WHAT TOOLS ARE

CHAPTER SUMMARY Tools are functions the agent can call to learn things or change things. In tool calling, also called function calling, the model writes a structured request and the harness runs the real code, which gives the harness a chance to check, approve, and log every action. Input and output schemas describe the exact shape of a tool's request and reply, enabling validation, clear understanding, and reliable parsing. Read-only tools retrieve information, while action tools change the world and need extra safety measures. KEY TERMS REVIEW Function: a named, reusable recipe with inputs and an output. Tool: a function the agent can call. Tool calling (function calling): the model writes a structured request that the harness executes. Schema: a formal description of fields, types, and rules. Input schema and output schema: the agreed shape of a tool's request and reply. Parsing: splitting structured data into named parts. Read-only tool: retrieves without changing anything. Action tool: changes something in the world. Audit log: a permanent record of tool use.