Define a Function for OpenAI Function Calling

Function calling (now called "tool use" in the OpenAI API) allows GPT models to invoke developer-defined functions with structured arguments. Instead of returning free-form text, the model returns a JSON object matching your function's parameter schema — which you parse and execute as application logic. This example defines a weather lookup function with properly typed parameters, required field constraints, and descriptive strings that help the model choose when and how to call the function. The function definition follows JSON Schema for the parameters property. Each parameter needs a type, description, and optionally an enum for constrained values. The description field is the most important part — it tells the model what the parameter means and what values are valid. A parameter described as "city name in English" produces different model behavior than one described as "full city name including country for disambiguation, e.g. Paris, France vs Paris, Texas". Tool choice can be set to "auto" (model decides when to call the function), "required" (model must call a function), or {"type": "function", "function": {"name": "..."}} (force a specific function). For extraction tasks, use "required" to guarantee structured output. For assistant tasks, use "auto" so the model can choose to answer directly when the function is not needed.

{
  "model": "gpt-4o",
  "messages": [
    {"role": "user", "content": "What's the weather like in Tokyo right now?"}
  ],
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "get_weather",
        "description": "Get current weather conditions for a city",
        "parameters": {
          "type": "object",
          "properties": {
            "city": {
              "type": "string",
              "description": "City name in English, e.g. Tokyo"
            },
            "units": {
              "type": "string",
              "enum": ["celsius", "fahrenheit"],
              "description": "Temperature unit"
            }
          },
          "required": ["city"]
        }
      }
    }
  ],
  "tool_choice": "auto"
}