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Do default polymorphic type handlers work also for cases where the class discriminator is just not present in the json?

I don't think so. The default handler isn't nullable.

It would also mean that you would accept any json object whatsoever.

you can extend <https://kotlin.github.io/kotlinx.serialization/kotlinx-serialization-json/kotlinx-serialization-json/kotlinx.serialization.json/-json-content-polymorphic-serializer/index.html|JsonContentPolymorphicSerializer> to do this