scala> sealed trait Op
defined trait Op

scala> case class Foo(s: String) extends Op
defined class Foo

scala> case class Ciao(i: Int) extends Op
defined class Ciao

scala> case class Bar(op: Op, b: Boolean)
defined class Bar

scala> Bar(Ciao(3), false)
res6: Bar = Bar(Ciao(3),false)

scala> res6.asJson
res7: io.circe.Json =
{
  "op" : {
    "Ciao" : {
      "i" : 3
    }
  },
  "b" : false
}

scala> decode[Bar](res7.toString)
res8: cats.data.Xor[io.circe.Error,Bar] = Right(Bar(Ciao(3),false))