Unused constants from constants.js do not get tree-shaked in the client bundles

Link to the code that reproduces this issue

https://github.com/seyoon20087/nextjs-constants-not-treeshaked-demo

To Reproduce

NOTE: This actually happens on all Next.js apps that use the current version at the time of this writing; but still I am reporting it here.

1. Clone the repo above;
2. Run yarn build
3. Inspect the bundles (especially .next/static/chunks/main-xxx.js); observe that unused constants are imported in the client bundle
4. To additionally confirm this, I ran yarn client:rsd, opened the link printed in the terminal, then go to the "bundle size" section (next to "compile analysis". I see that, when inspecting the "static/chunls/main-xxx.js" file deeply, I see that node_modules/next/dist/shared/lib/constants.js and node_modules/next/dist/lib/constants.js are included in the bundle but does not tree-shake any constants that are not actually used in the client

Current vs. Expected behavior

Expected behavior:

Unused constants get tree-shaked out from the production bundle

Current behavior:

Unused constants ends up in the production bundle

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 22.6.0: Tue Jul 15 08:22:28 PDT 2025; root:xnu-8796.141.3.713.2~2/RELEASE_X86_64
  Available memory (MB): 8192
  Available CPU cores: 4
Binaries:
  Node: 24.7.0
  npm: 11.5.1
  Yarn: 1.22.22
  pnpm: 10.19.0
Relevant Packages:
  next: 16.0.2-canary.3 // Latest available version is detected (16.0.2-canary.3).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: N/A
Next.js Config:
  output: N/A

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

Output

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

next build (local)

Additional context

This issue started all the way back from these commits:

• https://github.com/vercel/next.js/blob/f50ef62c54e36cd52684a72630531a87d846767b/packages/next/src/shared/lib/router/utils/route-regex.ts#L1-L4
• https://github.com/vercel/next.js/blob/e84b65b0d93f29ed26ef898e980a7e2aca2dae98/packages/next/src/shared/lib/router/router.ts#L47
• https://github.com/vercel/next.js/blob/1e332532c7e0036de2e09802b83eb7d692b2d654/packages/next/src/client/page-loader.ts#L12-L15

I think the exact reason for this issue is that, because you are "transpiling" all these files, and pointing all client entrypoints to, CJS (and not ESM) by default, which negates the possibility of tree-shaking.

Reproduce the issue with the linked demo using yarn build, then inspect .next/static/chunks/main-xxx.js for the constants modules and unused exports. Read the referenced route-regex.ts, router.ts, and page-loader.ts changes alongside the client bundle behavior. Done means production client bundles exclude constants that are not used by the client.