I am trying to deserialize a json object that has a recursive hierarchy, however, the json structure does not include type discriminators:

...
                    "ObjectName": {
                        "Child1": [
                            {
                                "Child2": {
                                    "Field": "Some Field"
                                }
                            },
                            {
                                "Child2": {
                                    "Field": "Some Field"
                                }
                            }
                        ]
                    }
...

So, I used this custom deserializer when processing the base class, which scans the first key of the jsonobject and inject it as a type discriminator into the json.
(idea of kotlinx serialization — best way to do polymorphic child deserialization)

@Serializable(with = BaseClassDeserializer::class)
@Polymorphic
sealed class BaseClass

@SerialName("Child1")
data class Child1(val pair: Pair<BaseClass, BaseClass>) : BaseClass()

@SerialName("Child2")
data class Child2(val Field: String) : BaseClass()

object BaseClassDeserializer :
    JsonTransformingSerializer<BaseClass>(PolymorphicSerializer(BaseClass::class)) {
    override fun transformDeserialize(element: JsonElement): JsonElement {
        val typeString = element.jsonObject.keys.first()
        return JsonObject(element.jsonObject.toMutableMap().also { it["type"] = JsonElement(typeString) })
    }
}

However, kotlin complains that it does not know about my class structure of BaseClass.

kotlinx.serialization.json.internal.JsonDecodingException: Serializer for subclass 'And' is not found in the polymorphic scope of 'Conditional'.
Check if class with serial name 'And' exists and serializer is registered in a corresponding SerializersModule.
To be registered automatically, class 'And' has to be '@Serializable', and the base class 'Conditional' has to be sealed and '@Serializable'.

I have to register the hierarchy by myself to stop this error.

    private val json = Json {
        serializersModule = SerializersModule {
            polymorphic(BaseClass::class) {
                subclass(Child1::class)
                subclass(Child2::class)
        }
    }

How should I arrange the annotations such that kotlin will help me automatically generate the class hierarchy structure, as I supposed?