the following constantly throws a ClassCastException

class java.util.LinkedHashMap cannot be cast to class com.expediagroup.graphql.examples.server.spring.query.ScalarQuery$PhoneNumber

fun convolutedExample(optListInput: OptionalInput<List<PhoneNumber>>): String {
        return patient
            .let { if (it is OptionalInput.Defined) it.value else null }
            ?.map { it.number }
            .toString()
    }

    data class PhoneNumber(val number: Long)

when called like this

@Test
fun test() {
    val query = "convolutedExample"
    val expectedData = "[111111]"

    <http://testClient.post|testClient.post>()
        .uri(GRAPHQL_ENDPOINT)
        .accept(APPLICATION_JSON)
        .contentType(GRAPHQL_MEDIA_TYPE)
        .bodyValue("""query { $query(optListInput: [{
            |   number: 111111,
            |   }]
            |) }""".trimMargin())
        .exchange()
        .verifyData(query, expectedData)
}

Have I gone wrong somewhere or is this just too many generics deep for jackson to figure out?

that error indicates that jackson doesn't understand how to correctly parse the input

I've reproduced it in graphql-kotlin by only adding those two snippets I sent. Would you like me to make a PR with the added test?

yeah PR would be great

made the PR, first PR so forgive the stumbling around the whole process.