kotlinx.serlialization + Array<Any>

sample.kt

package sample

import kotlinx.serialization.*
import kotlinx.serialization.internal.StringDescriptor
import kotlinx.serialization.json.*

object ToStringSerializer: KSerializer<Any> {
    override val descriptor: SerialDescriptor =
        StringDescriptor.withName("toString")

    override fun serialize(encoder: Encoder, obj: Any) {
        encoder.encodeString(obj.toString())
    }

    override fun deserialize(decoder: Decoder): Any = error("This serializer doesn't support deserialization")
}

@Serializable
data class Message(
    val action: String,
    val method: String,
    val args: Array<@Serializable(with = ToStringSerializer::class) Any>,
    val id: String
)

@UseExperimental(UnstableDefault::class)
fun main() {
    val json = Json
    println(json.stringify(
        Message.serializer(),
        Message(
            action = "Hello",
            method = "world!",
            args = arrayOf(1, "two", 3f),
            id = "some id")))
    // output: {"action":"Hello","method":"world!","args":["1","two","3.0"],"id":"some id"}
}