I've developed several plugins for different harnesses, and I needed some upstream change for most of them.

The deciding factor for me whether or not I will work on the feature of the plugin is whether I (or rather, my agent) can look in upstream source and evaluate if it can be done with minimal upstream change, which I then contribute. And generally, even if no upstream change is needed, agents work so much better when they can read the code.

So why not just make Claude code open source? Considering also that source code was leaked once anyway.