Chapter 1: Why a harness?

A model reads text and writes text. It cannot run code, read files, or remember past calls. The harness adds those abilities. It does five jobs. It sends the prompt. It reads the reply. It runs the tools the model asks for. It returns the tool results. It decides when the run stops.

Example. Asked alone, a model can only guess whether checkout is slow. With a harness, the model calls a countErrors tool, reads the real count, and answers from that data.

Objectives: describe a model call, name the five harness jobs, and explain what the course builds.