I'm using opencode v2, if it matters. I don't have any special skills / prompts for subtasks. Instead, I found new models (I usually have sol or opus as my main model) have the tendency to trigger subtasks, even to a level I need to ask it "please do this task XYZ yourself". The only special prompt I gave is just something like "please use gpt sol or luna for subtasks, base on your judgement". And in my experience it can handle more than 5 async subtasks out of the box