A harness is the software wrapped around an AI model that lets it actually do things: touch your files, run your tools, and remember what happened. On its own a model is a brain in a jar — it can describe what it would do in convincing detail, and do none of it.
The harness handles what goes into the model's context, what tools it can call, how it retries, and what it remembers between steps. None of that is machine learning; it's ordinary software engineering, which is exactly why it turned out to be where the competitive advantage went.
