return httpClient.newWebSocket(new Request.Builder()
        .url(url)
        .build(), listener);
  }

  static final class SlackJsonAdapters {
    @ToJson String urlToJson(HttpUrl httpUrl) {
      return httpUrl.toString();
    }

    @FromJson HttpUrl urlFromJson(String urlString) {
      if (urlString.startsWith("wss:")) urlString = "https:" + urlString.substring(4);