Undici v7(+?) `Request` cannot parse `FormData` from Undici v5

[penalosa]

Bug Description

I'm trying to construct a Request with a FormData body, using the global Request in Node.js v24 (which seems to be Undici 7.10.0), but a FormData imported from Undici 5.28.5.

Reproducible By

import { FormData } from 'undici';

const r = new Request('https://example.com', {
  body: new FormData(),
  method: 'POST',
});

console.log(r.headers.get('content-type'));

This outputs multipart/form-data in Node.js 20 & 22, but text/plain in Node.js 24

Additional context

This may well be expected behaviour! However, if it is I would've expected to see some documention mentioning that Undici 5 isn't supported in Node.js 24+.

[anonrig]

cc @Uzlopak @KhafraDev @nodejs/undici

[KhafraDev]

This is expected.

[penalosa]

@KhafraDev does that mean that Undici 5 is deprecated? I couldn't find any mention of that in the docs

[mcollina]

No. This is doing what the spec says we should be doing. If you use FormData from undici, you have to use fetch from undici as well. If you use FormData from global, use fetch from global.

I'll need to add a script to override all the globals here, so you can just replace all of them.