AI Agents — Book 1 · What an Agent Actually Is

"AI agent" is used to mean at least four different things, and the confusion is expensive: people build autonomous agents for jobs one prompt would have done better. This book fixes the vocabulary. The agent loop in four steps — think, act, observe, decide — and why step four is the whole definition. The real distinction between a workflow and an agent, in Anthropic's own terms. A three-rung ladder from single prompt to workflow to agent, with a test for each rung. The four conditions that justify climbing, the four costs nobody mentions, and the three principles — simplicity, transparency, and the agent-computer interface — that keep agents working.