Webpack production build includes ~840KB of dev-only Next.js Devtools modules when using certain Browserslist configs

[AndreiBalan-dev]

Link to the code that reproduces this issue

https://github.com/AndreiBalan-dev/nextjs-webpack-issue-repro/

To Reproduce

1. Clone the reproduction repository:

   git clone https://github.com/AndreiBalan-dev/nextjs-webpack-issue-repro.git
   cd nextjs-webpack-issue-repro

2. Install dependencies using npm ci:

   npm ci

3. Build the project using webpack mode:

   npm run build:webpack

4. Check the production client bundle for dev-only strings:

   grep -o "next-devtools" .next/static/chunks/ed9f2dc4-.js | wc -l
   grep -o "dev-overlay" .next/static/chunks/ed9f2dc4-.js | wc -l

5. Observe that ~840–860KB of dev-only modules are present inside the production bundle.

Current vs. Expected behavior

Current Behavior

The production webpack output includes dev-only code such as:

• next-devtools/userspace/*
• client/dev/*
• client/dev-overlay/*
• client/app-link-gc

Counts inside the affected chunk (ed9f2dc4-*.js):

• "next-devtools": 27 occurrences
• "dev-overlay": 29 occurrences
• "dev-tools-indicator": 16 occurrences

Chunk size: ~840–860KB of dev-only content included in production.

Expected Behavior

These modules should never appear in a production client bundle, regardless of Browserslist,
dependency versions, or SWC/Babel transformations.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Home
  Available memory (MB): 32539
  Available CPU cores: 24
Binaries:
  Node: 22.14.0
  npm: 11.6.4
  Yarn: N/A
  pnpm: 10.28.2
Relevant Packages:
  next: 16.2.0-canary.16
  eslint-config-next: N/A
  react: 19.3.0-canary-10680271-20260126
  react-dom: 19.3.0-canary-10680271-20260126
  typescript: 5.9.2
Next.js Config:
  output: N/A

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

Webpack

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

next build (local)

Additional context

I haven’t bisected the entire canary history yet, but I can confirm the issue exists on both 16.2.0-canary.16 and 16.2.0-canary.35.
The minimal reproduction consistently reproduces it across all recent canary versions I tested.

I have opened a PR with more information here: #89244