@Test
  void encodeLogicSupportsByteArray() throws Exception {
    byte[] expectedRequest = {12, 34, 56};
    server.enqueue(new MockResponse());

    OtherTestInterfaceAsync api =
        AsyncFeign.builder()
            .target(OtherTestInterfaceAsync.class, "http://localhost:" + server.getPort());

    CompletableFuture<?> cf = api.binaryRequestBody(expectedRequest);

    assertThat(server.takeRequest()).hasBody(expectedRequest);

    checkCFCompletedSoon(cf);
  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/AsyncFeignTest.java