Tool_choice = auto sending content and tool_calls

API
2

You have the answer in diagram and more details in the link.

Tool choice

By default the model will determine when and how many tools to use. You can force specific behavior with the tool_choice parameter.

  1. Auto: (Default) Call zero, one, or multiple functions. tool_choice: "auto"

  2. Required: Call one or more functions. tool_choice: "required"

  3. Forced Function: Call exactly one specific function. tool_choice: {"type": "function", "name": "get_weather"}

The image compares three programming modes—Auto, Required, and Forced Function—all featuring function calls like , , and , with variations in necessity and enforcement of these calls. (Captioned by AI)
The image compares three programming modes—Auto, Required, and Forced Function—all featuring function calls like , , and , with variations in necessity and enforcement of these calls. (Captioned by AI)3296×1768 215 KB

https://platform.openai.com/docs/guides/function-calling?api-mode=responses#additional-configurations