With a wrapper around my class it looks like this:

{
  "solution": {
    "type": "com.klitsie.pubquiz.common.question.solution.SolutionV0.SingleSolutionV0",
    "solution": "123",
    "id": "8db60275-45ae-4ba3-a463-8cac0cd5afce"
  }
}

If I directly try to serialize the class, it looks like this:

{
  "solution": "123",
  "id": "514df2d1-ffc9-4691-9087-6af334721811"
}

This is my json setup:

Json {
            ignoreUnknownKeys = true
            isLenient = true
            prettyPrint = true
        }

Does anyone know why the discriminator is missing if I try to send out a body directly without wrapping it?

If I persist the Json to my Database in jsonb then I can clearly see the type discriminator being added to my object (without wrapper) but when I use it in the context of ktor it is not there

I figured it out: I should specifically define the type of the parent in my respond call:

// WRONG
val mySeleadClass = SealedClass.Whatever("whatever")
call.respond(mySeleadClass )

// RIGHT
val mySeleadClass = SealedClass.Whatever("whatever")
call.respond<SealedClass>(mySeleadClass)

Because if you do not infer the type of the actual parent sealed class, it will simply serialize the child class as if it would be a stand alone the child class (and it will not be treated as a sealed class at all). https://github.com/Kotlin/kotlinx.serialization/issues/1194