return jsonAdapter.fromJson(response.body().source());
    }
  }

  /** See https://api.slack.com/methods/rtm.start. */
  public RtmStartResponse rtmStart(String accessToken) throws IOException {
    HttpUrl url = baseUrl.newBuilder("rtm.start")
        .addQueryParameter("token", accessToken)
        .build();
    Request request = new Request.Builder()
        .url(url)