Though it had a rocky launch, Google is beginning to gain its footing with Stadia and is now adding some functionality that had been previously promised but missing. The Google Assistant button on the controller had until now been inactive, displaying only a coming soon message on your screen upon being pushed. Today that's finally going to change, as Google activates Stadia's Assistant button.
To activate the feature, you must navigate to the new Google Assistant section of settings within the Stadia app.
Pressing the button on the controller will now trigger the Assistant on your display, and the button's light will pulse as it's listening. Currently, this will only work on the Stadia homepage of your Chromecast. This version of the Assistant also doesn't carry the full set of features from your other Google devices; it cannot control your smart home, and it can't trigger any media playback. It's limited to simple queries, which does include those that access your personal information. It can also be used to launch games provided they're in your Stadia library.
Hopefully this is a sign that Google will continue to add the features that didn't get included with Stadia at launch and give us a more fully-functioning service.