Function calling: describing functions with JSON schema and handling tool calls in code

Function calling (also called tool calling) is the mechanism that lets a language model use your code. The model never runs anything itself. You describe the functions it may use; when it decides one would help, it returns a structured request naming the function and its arguments, your code runs the function, and you send the result back so the model can continue. This works both in plain chat completions and inside Agent Service.