AI Agents — Book 2 · Tools

An agent without tools is a chatbot with extra steps. This book covers the only mechanism by which an agent touches anything: the three fields of a tool definition, and why the `description` does more work than the code behind it — it is the only thing the model has to decide with, and a perfect tool with a three-word description is never called. Then the round trip from `tool_use` to `tool_result`, the step in it that is yours and where every permission gate has to live, the difference between client and server tools, the full list of what Anthropic already provides free, and how to make a tool fail usefully instead of crashing.