IRunnable.kt

interface IRunnable {
  fun run()
}

class Horse : IRunnable {
  override fun run() {
    println("horse running")
  }
}

class Dog : IRunnable {
  override fun run() {
    println("dog running")
  }
}

@Serializer(forClass = Horse::class)
object HorseSerializer {}

@Serializer(forClass = Dog::class)
object DogSerializer {}

@Serializable
data class Zoo(val runnable : IRunnable)

RunnableTest.kt

class RunnableTest {

  private val logger = KotlinLogging.logger { }

  @Test
  fun testJson() {

    val runnableModule = SerializersModule {
      polymorphic(IRunnable::class) {
        Horse::class with HorseSerializer
        Dog::class with DogSerializer
      }
    }

    val zoo = Zoo(Dog())

    Json(context = runnableModule).stringify(Zoo.serializer() , zoo).also {
      logger.info("zoo = {}" , it)
    }
  }
}

// 23:56:42.407 [main] INFO RunnableTest - zoo = {"runnable":{"type":"Dog"}}