Tool Use (Function Calling)

On its own, a language model can only produce text. Tool use closes the gap to the real world: the model is told what functions it can call and, when it decides one is needed, it outputs a structured request — search_orders(customer_id) — that your code executes and hands the result back for the model to use. The model chooses which tool and what arguments; your system runs it.

This is the mechanism that turns a model into an agent. Planning is useless without the ability to act, and tool use is that ability — reading a file, querying a database, calling an API, triggering a workflow. The reliability questions that dominate agent-building (does it pick the right tool? pass valid arguments? recover when a call fails?) are all tool-use questions. Open standards like the Model Context Protocol exist to make the same tools work across models and apps without bespoke wiring each time.