Link to the code that reproduces this issue

https://github.com/aleewains/next-app

To Reproduce

1. Create a new Next.js 16 project: npx create-next-app@latest
2. Create the following file structure:
   • app/users/@modal/(.)[id]/page.tsx (intercepting route)
   • app/users/@modal/default.tsx (returns null)
   • app/users/[id]/page.tsx (full page)
   • app/users/layout.tsx (renders {children} and {modal} slots)
   • app/users/page.tsx
3. Start dev server: npm run dev
4. Navigate to http://localhost:3000/users
5. Then click on a cardm it open a modal,
6. Now, save any file in the project (add a space and save)
7. Observe the error in the terminal
8. also oberve interception-route-rewrite-manifest file in .next/server
9. video link explaining https://drive.google.com/file/d/1PBaVuGAvnZzVYZF5IY4NC_9Oe9fJs6Ez/view

Current vs. Expected behavior

Expected: Interception routes continue working normally after HMR file saves.

Actual: The following error is thrown on every file save:

Error: Invalid interception route: /users/(.)(.)(.)1
Must be in the format /<intercepting route>/(..|...|..)(..)/<intercepted route>

The number of (.) markers increases with each save (1 save = 1 marker, 3 saves = 3 markers), proving entries accumulate. A full dev server restart is required each time to temporarily resolve it.

Root Cause (investigated): In setup-dev-bundler.js, interception routes are pushed into beforeFiles on every HMR update without clearing old entries:

// Runs on EVERY HMR update — never clears!
opts.fsChecker.rewrites.beforeFiles.push(...interceptionRoutes); // BUG

Fix — filter before pushing:

opts.fsChecker.rewrites.beforeFiles = opts.fsChecker.rewrites.beforeFiles
    .filter(r => !_generateinterceptionroutesrewrites.isInterceptionRouteRewrite(r));
opts.fsChecker.rewrites.beforeFiles.push(...interceptionRoutes);

This is a regression — Next.js 15.2.4 does not have this code at all and works correctly.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 8039
  Available CPU cores: 8
Binaries:
  Node: 24.12.0
  npm: 11.6.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.1.6 // Latest available version is detected (16.1.6).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: N/A
Next.js Config:
  output: N/A

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

Turbopack, Parallel & Intercepting Routes

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

next dev (local)

Additional context

• Confirmed working in Next.js 15.2.4 (the beforeFiles.push pattern does not exist there)
• Confirmed broken in Next.js 16.1.6
• Manually applying the filter-before-push fix to node_modules resolves the issue completely
• Reproducible locally on Windows 11 with Turbopack (default in Next.js 16)
• Does NOT require deployment — purely a local dev server issue