Function calling

Function calling (also called tool use) is the mechanism by which an LLM produces a structured request to invoke an external function. The model returns JSON matching a schema you provide ({"name": "search_orders", "arguments": {"customer_id": "..."}}), your code executes that function, and the result is fed back to the model. This is the foundation of agentic systems and is the right abstraction for almost any "the LLM needs to do something in the real world" use case.