---
title: "Tool use (function calling): Definition"
description: "The ability of a language model to call external functions or APIs, such as search, databases or business systems, and use the results in its next step."
canonical: https://sudosapient.si/terms/tool-use
updated: 2026-10-27
publisher: Sudo Sapient (https://sudosapient.com/)
---
# Tool use (function calling)

**Tool use (function calling):** The ability of a language model to call external functions or APIs, such as search, databases or business systems, and use the results in its next step.

## Why it matters

Tool use is what turns a language model from a writer into an actor, which is why it is also where most risk lives. Give the model the narrowest tools that do the job, validate every argument in code, log every call, and require human approval for any tool that changes money, data or external communication.

## Related terms

- [AI agent](https://sudosapient.si/terms/ai-agent): A system in which a language model decides which actions and tools to use, within defined limits, to complete a task, rather than following a fixed sequence of steps.
- [Model Context Protocol (MCP)](https://sudosapient.si/terms/mcp): An open protocol that standardises how AI applications connect to external tools and data sources through MCP servers exposing tools, resources and prompts.

## In practice

In a Sudo Sapient sprint, this is a building block of AI Product Development: agents, copilots and AI-native features shipped in 6–10 weeks. [Sudo Sapient's services](https://sudosapient.com/services/) describe the scope.