schema.json

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "definitions": {
        "MyUnion": {
            "type": "array",
            "items": {
                "anyOf": [
                    {
                        "type": "string"
                    },
                    {
                        "$ref": "#/definitions/MyObject"
                    }
                ]
            }
        },
        "MyObject": {
            "type": "object",
            "properties": {
                "array": {
                    "type": "array",
                    "items": {
                        "type": [
                            "string",
                            "number"
                        ]
                    }
                }
            },
            "required": [
                "array"
            ],
            "additionalProperties": false
        }
    },
    "$ref": "#/definitions/MyUnion"
}