Tool use / function calling

Tool use (also called function calling) is a mechanism that lets Claude interact with external functions, APIs, and services you define. When you include tool definitions in your API request, Claude can decide to call one of those tools during its response — pausing text generation and returning a structured JSON request for your application to execute. Your application runs the function, returns the result, and Claude uses that result to continue its response. There are two categories of tools: client tools, where your application handles execution (custom functions, database lookups, third-party API calls), and server tools, where Anthropic handles execution on its own infrastructure (web search, code execution, web fetch). Client tools require you to build an 'agentic loop' — a cycle of sending a request, receiving a tool_use block, executing the function, returning a tool_result, and sending the updated conversation back to Claude. Tool use extends Claude beyond text generation into real-world task completion. Claude can retrieve live data it was not trained on, execute code, query databases, call APIs, and chain multiple tools together across complex multi-step workflows. The model never executes tools directly — it only decides when to call them and with what arguments, while your application or Anthropic's infrastructure handles the actual execution.