Link to the code that reproduces this issue

https://github.com/scinscinscin/instrepro

To Reproduce

1. Clone repo and push to vercel as project.
2. Hit the /api/hello endpoint
3. Check logs and see that the console.log() call inside instrumentation.server.ts file is not called

Current vs. Expected behavior

I expect "Registering instrumentation (inside register fn)..." to be printed in the logs of the function

Currently, there's nothing printed, leading me to suspect that instrumentation.ts is never called.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP Tue Nov 5 00:21:55 UTC 2024
  Available memory (MB): 14435
  Available CPU cores: 16
Binaries:
  Node: 24.8.0
  npm: 11.6.0
  Yarn: 1.22.22
  pnpm: 8.15.7
Relevant Packages:
  next: 16.0.3 // Latest available version is detected (16.0.3).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A
Done in 2.93s.

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

Pages Router, TypeScript, Turbopack

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

Vercel (Deployed)

Additional context

This is all in pages router. I do not know if this happens for app router.

WHERE IT WORKS:
instrumentation.ts is called (when you're not using page extensions). You can deploy the initial commit in the repository and see that it works. It also works in local dev and local prod builds.

WHERE IT DOESN'T WORK:
If you add a page extension in next.config.ts like server.ts and rename the instrumentation file as well, it doesn't work in Vercel BUT it works in local prod and local dev builds.

So the breaking scenario is Vercel + Pages Router (unsure if it happens in App Router) + With Custom Page Extensions.

I wanted to report this to Vercel but there's this post in their forum that says its a next.js feature, so I don't know where to put it.