`Request` object behaves differently in fetch

Link to the code that reproduces this issue

https://github.com/lennartkerkvliet/next-fetch-request-bug

To Reproduce

When sending a request body using the Request object in a Next.js application while using server actions, the content-length parameter is not included in the request and instead chunked transfer encoding is used. This behavior differs from the expected functionality of the Fetch API, which internally is also constructing a Request object.

Steps to reproduce the behavior:

1. Create a Request object with a body.
2. Pass the Request object to the fetch function.

Example code:

const body = JSON.stringify({ username: "test", password: "password" });
const request = new Request("https://www.my-site.com/api/login", { 
    method: "POST", 
    headers: { "Content-Type": "application/json" },
    body: body
});
const response = await fetch(request); // Server receives an empty body

Current vs. Expected behavior

I would expect the parameters in Request to be identical to when using the more traditional use of fetch API:

const body = JSON.stringify({ username: "test", password: "password" });
const response = await fetch("https://www.my-site.com/api/login", { 
    method: "POST", 
    headers: { "Content-Type": "application/json" },
    body: body
});

but instead will return the latter

Provide environment information

Operaring System:
  Arch: arm64
  Version: macOS Sonoma 14.4.1

Binaries:
  Node: v22.2.0
  npm: 10.7.0

Relevant Packages:
  next: 14.2.3

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

Not sure, Runtime

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

next dev (local), Vercel (Deployed), Other (Deployed)

Additional context

• Using plain Node.js, the above code works as expected.

It seems to be somewhat related to the changes NextJS is making to the Response object and in particular the duplex option. The Django server I am using for my own project does not support transfer-encoding, and will mark the request as a "Bad request syntax".

Start with the reproduction linked in the issue and compare the two fetch forms: passing a Request object versus passing the URL and options directly. Investigate the server-action/runtime path involved in constructing and sending the Request. Done means the Request form sends the body with Content-Length rather than chunked transfer encoding and is accepted by the described Django server.