Constant TOOL_CALL_TRIGGER 

const TOOL_CALL_TRIGGER: &str = "!MOCK_TOOL_CALL!";

Anywhere in a chat message, this makes the mock answer with a function call instead of a text answer, which is how a test drives the chatbot’s tool loop. The chatbot then runs the tool and asks again with the tool output as the last input item, and the mock answers that with a text round, completing a two-round turn.