Everything you build around the model to turn it into a working agent: the loop itself, the tool implementations and what they are permitted to touch, the care of the message history, the stop rules, and, as the system matures, the logging, recovery, and guardrails. An agent, in one line, is a model plus a harness. The field uses the word with elastic scope; this book anchors the definition in Chapter 3 and reconciles the wider senses in Chapter 18.