what I've done is have a

Data

class with nullable fields, and a

Base

class with a custom serializer which calls

input.readValue() as Data

in

load

and checks the null fields and constructs the right subtype

basically,

input.readValue()

is not sufficient for deserializing, logic there is much more complicated – you should call

readBegin()

, then read primitives field-by-field and so on. You can take a look on standard collection serializers (https://github.com/Kotlin/kotlinx.serialization/blob/master/runtime/common/src/main/kotlin/kotlinx/serialization/internal/CollectionSerializers.kt#L58 and https://github.com/Kotlin/kotlinx.serialization/blob/master/runtime/jvm/src/main/kotlin/kotlinx/serialization/PolymorphicSerializer.kt#L37) to get an idea what happens there

oh, it seems like changing it to

input.read(Data::class.serializer())

worked, but I'm still testing it