I was facing same issue, so I used Transform component and this object:

{
  "type": "object",
  "properties": {
    "message_ids": {
      "type": "array",
      "items": {
        "type": "string",
        "default": ""
      },
      "description": "A list of unique message identifiers"
    }
  },
  "additionalProperties": false,
  "required": [
    "message_ids"
  ],
  "title": "message_ids"
}