Tool (Function Calling)
Also known as: Function, Function Calling, Tool Use
In one sentence
A capability that allows an AI model to call external functions or APIs — like searching the web, querying databases, or running calculations — instead of relying solely on its training data.
Explain like I'm 12
Like giving the AI a toolbox full of gadgets. Instead of guessing the weather, it can check a weather app. Instead of guessing at maths, it can use a calculator. The AI picks the right tool for each job.
In context
When you ask an AI assistant 'What's the weather in London?', the model doesn't actually know — it was trained months ago. With tool use, it recognises the question needs live data, calls a weather API, gets the current forecast, and includes it in its response. Modern platforms like OpenAI, Anthropic, and Google define tools as JSON schemas that describe available functions, their parameters, and expected outputs. The model decides when to call which tool based on the conversation.
See also
Related Guides
Learn more about Tool (Function Calling) in these guides:
Structured Output and Function Calling: Getting Reliable JSON from AI
IntermediateLearn how to get reliable, parseable JSON output from AI models using structured output, function calling, and JSON schema. Essential for production AI applications.
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AdvancedEnable AI to call APIs, query databases, and use external tools. Build agentic systems that take actions in the world.
8 min readAgents & Tools: What They're Good For (and What to Watch For)
IntermediateUnderstand AI agents that use tools to complete tasks. When they work, when they fail, and how to use them safely.
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