Either

Either a JSON object or a stream of server-sent events (SSE), each containing a JSON object.

{
    "blocks": {
        "property1": {
            "kind": "html",
            "content": "string"
        },
        "property2": {
            "kind": "html",
            "content": "string"
        }
    },
    "attachments": [
        {
            "blockId": "fcde1d7c-c9cf-4365-8b6f-51bb6a8bc460",
            "attachmentId": "96b9bbac-86d3-4497-9e0c-1f8e3803eddb"
        }
    ],
    "totalCost": 0
}