What is…
Tool use (function calling)
A model's ability to call external tools — search, calculators, code, APIs — and use the results.
Models can't browse or run code on their own. With tool use, the app tells the model which tools exist; the model replies with a structured request ("call get_weather with city=Austin"), the app runs it, and the result goes back into the conversation.
Every agent is built on this. It's also why an assistant can check today's news or analyze your spreadsheet accurately.
🧠 Test yourself
Which of these describes Tool use?
Related terms
Agentic AI (AI agents)
AI that doesn't just answer — it plans, takes actions with tools, checks results and keeps going until the job is done.
MCP (Model Context Protocol)
An open standard for plugging AI apps into tools and data — a 'USB-C port' for AI.
Structured output
Getting answers in a strict, machine-readable format like JSON or a fixed table.
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