First off you need to pick a harness that you can use with every model. This way you isolate model problems from harness problems. If you had to switch harnesses (Claude Code) in order to try out Fable, you're doing something wrong.

Secondly yeah, all models have issues with foreground vs background subagents. So I just modified my harness to delete foreground subagents. No more confusion. If they need the subagent's output tokens they can read it from the session transcript.

"Take away tools" is often the best solution for model-gets-confused problems. Narrow down your toolset until there's only one way to do things, and then make it the maximally-powerful way.