The nomenclature that makes sense for me is that the agent is the combination of the harness and the model. The model provides text-completion, the harness provides the loop around it, and the agent is the full structure of both.

However, nomenclature evolves over time. I recall (perhaps falsely) that The Cloud was specifically a term for elastic on-demand provider-managed compute/storage/network. Over time, it came to mean many other things. e.g. Salesforce Data Cloud.

I imagine if you step away from this for a year and come back, an agent will be something entirely different, perhaps a robotic horse, and a harness will be your saddle on the horse. Who knows?

The harness isn't either of those; the harness is quite literally a harness, giving the model/agent sensors and actuators (aka "skills") to interact with its environment. Compare with e.g. the Power Loader from Aliens: https://www.deviantart.com/pynion/art/Aliens-Power-Loader-11...

The model is still the model, and the agent is still the user<->model interface.

Here's how I see it: "Agent" isn't really describing a component, it's describing how you use the LLM. You have the model, and you have a harness around it that might be minimal or might have more features. If it's directly responding to user actions then it's not an agent, if it's semi-autonomous then it's an agent. (Yes this line is sometimes fuzzy.)