Next.js 16.0.1: next build and next dev generate different next-env.d.ts file

[rtivital]

Link to the code that reproduces this issue

https://github.com/mantinedev/next-app-template

To Reproduce

1. Clone the repository and install dependencies
2. run npm run dev
3. See that next-env.d.ts file changed
4. run npm run build
5. See that next-env.d.ts file changed again

As I understood, the next-env.d.ts file must be commited to the vcs and it changing depending on the script might polute the git history.

Current vs. Expected behavior

Current behavior: The next-env.d.ts file changes depending on whether next build or next dev script.
Expected behavior: (same as prior Next.js 16) next-env.d.ts file does not change depending on the script.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.0.0: Wed Sep 17 21:41:45 PDT 2025; root:xnu-12377.1.9~141/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 24.8.0
  npm: 11.6.0
  Yarn: 4.10.3
  pnpm: N/A
Relevant Packages:
  next: 16.0.1
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Turbopack

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

next dev (local)

Additional context

Information with more details here – mantinedev/next-app-template#46

[CHC383]

Not sure if the behavior is expected or not, but next-env.d.ts is not supposed to be committed based on the Vercel template

[icyJoseph]

With isolatedDevBuild, https://nextjs.org/docs/app/api-reference/config/next-config-js/isolatedDevBuild, the dev command outputs to .next/dev, which means that Turbopack traces and typegen output goes to .next/dev. At build time, the typegen goes into .next - you may disable isolatedDevBuild, but you also don't have to track next-env.d.ts in git history.

To have next-env.d.ts available in CI and such, you can run npx next typegen, and that'll make a definitions file available as needed. https://nextjs.org/docs/app/api-reference/cli/next#next-typegen-options

[rtivital]

Thanks for the clarification, I'll add next-env.d.ts to .gitignore