It seems that spring AMQP has built-in MessageConverter support for Jackson with the Jackson2JsonMessageConverter class, but is there an equivalent for the kotlinx serialization lib, or would it be necessary to write a custom message converter for it?
Trying to deserialize a JSON AMQP message without providing a MessageConverter Bean will cause an exception
Yes, you have to write your own org.springframework.amqp.support.converter.MessageConverter implementation for Kotlin Serialization.
It looks like we can borrow ideas from the org.springframework.messaging.converter.KotlinSerializationJsonMessageConverter.
However you also can use a org.springframework.amqp.support.converter.MessagingMessageConverter with a delegate to the mentioned KotlinSerializationJsonMessageConverter.
See AbstractRabbitListenerContainerFactory.setMessageConverter(MessageConverter messageConverter).