Serialize JSON String as JSON Object - json

Question

I am writing a custom Serializer for kotlin.serialization which interprets a JSON variable as a String no matter of its type. Deserializing works out fine. But I have trouble serializing the String into its original structure again.
At the moment I just deserialize the value as a String. But that adds extra quotes around the value and leads to problems.
Here is a quick example:
//
//Custom serializer:
//
object JsonToStringSerializer : KSerializer<String> {
override val descriptor: SerialDescriptor = PrimitiveDescriptor("JsonToStringSerializer", PrimitiveKind.STRING)
override fun serialize(encoder: Encoder, value: String) {
if(value.startsWith("[") || value.startsWith("{")) {
encoder.encodeString(value) //<-- bad solution
println("Oh no! Object has been turned into a String!")
}
else //will also encode Int, Float, Boolean, ... into String - but that is ok
encoder.encodeString(value)
}
override fun deserialize(decoder: Decoder): String {
val output = (decoder as JsonInput).decodeJson().toString()
return if(output.startsWith("\""))
output.substring(1, output.length-1)
else output
}
}
//
//Data structure for test:
//
#Serializable
class Test (
#Serializable(with = JsonToStringSerializer::class) val objValue: String,
val stringValue: String
)
//
//test logic:
//
fun test() {
val jsonString = "{\"objValue\": [1,2], \"stringValue\": \"test\"}"
val obj = Json(JsonConfiguration.Stable).parse(Test.serializer(), json)
val newJsonString = Json(JsonConfiguration.Stable).stringify(Test.serializer(), obj)
println(newJsonString)
//expected result: {"objValue": [1,2], "stringValue": "test"}
//actual result: {"objValue": "[1,2]", "stringValue": "test"}
}
After some digging I found that encoder in serialize() belongs to the class StreamingJsonOutput.
I can think of three possible solutions. But I ran into a wall with each of them:
If I had a way to directly print() to composer from StreamingJsonOutput I could add the String manually to the output. But unfortunately composer is private (and StreamingJsonOutput is internal as well).
Another way would be to use the configuration unquotedPrint which would create Strings without quotes. But then stringValue would not have quotes either. So I need a way to just change the configuration only temporariy.
I could also just deserialize the value String from serialize() into a real object and then just let the encoder serialize it correctly. The problem, apart from the performance overhead, is that I don't know which datastructure to expect in value. So I am not able to deserialize it.
I hope somebody has an idea how to deal with that.
Thank you very much! :)