Being able to mange context over long running sessions is a function of the harness, not the model. Are you using Claude Code with GPT5.5? Codex? piclaw? They’ll all have different context management strategies to let you keep going when you would otherwise have filled up context and be forced to stop.
It doesn’t matter how good the harness is if the model does a bad job of planning and continuing from long context. A good harness cannot overcome a weak model.