What is Tool use?

Tool use lets a model such as Claude decide, based on the user's request and each tool's description, that it needs an outside function. Instead of answering in prose, the model returns a structured call naming the tool and its arguments. Anthropic's documentation says tool use is also called function calling.

The docs divide tools by where the code runs. Client tools run in your own application. That group covers tools you define yourself and tools with schemas published by Anthropic, such as bash and the text editor. For a client tool, the response ends with a stop reason of tool_use and one or more tool_use blocks. Your code performs the operation and sends the output back in a tool_result block, and the model then continues with that information. Server tools, such as web search, run on Anthropic's infrastructure, so the results arrive without any handler code on your side.

You define a custom tool with a name, a description and a JSON input schema. By default the model chooses on each turn whether to call a tool or answer directly, and the tool_choice setting can force a call. Tool definitions and tool blocks count as input and output tokens, so they add to request cost. The docs also note that when a required parameter is missing, a model may ask for it or may guess a value, so the behavior is not guaranteed.