I've made a web-app that allows churches to make YouTube playlists of the songs they're doing on Sunday.
I've written a Google Assistant intent that accepts input like "Play the songs we're doing this weekend on my television."
This intent makes use of a webhook, asking my server to look up the songs and hymns for the church in question. My server successfully returns a URL of a YouTube Playlist.
However, it is just reading the playlist URL back because I don't know the way to communicate to actions to show the URL on Chromecast.
When asking Google Support for help, I was told:
Yes, there is a way to send a YouTube URL to Chromecast, or a song-name/artist-name to Google Play or Spotify using a webhook. If you trigger intents via events (as described at https://api.ai/docs/events#invoking_event_from_webhook), you should be able to see a response defined in such intent in the Actions on Google test console. It may look like this:
User via test console -> Query request -> Intent A -> Webhook -> Web service logic -> Event is sent back to API.AI -> Intent B -> Response defined in Intent B is shown in test console.
Let us know if this helps or if it's different from what you are trying to accomplish.
Unfortunately, I have not been able to decipher the meaning of the above quote.
Thanks.
UPDATE FROM GOOGLE:
Hi Jason,
Sorry you can't use your app to interact with inbuilt music apps. Alternate option is to provide a link to the user in the Google Assistant that opens the app a user wants to use. Hope this helps!
Thanks, Lalitha Actions on Google Support Team
So to answer your specific question, about Chromecast, that feature is not available for third party apps. You will not be able to stream the music directly.