Inconsistent 404 Handling with Parallel Routes

Link to the code that reproduces this issue

https://github.com/r4sheed/nextjs-parallel-routes-404-repro

To Reproduce

1. Set up the project:

   • Option A: Clone the repository:

     git clone https://github.com/r4sheed/nextjs-parallel-routes-404-repro.git
     cd nextjs-parallel-routes-404-repro
     

   • Option B: Use CodeSandbox:
     Open the project directly in CodeSandbox: https://codesandbox.io/p/github/r4sheed/nextjs-parallel-routes-404-repro/master?import=true

   Ensure you have the project files as structured. The key files for reproducing the bug are:

   • src/app/(main)/layout.tsx (defines the main layout with the parallel route slot)
   • src/app/(main)/@breadcrumb/default.tsx
   • src/app/(main)/@breadcrumb/[...catchAll]/page.tsx (provides the content for the breadcrumb slot)
   • src/app/(main)/not-found.tsx (custom 404 page for the (main) group)
   • src/app/page.tsx (root page with a link to a non-existent page)
   • src/app/(main)/main-group-page/page.tsx (a page within the group, also with a link to a non-existent page)
   • src/app/(main)/exists/page.tsx (another page within the group)

2. Install dependencies:

   npm install
   

3. Run the development server:

   npm run dev
   

4. Open the application:
   Open http://localhost:3000 in your browser.

5. Test Scenario 1 (Correct 404):

   • On the home page (/), you will see a "Go to a page that does not exist (from root)" link.
   • Click this link.
   • Expected: You should see the "Not Found" page defined in src/app/(main)/not-found.tsx. The browser's network tools should show a 200 status for the navigation.

6. Test Scenario 2 (Incorrect 200 OK):

   • Navigate back to the home page (/).
   • Click the "Go to Main Group Page" link to navigate to /main-group-page.
   • On the /main-group-page, you will see a "Go to a page that does not exist (from main-group-page)" link.
   • Click this link.
   • Expected: You should see the "Not Found" page.
   • Actual: The "Not Found" page is not displayed. The browser's network tools will show an HTTP 200 status for the navigation to /does-not-exist. The content might remain the same as /main-group-page or appear as an empty page, but it's not the custom 404 page.
   • You can repeat this by navigating to /exists first and then clicking the link to /does-not-exist from there; the behavior will be the same.

Current vs. Expected behavior

In Next.js version 15.3.2, there's an issue with inconsistent 404 error handling when using parallel routes (specifically with a @breadcrumb slot and a [...catchAll]/page.tsx segment) during client-side navigation.

What Happens (The Bug):

If you navigate from a page within a route group (e.g., from /main-group-page or /exists) to a non-existent page within that same group (e.g., /does-not-exist), the application incorrectly returns an HTTP 200 status. The custom not-found.tsx page for that group is not rendered. Instead, the content of the previous page might remain visible, or an empty page might be shown. This only happens during client-side navigation; a full page refresh of the non-existent URL correctly shows the 404 page.

What You Expected to Happen:

When navigating from any page (either the root / or a page within the (main) group like /main-group-page) to a non-existent page (e.g., /does-not-exist), the application should consistently display the custom not-found.tsx page defined for the (main) route group. The browser's network tools should indicate a 404 status for the attempted navigation to the non-existent page

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): 2194
  Available CPU cores: 1
Binaries:
  Node: 20.12.1
  npm: 10.5.0
  Yarn: 1.22.19
  pnpm: 8.15.6
Relevant Packages:
  next: 15.3.2 // Latest available version is detected (15.3.2).
  eslint-config-next: 15.3.2
  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)

Parallel & Intercepting Routes

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

next dev (local)

Additional context

No response

Clone the linked reproduction, install dependencies, and run npm run dev. Compare client-side navigation from / and from /main-group-page or /exists to /does-not-exist, focusing on the listed parallel-route files and src/app/(main)/not-found.tsx. Done means every navigation renders the custom Not Found page with the expected HTTP status instead of returning 200 or stale content.