Tool Call

A tool call or function call is a directive issued by a large language model to execute a function with specific arguments, to obtain information or perform an action.

The developer tells the model which functions it can request by providing tool definitions. Each definition describes the function's name, purpose, and parameters, including what each parameter means.

The model returns the function name, concrete arguments, and a call ID identifying the call.

The client executes the tool locally or remotely. In the next inference request, the developer provides the tool call output item alongside the matching call ID, so the model can connect the result to the original call and continue reasoning, request more tools, or provide a final answer.

The model may request multiple function calls in one response or across successive requests to complete a task.

Tool calls are a key step toward agent autonomy, allowing agents to request actions such as using a computer, browsing the internet, or executing other available functions as needed to complete a task.