typescript should be added to dependencies not devDependencies since it is used to parse the next.config.ts

[modten]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/late-paper-vwtflr?workspaceId=ws_YRKBG6vgzA8hNCZ8dUsZEh

To Reproduce

In our workflow, we remove the devDependencies during the CI phase to reduce the size of the final deploy image.

npx create-next-app@latest 
npm install
npm run build
npm prune --production
npm start

It reports the following problem

➜  workspace git:(main) ✗ npm start       

> start
> next start

   ▲ Next.js 15.4.2-canary.7
   - Local:        http://localhost:3000
   - Network:      http://192.168.241.2:3000

 ✓ Starting...
 ⚠ Installing TypeScript as it was not found while loading "next.config.ts".

Installing devDependencies (npm):
- typescript


added 5 packages, and audited 35 packages in 1s

8 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

 ⨯ Failed to load next.config.ts, see more info here https://nextjs.org/docs/messages/next-config-error
[Error: Cannot find module 'typescript'
Require stack:
- /project/workspace/node_modules/next/dist/build/next-config-ts/transpile-config.js
- /project/workspace/node_modules/next/dist/server/config.js
- /project/workspace/node_modules/next/dist/cli/next-test.js
- /project/workspace/node_modules/next/dist/bin/next] {
  code: 'MODULE_NOT_FOUND',
  requireStack: [Array]
}

Current vs. Expected behavior

In version 15.4.1, due to changes in the transpileConfig file #79055, next now relies on typescript to parse next.config.ts. In this case, typescript should be placed in dependencies to avoid being cleaned up.

And installing dependencies at runtime does not seem to be a good practice. In production environments, it is often forbidden to connect to the public network or perform any operations that change the deployed artifacts.

Why not compile next.config.ts into next.config.js when executing the build command? Starting from Node.js 22, typescript file parsing is natively supported, which may be another solution.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.4.2-canary.7 // Latest available version is detected (15.4.2-canary.7).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: N/A

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

create-next-app

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

next start (local)

Additional context

No response

_NEXT-4657

[vickrant-dev]

Yep, I did try and reproduce this same issue (almost). I did fall into the same error as u mentioned but then afterwards I ran npm start one more time and it works perfectly fine and even If I were to kill the terminal and redo npm start it works fine without causing any issues.

However, If I run this cmd npm prune --production and then run npm start the issue persits again and If I do npm start one more time after this, it works normally again.

I looked up in AI by mentioning the issue and where it exactly breaks and where it exactly works fine, this is what it has to say:

The issue arises because Next.js may rely on certain dev dependencies for the production build, and npm prune --production can remove those dependencies, leading to errors.

[darvids0n]

We just had an app crash in test and production environments because Next.js attempted to call npm install --save-exact --save-dev typescript inside our distroless container which deliberately does not contain NPM for security reasons. Our Next.js is currently pinned to 15.3.x.

Please change this behaviour so that we can opt out of this attempted NPM call in production builds where the code has already been compiled to JavaScript. We should not have to bundle TypeScript as a dependency just for the config file.