{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "properties": {
    "myArray": {
      "type": "array",
      "maxContains": 1,
      "contains": {"properties": {"itemType": {"const": "type A"}}
      },
      "items": {"properties": {"itemType": {"type": "string"}}}
    }
  }
}