But the runtime is missing things too, unfortunately. Like polymorphism, which is absolutely critical to what I'm doing right now.

Polymorphism is sort of supported. The problem is serializer discovery. Javascript has no reflection/annotation support yet. It has no Class.forName. Basically the candidate types need to be registered in the context so that they can be found.

Is that so? I may be able to use it then, which I would honestly prefer… could you point me to how it works exactly then? I researched into it for a bit and all I found was a file in the JVM codebase: https://github.com/Kotlin/kotlinx.serialization/blob/master/runtime/jvm/src/main/kotlin/kotlinx/serialization/internal/Polymorphic.kt Which lead me to believe that it only worked on the JVM. My system requires registration too, so maybe I could slide my stuff over into it. Mirror would then become a temporary patch to serialization, which would be better for everyone. I’d prefer to stick to the standard, but I also need something that works within the next couple of months. Required for an upcoming project.