Spring Boot 3 WebFlux + Kotlin + @JsonValue wrapper fails for String / Boolean / Enum responses (ClassCastException)

I'm migrating a Kotlin + Spring WebFlux application from Spring Boot 2.5.x to Spring Boot 3.5.x, and several controller endpoints that used to work now consistently fail with ClassCastException.

🔍 What breaks

Any endpoint that returns a wrapped response of type:

• String

• Boolean

• numeric primitives (Int, Long, etc.)

• Enum

• or lists of these (e.g. List<Enum>)

throws exceptions such as:

ClassCastException: APIResponse$Success cannot be cast to CharSequence
ClassCastException: APIResponse$Success cannot be cast to java.lang.Boolean
ClassCastException: APIResponse$Success cannot be cast to java.lang.Enum

🔧 How we wrap responses

We use a generic wrapper for all controller responses:

val result = block()
APIResponse.Success(result)

Success is defined as:

class Success<T>(
    private val response: T,
    val status: HttpStatus = HttpStatus.OK
) : APIResponse<T>(status) {

    @JsonValue
    fun value() = response
}

@JsonValue is used so the client receives the raw value T (e.g. "string" instead of { response: "string" }).

What changed after upgrading

• This worked fine in Spring Boot 2.5.x

• After upgrading to Spring Boot 3 / Spring Framework 6, only scalar or enum types fail