Stadia already confirmed last year that it would support Bluetooth gamepads. At the time though official support was not mentioned for Razer's mobile offerings.
Now that Stadia has been extended to work on the Razer Phone and Razer Phone 2, support for Razer mobile controllers also makes sense.
Gamepads like the Kishi are perfect for Stadia on the Razer Phone 2
If you're playing games on Stadia using a Razer Phone or Razer Phone 2, then either the Junglecat or Kishi are probably your best bet. Both controllers allow the phone to be sandwiched between both sides of the controller.
Which makes for a more compact form factor and brings the screen a little closer to your face. If you like the feel of a traditional controller, the Raiju Mobile is a good option too. It even comes with a built-in clip to hold your phone. Not too unlike the CLAW phone mount for the Stadia controller.
Of course you don't have to use any of these options. The Stadia controller can be used with any of the newly supported phones. The downside, is that the Stadia controller doesn't support wireless connection. So you have to take up the charging port with a USB C cable to connect the Stadia controller to your device.
With something like the Razer Kishi, you could play games and charge your phone at the same time. Thanks to the controller's power sharing feature. Ultimately, that will just extend your gameplay sessions. And that's not a bad thing.
The Razer Kishi is not yet available
Razer hasn't released the Kishi yet so although support for Stadia is there, you can't actually use it.
Both the Junglecat and the Raiju Mobile are available though and retail for $99 and $149 respectively. The Kishi will be $99 when it launches as well.
In addition to gaining support for new phones and new gamepads this week, Stadia is also getting new games. Google recently announced that four SteamWorld games would be coming to Stadia soon.
It hasn't announced dates for those games, but a launch is likely just around the corner.
Stadia will also be getting Spitlings soon as Google just announced today that the launch is on February 25. Which is next Tuesday. Overall, Stadia is ramping things up after a little bit of a slow start.