{
  "type": "function",
  "function": {
    "name": "cancel_reservation",
    "description": "Cancel an existing reservation",
    "parameters": {
      "type": "object",
      "properties": {
        "user_id": {
          "type": "string",
          "description": "User ID"
        },
        "reservation_id": {
          "type": "string",
          "description": "Reservation ID to cancel"
        },
        "reason": {
          "type": "string",
          "enum": ["change_of_plan", "airline_cancelled", "other"],
          "description": "Reason for cancellation"
        }
      },
      "required": ["user_id", "reservation_id", "reason"]
    }
  }
}