Also, from a quick google I found this example: protocol-buffers-spring-cloud-stream/protocol-buffers-spring-cloud-stream-starter/src/main/java/io/disc99/protocol/buffers/spring/cloud/stream at master · disc99/protocol-buffers-spring-cloud-stream · GitHub
Take a look and see if that helps create the MessageConverter, but keep in mind that it’s using the old annotation style for the actual coding of the app. I think you should be able to re-use the MessageConverter though.
Let us know how it goes 