Am I right that

@Polymorphic

doesn't work with generics well?

class ParserTest {
    open class Field

    @Serializable data class FieldB(val a: String, val b: Int) : Field()

    @Serializable data class FieldC(val a: String, val b: Int) : Field()

    @Serializable data class Data(@Polymorphic val a: List<Field>)

    @Test
    fun testPoly() {
        val obj = JSON.parse<Data>("""{"a":[{a:"a", b: 1}]}""")
        println("obj = ${obj}")
    }
}

produces

java.lang.IllegalArgumentException: JSON at 6: Expected '[, kind: kotlinx.serialization.UnionKind$POLYMORPHIC@33e5ccce'

There’s no type information provided in your input. It is recorded in form [“fqClassName”, class_body], so input should look like

"""{"a":[["FieldC", {a:"a", b: 1}]]}"""

ic, thank you. Is there any simple way to provide such info without writing full serializer? I need to distinguish

{"a":[{a:"a", b: 1}]}

and

{"a":[{a:"a", c: "1"}]}

I’m afraid not, you need to peek in data before making decision, and this is not what abstract codegen mechanism can do