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What is tool use (function calling), and why is it called the foundation of agents?

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Tool use lets a model request a function through structured output. The application describes available tools and their argument schemas. The model selects a tool and proposes arguments, but the application performs the real action.

Tools connect a model to live data and capabilities such as search, databases, calculators, code execution, and business APIs. Without them, the model can only generate content from its context.

Tool use is foundational because agents learn from results and decide what to do next. Every request must still pass validation, authorization, timeouts, and approval rules. A model-generated tool call is untrusted input, not permission to execute.

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