Prod p = new Car { Wheels = 4 };

var opt = new JsonSerializerOptions { TypeInfoResolver = new DefaultJsonTypeInfoResolver() };

string js = JsonSerializer.Serialize<Prod>(p, opt);

System.Console.WriteLine(js.Contains("Car"));

[JsonPolymorphic] [JsonDerivedType(typeof(Car))] abstract class Prod { }

class Car : Prod { public int Wheels { get; set; } }