Function Calling: The Request, Execute, Return Cycle

Here is what a model produces when it "uses a tool":

{
  "type": "tool_call",
  "id": "call_01",
  "name": "lookup_order",
  "input": { "order_number": "8841-B" }
}

That block is text. No order was read and nothing has changed. The model has written down a request, which does nothing until your code acts on it.

The arrangement is called function calling, or tool use, and it is the only way an agent can read or change anything outside the model.

The Tool Definition

**A tool definition is the description of a tool that your code sends to the model with each call