Tool use
Also: function calling
A model calling external functions, such as search, code execution, or an API, by emitting a structured request that the surrounding software runs. It is the foundation of agents.
Tool use, also called function calling, is a model’s ability to ask for an action rather than answer from memory. Given a set of available tools, the model can emit a structured request, “search for X”, “run this code”, which the surrounding program executes, returning the result for the model to continue from.
It is what lets a model fetch fresh information, do exact calculation, or change something in the world, and it is the mechanism every agent is built on. MCP is an increasingly common way to expose tools to a model in a standard form.