@SuppressWarnings("deprecation")
  private Response responseWithText(String text) {
    return Response.builder()
        .body(text, Util.UTF_8)
        .status(200)
        .request(Request.create(HttpMethod.GET, "/api", Collections.emptyMap(), null, Util.UTF_8))
        .headers(new HashMap<>())
        .build();
  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/AsyncFeignTest.java