From the FAQ, seems like a Chromecast Ultra is required at least for now.
"For launch in November, only the Google Chromecast Ultra will be compatible with Stadia, and you will also need a Stadia Controller and access to Stadia to play with Chromecast. Stay tuned for future announcements regarding additional device compatibility."
Also worth noting that most existing Chromecast devices (those not bundled in the founder or premiere edition) will not support Stadia until they're updated over the next few weeks. You can check if a Chromecast Ultra can run Stadia by checking if it's running version 1.42.180348 or above. If your Chromecast is not on that version, attempting to play games on that Chromecast will fail until it receives the upcoming Stadia update.
I have ViZio 4K TV's with Chromecast built-in, but it is not working on any of those. Does anyone have any information about how to get Stadia working on a TV that has Chromecast built-in? It just feels like a waste to connect a Chromecast Ultra to a TV that already has Chromecast built-in.
Is Google working with TV manufactures to ensure that the built-in Chromecast will be updated to be able to play Stadia?
I believe it comes down to the version mentioned above. I have 3 other Ultras around the house and none would get an update beyond what the one that came with the Stadia has. My guess is that updates will roll out as it goes, which eases devices in and allows them to match support for them as they go..
In addition, you can get preview/early access to Chromecast updates, so maybe it is worth a try there?
Just to confirm for anyone curious..
It also doesn't work on Philips Android 4K TV's with Chromecast 'Ultra/4K' built in either.
I hope Android TV/Chromecast Built in Support does get added, as its a major blunder if it doesn't, as it would reduce the barrier to entry even further.