`Content-Type` header not automatically set with `FormData` + `Request` on Node 24

[rossipedia]

Describe the bug

On Node v24, when passing a FormData instance as the body of a Request, the standard behavior is that the Content-Type header is automatically set to multipart/form-data (with diver) unless explicitly set by the Request constructor's init param.

However, this does not seem to be the case in the jsdom environment. Instead, an error is thrown if you attempt to consume the Request instance using Request.prototype.formData()

Note

This issue is not present (per my testing) on Node versions prior to 24

Reproduction

https://github.com/rossipedia/vitest-formdata-jsdom-issue

The following test file will demonstrate the issue:

// @vitest-environment jsdom
import { test, expect } from "vitest";

test("foo", async () => {
  const formData = new FormData();
  formData.set("foo", "bar");

  const req = new Request("http://localhost:3000/", {
    method: "POST",
    body: formData,
  });

  await expect(req.formData()).resolves.toBeInstanceOf(FormData);
});

This test would be expected to pass, however instead it fails with the error:

TypeError: Content-Type was not one of "multipart/form-data" or "application/x-www-form-urlencoded".

System Info

System:
    OS: macOS 26.0.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 23.17 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.10.0 - /Users/bryanross/.local/state/fnm_multishells/47943_1761602626870/bin/node
    Yarn: 1.22.22 - /Users/bryanross/.local/state/fnm_multishells/47943_1761602626870/bin/yarn
    npm: 11.6.1 - /Users/bryanross/.local/state/fnm_multishells/47943_1761602626870/bin/npm
    pnpm: 10.18.1 - /Users/bryanross/.local/state/fnm_multishells/47943_1761602626870/bin/pnpm
    bun: 1.3.1 - /opt/homebrew/bin/bun
    Deno: 2.5.4 - /opt/homebrew/bin/deno
  Browsers:
    Chrome: 141.0.7390.123
    Firefox: 143.0.3
    Safari: 26.0.1
  npmPackages:
    vitest: ^4.0.3 => 4.0.3

Used Package Manager

npm

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[waldemarennsaed]

Facing the same issue here, mocking fetch and expecting it's mock.calls[0][0] request to have certain formData values. Suddenly stopped working after switching from node 22.20.0 to node 24.11.0.

Which node version are you using?

[rossipedia]

Tested on 24.10.0

Pretty sure it's because this list is incomplete, and all of Node 24's Fetch API types are now a lot stricter about what kind of objects they'll accept. However, I couldn't quite get it to work using patch-package so I'm not 100%