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GlossaryAgents

Tool use

An LLM's ability to invoke external functions / APIs as part of generating its response.

Tool use (sometimes "function calling") is the capability where an LLM produces a structured request to call a function instead of just text. The host application executes the function and feeds the result back into the next LLM turn. Tool use is the building block for agents and is supported natively by Claude, GPT, Gemini, and most modern models.

Related terms
AI agentAn LLM-driven system that takes actions in the world, often by calling tools and APIs in a loop.Context windowThe maximum amount of text (in tokens) an LLM can attend to at once.