Bug: `expected non-null body source` when passing a `Request` object to fetch AND receiving an error response (Node >= 24.14.0)

Why this is a good first issue

The issue involves a specific Node.js version interaction with fetch that crashes on error responses.

AI Summary

The issue occurs when passing a Request object with a body to fetch in Node.js >= 24.14.0, causing a crash on error responses. The fix likely involves adjusting how the patched fetch handles Request objects with bodies during error responses. The main blocker is understanding Next.js's fetch patching mechanism.

Issue Description

Link to the code that reproduces this issue

https://github.com/masakij/nextjs-expected-non-null-body-source-repro

To Reproduce

The reproduction is very simple. Execute this inside an App Router handler (e.g., Server Action or Route Handler) targeting an endpoint that returns a 4xx or 5xx error:

// 1. Create a Request object with a body
const req = new Request('https://httpstat.us/401', { // An endpoint that returns 401
  method: 'POST',
  body: JSON.stringify({ test: true }),
  headers: { 'Content-Type': 'application/json' }
});

// 2. Pass the Request object directly to global fetch
// 💥 This crashes immediately on Node >= 24.14.0 when the response is an error
const res = await fetch(req);

Current vs. Expected behavior

Expected Behavior:

The patched fetch should return the Response object with res.ok === false without crashing, just as it does in a pure Node.js environment.

Actual Behavior:

The application crashes with TypeError: expected non-null body source at ignore-listed frames.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:53:31 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T8122
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 24.14.0
  npm: 11.9.0
  Yarn: N/A
  pnpm: 10.29.3
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: 5.9.3
Next.js Config:
  output: standalone

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

Not sure

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

next dev (local)

Additional context

Description:

In the App Router environment, using the global patched fetch crashes with TypeError: expected non-null body source under a very specific set of conditions:

You pass a Request object (that contains a body, e.g., POST/PUT) dir