`The router state header was sent but could not be parsed` error occurs during version skew (v15 => v16.2)

[mizdra]

Link to the code that reproduces this issue

https://github.com/mizdra-sandbox/repro-next-version-skew-prefetch-bug

To Reproduce

1. Start the old version of Next.js: cd next-old && pnpm run build && pnpm run start &
2. Open http://localhost:3000/ in the browser.
3. Stop the old version of Next.js: killall -9 "next-server (v15.5.15)"
4. Start the new version of Next.js: cd next-new && pnpm run build && pnpm run start &
5. Scroll down in the browser until you can see the link to Sub Page

Current vs. Expected behavior

Actual behavior

The new version of Next.js server outputs the following error:

Expected behavior

The new version of Next.js server outputs no errors.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.4.0: Thu Mar 19 19:33:25 PDT 2026; root:xnu-12377.101.15~1/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 14
Binaries:
  Node: 24.15.0
  npm: 11.12.1
  Yarn: 1.22.22
  pnpm: 10.33.0
Relevant Packages:
  next: 16.2.4 // Latest available version is detected (16.2.4).
  eslint-config-next: N/A
  react: 19.2.5
  react-dom: 19.2.5
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Runtime, Headers, Not Found

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

next start (local), Other (Deployed)

Additional context

• The error occurs when a prefetch request is sent from a page rendered by an older version of Next.js to a newer version of Next.js server.
• The following conditions are required to reproduce:
  • The older version of Next.js must be version 15 (reproduced confirmed in 15.0.0 and 15.5.15, though not all versions have been tested)
  • The newer version of Next.js must be version 16.2 (reproduced in 16.2.0 and 16.2.4, but not in 16.1.7)
  • Dynamic API (e.g., headers(), cookies()) is called within not-found.tsx
  • The server is running in the background
• The prefetch request returns 500.
• On pages with many embedded links, a large number of prefetch requests are sent. As a result, many errors occur.
• Errors only occur on the server side, with no particular impact to the user. When the user clicks the link to Sub Page, the browser navigates to Sub Page.
• I've also confirmed the same error in Next.js running on Google Cloud Run.
  • I'm sorry, but I don't have a minimal reproduction code.
• Searching X, it appears that several users have encountered this error.
  • https://x.com/jespertwitties/status/2034883372149793123
  • https://x.com/themcmxciv/status/2037473792352829564
  • https://x.com/abdushbag/status/2039333688190595359

[mizdra]

This might be a duplicate of #92907. However, while #92907 results in an error when upgrading from 16.1 to 16.2, this issue does not produce an error.