Ahh, so I was a noob after all.

sumTypes.scala

import scalaz._
import argonaut._,Argonaut._

object JsonDb {

  sealed trait SumType
  case class Option1( foo:String ) extends SumType
  case class Option2( bar:Int    ) extends SumType

  private implicit val option1Codec = casecodec1( Option1.apply , Option1.unapply _ )("foo")
  private implicit val option2Codec = casecodec1( Option2.apply , Option2.unapply _ )("bar")

  implicit def sumTypeCodec  = CodecJson[SumType](                                                                                                                                                                                                                              
    (sumType) => Json.obj( sumType match {
      case o1:Option1 => "option1" := o1.asJson
      case o2:Option2 => "option2" := o2.asJson
    } ) ,
    c => ( (c --\ "option1").as[Option1] ||| (c --\ "option2").as[Option2] )
  )

}