Illuminate\Http\Client\PendingRequest : set headers in beforeSending callback

Hello, I'm trying to add headers to outgoing PendingRequests from within beforeSending callback. I guess it's the right moment to be sure everything I need is set to populate my headers' data.

Here is my callback:

public function buildRequestToHeaders(\Illuminate\Http\Client\Request $currentRequest, array $options, PendingRequest $requestTo): PendingRequest
{
    $headers['x-forwarded-for'] = request()->ip();
    $headers['x-my-custom-header'] = 'custom data computed from within the app';
    $requestTo->withHeaders($headers);
    
    return $requestTo;
}

The problem is that my headers go into $options but not into the embedded Guzzle client.

Do you know if it's possible ? How can I rebuild the Guzzle client so that it takes the new headers ?