try {
      OAuthSession session = slackApi.exchangeCode(code, redirectUrl());
      listener.sessionGranted(session);
    } catch (IOException e) {
      return new MockResponse()
          .setResponseCode(400)
          .setBody("code exchange failed: " + e.getMessage());
    }

    synchronized (this) {
      listeners.remove(state);
    }

    // Success!
    return new MockResponse()