observed:

Bug 1 — Extension affects proxy requirement.
With a custom extension, instrumentation.universal.ts is silently ignored unless a matching proxy.universal.ts is present. Meanwhile, proxy.ts does not work as a trigger even though .ts is explicitly listed in pageExtensions.

Bug 2 — Extension forces Edge runtime.
Once the proxy file is in place and instrumentation.universal.ts is executed, NEXT_RUNTIME is always edge — regardless of the actual server runtime context. Standard instrumentation.ts correctly reports NEXT_RUNTIME=nodejs.

The pageExtensions config is documented as controlling which file extensions are treated as pages/routes. It should have no side-effects on the instrumentation lifecycle or the runtime environment reported to register().