I’m trying to encode a sealed class

        json.encodeToString(
            Filter.NOT(
                Filter.Field(
                    name = "foo", value = "bar"
                )
            )
        )

and output something completely different like

{   
    "not": {
        "foo": "bar"
    }
}

to exactly match the expected JSON of an API I’m integrating into.

The data structure is a sealed interface (with and, or, xor, etc, but removed for simplicity):

@Serializable
sealed interface Filter {

    ... 
    
    @Serializable
    @SerialName("not")
    class NOT(val not: Filter): Filter

    @Serializable(with = Field.Companion::class)
    data class Field(
        val name: String,
        val value: String
    ) : Filter {

        companion object : KSerializer<Field> {

            override val descriptor: SerialDescriptor
                get() = mapSerialDescriptor<String, String>()

            override fun serialize(encoder: Encoder, value: Field) {
                println(value)
                encoder.encodeStructure(mapSerialDescriptor<String, String>()) {
                    mutableMapOf("a" to "b", value.name to value.value)
                }
            }
            override fun deserialize(decoder: Decoder): Field = TODO()
        }
    }

If I encodeToString, I currently get:

{
    "not": {
        "type": "kotlin.collections.HashMap"
    }
}

if I remove the custom serializer

{
    "not": {
        "type": "com.jvaas.testing.model.filter.Filter.Field",
        "name": "foo",
        "value": "bar"
    }
}

So the type should not be there, and name + value should be encoded “name”: “value” instead of “name”: name, “value”: value to achieve

{   
    "not": {
        "foo": "bar"
    }
}

Any idea how this can be done?