The harness is the tool you use to communicate with the model.

It can be as basic as a simple conversation tool (like ChatGPT back in the day), or more advanced like GitHub Copilot, Claude Code, etc.

Advanced harnesses will give precise instructions to the model to tell him what tooling / skills / MCPs / etc are available, and will orchestrate all the actions around the model (like starting subagents, calling external tools, etc).