Tool Use (Function Calling)
Tool use is a model's ability to call external functions — look up an order, send an email, query stock — by returning a structured request that your code executes, then continuing with the result. It's what lets a model act on real systems.
Mechanically, you describe the available functions and their parameters; the model returns a structured call rather than prose; your code runs it and hands back the result; the model continues. The model never touches your systems directly, which is the security boundary that makes this workable.
The failure mode is calling the right function with wrong arguments — a mistyped date, a customer ID that doesn't exist. Validate arguments before executing, never after.
Why it matters
Tool use is the whole reason AI can do work rather than just describe it. It's also where the integration effort lives: every tool is an API to connect, authenticate and handle failures for.
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