@Override
                  public Object decode(Response response, Type type) throws IOException {
                    String string = super.decode(response, type).toString();
                    if ("retry!".equals(string)) {
                      throw new RetryableException(
                          response.status(),
                          string,
                          HttpMethod.POST,
                          NON_RETRYABLE,
                          response.request());
                    }
                    return string;
                  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/FeignTest.java