byte[] bytes = new byte[16];
    secureRandom.nextBytes(bytes);
    return ByteString.of(bytes);
  }

  private HttpUrl redirectUrl() {
    return mockWebServer.url("/oauth/");
  }

  /** When the browser hits the redirect URL, use the provided code to ask Slack for a session. */
  @Override public MockResponse dispatch(RecordedRequest request) {
    HttpUrl requestUrl = mockWebServer.url(request.getPath());