This would merge capabilities into the params object which should be of type Options.WebDriver, can you explain your intention doing this? It feels like this is not what we want, right?

Yeah, the merging is what I aimed for. Because of this error Error: Invalid or unsupported WebDriver capabilities found ("hostname", "port")., I need a way to separate out fields like host, port, etc.. from an object that is passed into reloadFunction and use them to overwrite the old values existed in instance.options. So, assuming you pass something like this in reloadFunction,

{
host: 'xxxx',
port: 1234,
browserName: 'chrome'
}

That top line would make capabilities field look like this

host: 'xxxx',
port: 1234,
capabilities: {
    browserName: 'chrome'
}

And I can just use this to overwrite the corresponding value in instance.options.

Not sure if I answer your question, but let me know.

I would suggest to just have some simple if statements and move the connection parameters into the params object, e.g.:

for (const prop of ['protocol', 'hostname', 'port', 'path'] as Option.Connection) {
    if (prop in capabilities) {
        params[prop] = capabilities[prop];
        delete capabilities[prop];
    }
}

better?

Just for understanding: this would completely get rid of the mobile session so you couldn't continue anything on the native app side, correct?

Yeah, I tested it out locally. This is not meant to be for concurrent session like multi-remote, if that's what you're asking. It's meant to be used within a single session. Btw, I made a small change again, I removed deepmerge because I don't want caps from original session to be mixed with a new one.