Comment by TeMPOraL

I'd say the core is that the harness/runtime/${whatever you call it} doesn't just unconditionally sends model output to the user, and user input to the model, +/- some post-processing, but instead runs a loop that feeds the output back to the model if some conditions are met. That gives you basic "thinking" and single "function calling" a-la early ChatGPT. However, if you allow it to loop arbitrary number of times and allow the output to decide whether to loop or to stop, you get a basic agent.