In the code below I expected the consumer to only receive one message with list of messages, but I’m actually getting the messages separately.
How can I turn on consumer batching to only receive a single message with a list of messages?
I used spring boot 3.2.9 and cloud 2023.0.3.

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Bean
    public Function<Message<List<String>>, String> route() {
        return input -> {
            System.out.println("processing: " + input);
            return input.getPayload() + " response";
        };
    }
}

application.yaml:

spring:
  application:
    name: demo
  cloud:
    function:
      definition: route
    stream:
      bindings:
        route-out-0:
          destination: out-topic
        route-in-0:
          destination: in-topic
          group: group1
          consumer:
            batch-mode: true

in test dir:
application-local.yaml:

spring:
  cloud:
    stream:
      bindings:
        test-out-0:
          destination: in-topic

@SpringBootTest
@EmbeddedKafka
@ActiveProfiles("local")
@Import(TestChannelBinderConfiguration.class)
class DemoApplicationTests {

    @Autowired
    StreamBridge streamBridge;

    @Autowired
    OutputDestination outputDestination;


    @Test
    void test() throws InterruptedException {
        System.out.println("sending");
        streamBridge.send("test-out-0", "some message1".getBytes());
        streamBridge.send("test-out-0", "some message2".getBytes());
        streamBridge.send("test-out-0", "some message3".getBytes());
        System.out.println("received: " + new String(outputDestination.receive(1000, "out-topic").getPayload()));
        System.out.println("received: " + new String(outputDestination.receive(1000, "out-topic").getPayload()));
        System.out.println("received: " + new String(outputDestination.receive(1000, "out-topic").getPayload()));
    }

}

I thought consumer would only be called once with list of 3 messages I sent but I actually see this in the logs:

processing: GenericMessage [payload=byte[13], headers={source-type=kafka, id=8d2ff990-91d1-dde3-1874-525036375f80, contentType=application/json, timestamp=1725719498271, target-protocol=kafka}]
processing: GenericMessage [payload=byte[13], headers={source-type=kafka, id=305a409e-5e43-7c58-d1dd-215d3595f56d, contentType=application/json, timestamp=1725719498271, target-protocol=kafka}]
processing: GenericMessage [payload=byte[13], headers={source-type=kafka, id=cc6a41a3-9ffe-19cc-71ef-a211314f2e5f, contentType=application/json, timestamp=1725719498287, target-protocol=kafka}]

So not only the consumer is called 3 times, but it doesn’t even deserialize message to String.
If I change Function<Message<List<String>>, String> to Function<Message<String>, String> then consumer is called 3 times but at least I receive Strings.