AI Glossary

Tool use

tool use, function calling, tool calling, tool invocation

Tool use is a language model's ability to call external tools — a search engine, an API, a calculator or code — when generating text alone isn't enough to complete the task.

The model calls a tool instead of guessing the answer.
It works from a fixed description of the functions and their parameters.
It is the foundation of agents: it lets them act, not just answer.

On its own, a model only generates text. Tool use gives it access to defined tools: based on their descriptions, the model decides which one to call, supplies the parameters, and then uses the returned result in its answer. So instead of guessing an exchange rate or a date, it looks them up in a real source.

This is the foundation of how agents and data-connected assistants work. Safety depends on which tools you expose and with what permissions — the model should only be able to call what you have deliberately allowed.

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