@Test
  void encodedQueryParam() throws Exception {
    server.enqueue(new MockResponse());

    TestInterfaceAsync api =
        new TestInterfaceAsyncBuilder().target("http://localhost:" + server.getPort());

    CompletableFuture<?> cf = api.encodedQueryParam("5.2FSi+");

    assertThat(server.takeRequest()).hasPath("/?trim=5.2FSi%2B");

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