Sample implementation of a serializer that, for json, encodes/decodes single element lists as their element directly.

JsonOrListSerializer.kt

import kotlinx.serialization.KSerializer
import kotlinx.serialization.builtins.ListSerializer
import kotlinx.serialization.descriptors.SerialDescriptor
import kotlinx.serialization.encoding.Decoder
import kotlinx.serialization.encoding.Encoder
import kotlinx.serialization.json.JsonArray
import kotlinx.serialization.json.JsonDecoder
import kotlinx.serialization.json.JsonEncoder

class JsonOrListSerializer<T>(private val elementSerializer: KSerializer<T>): KSerializer<List<T>> {
    private val listSerializer = ListSerializer(elementSerializer)

    override val descriptor: SerialDescriptor = SerialDescriptor("JsonOrListSerializer", listSerializer.descriptor)

    override fun deserialize(decoder: Decoder): List<T> {
        if (decoder !is JsonDecoder) return listSerializer.deserialize(decoder)

        val elem = decoder.decodeJsonElement()
        return when (elem) {
            is JsonArray -> decoder.json.decodeFromJsonElement(listSerializer, elem)
            else -> listOf(decoder.json.decodeFromJsonElement(elementSerializer, elem))
        }
    }

    override fun serialize(encoder: Encoder, value: List<T>) {
        when {
            // this is to show how this would work
            encoder is JsonEncoder && value.size == 1 ->
                encoder.encodeSerializableValue(elementSerializer, value.single())

            else -> listSerializer.serialize(encoder, value)
        }
    }
}