What is Tool Use (Function Calling) in agentic AI?

Tool use, often implemented through function calling, is how an agent extends beyond generating language and actually affects the outside world, calling APIs, databases, or code execution environments.

• Each tool is described with a schema, its name, its purpose, and the parameters it expects
• The agent's reasoning engine selects a tool based on that schema, without needing to know how the tool is implemented internally
• The tool's response becomes an observation the agent factors into its next reasoning step

This separation matters, the agent only ever sees a tool's name and description, so the quality of that description effectively acts as the API contract between the agent and the tool.

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