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I've been through the serialization guide (very impressive piece of work) but how to deal with custom deserializers for polymorphic classes (cannot use "type" discriminant) has eluded me. Can someone refer me to which section(s) of the guide might contain the information I need?

Then I think you are after <https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/json.md#content-based-polymorphic-deserialization>

<@U0152P3VB5J> Your suggestion works well with a single level abstraction, something like this:
```
object ServerInfoSerializer : JsonContentPolymorphicSerializer&lt;ServerInfo&gt;(ServerInfo::class) {
    override fun selectDeserializer(element: JsonElement) = ServerInfoImpl.serializer()
}

interface ServerInfo {
    val name: String
    val version: String?
}

@Serializable
class ServerInfoImpl(override val name: String, override val version: String?) : ServerInfo```
But how to handle a hierarchy is eluding me at the moment. Something simple like:
```object InitializeResultSerializer : JsonContentPolymorphicSerializer&lt;InitializeResult&gt;(InitializeResult::class) {
    override fun selectDeserializer(element: JsonElement) = when {
        //"serverInfo" in element.jsonObject -&gt; ServerInfoImpl.serializer()
        else -&gt; InitializeResultImpl.serializer()
    }
}

interface InitializeResult {
    val serverInfo: ServerInfo?
}

@Serializable
class InitializeResultImpl(
    override val serverInfo: ServerInfo?
) : InitializeResult```
which, when I try to use
```        val result: InitializeResult = InitializeResultImpl(serverInfo)
        assertEquals(jsonResult, Json.encodeToString(InitializeResultSerializer, result))```
I get a failure and a message that `serverInfoImpl` is not serializable in the scope of `serverInfo` which I see clearly. What I don't see is how to satisfy this requirement. Suggestions?