Hmm, you don't need to parse the headers by yourself, this is done by undici automatically, we just need to standardise the way that these are forward to undici.

If Array, push host to the headers array, if an object, attach host as a new property

I've simplified the code!

A header case I'm slightly worried about though is non-array iterables.

request({ origin, path: '/', method: 'GET', headers: new Set([['f0', 'b0'], ['f1', 'b1']]) }).

These headers are valid under UndiciHeaders and a request like this (though rare) is possible; but the proposed array/object check would break this.

Another issue I've noticed is [string,string][] headers always raise an error due to the way header processing is handled in Request.js. Say headers = [['f0', 'b0']] or [['f0', 'b0'], ['f1', 'b1']]

if (Array.isArray(headers)) {
      if (headers.length % 2 !== 0) {
        throw new InvalidArgumentError('headers array must be even')
      }
      for (let i = 0; i < headers.length; i += 2) {
        processHeader(this, headers[i], headers[i + 1])
      }
}