isolatedDevBuild causes typescript issues when running next build with stale dev types

[SantoJambit]

Link to the code that reproduces this issue

https://github.com/SantoJambit/next-dev-build-issue

To Reproduce

1. npm ci
2. npm run dev
3. Open http://localhost:3000/api/hello in a browser
4. git checkout issue
5. npm run build
6. notice an error

Current vs. Expected behavior

I'm getting the following error:

.next/dev/types/validator.ts(62,39): error TS2307: Cannot find module '../../../app/api/hello/route.js' or its corresponding type declarations.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sat, 04 Jul 2026 20:51:33 +0000
  Available memory (MB): 31395
  Available CPU cores: 16
Binaries:
  Node: 22.23.1
  npm: 10.9.8
  Yarn: 4.1.1
  pnpm: N/A
Relevant Packages:
  next: 16.3.0 // Latest available version is detected (16.3.0).
  eslint-config-next: N/A
  react: 19.2.1
  react-dom: 19.2.1
  typescript: 5.9.3
Next.js Config:
  output: standalone

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

TypeScript

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

next build (local)

Additional context

When I delete .next/dev/types, the error goes away.

I'm assuming this is due to the tsconfig entries:

  "include": [
    ...
    ".next/types/**/*.ts",
    ".next/dev/types/**/*.ts"
  ],

Removing them doesn't work though, since next.js just keeps bringing them back on build.

[tejaraju357]

Hey! I'm taking a look at this now and working on a fix. Will update in a few minutes once verified.