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This works for me:
{
        "type": "make",
        "function": {
            "name": "get_practitioners",
            "async": false,
            "description": "get list of practitioners who can be booked with the given appointment type",
            "parameters": {
                "type": "object",
                "properties": {
                    "clinic_id": {
                        "description": "id of chosen clinic",
                        "type": "string"
                    },
                    "appointment_type_id": {
                        "description": "the id of the chosen appointment type",
                        "type": "string"
                    }
                },
                "required": [
                    "appointment_type_id",
                    "clinic_id"
                ]
            }
        },
        "messages": [
            {
                "type": "request-start",
                "content": "Thanks for that."
            }
        ],
        "server": {
            "url": "xxx",
            "timeoutSeconds": 30
        },
        "async": false
    }