circe-either-codec.scala

implicit def eitherEncoder[A: Encoder, B: Encoder]: Encoder[Either[A, B]] =
  Encoder[Json].contramap[Either[A, B]](_.asJson)

implicit def eitherDecoder[A: Decoder, B: Decoder]: Decoder[Either[A, B]] =
  Decoder[Json].emap[Either[A, B]] { js =>
    js.as[A] match {
      case Left(_)  => js.as[B].map(_.asRight[A]).leftMap(_.message)
      case Right(x) => Right(x.asLeft[B])
    }
  }