Hi @mitchellecm7 — thanks for taking this on. It's a big, ambitious piece of work, and Cast support is something a lot of people have wanted for years.

Up front, for transparency: I've been working on the same problem independently (#3567 + #3570), so I have an obvious bias here — please read the below as notes from someone who went down the same path, not a competing claim. I'd genuinely rather the best implementation land than mine specifically.

I pulled this branch and tried to build and test it on a real Chromecast. A few things I ran into, in case they're useful:

Build (clean checkout of the PR head, standard :play-services-core:assembleVtmDefaultDebug):

• chromecast.connect() throws GeneralSecurityException as well as IOException (it's in the su.litvak raw-request fork microG uses), but in a few spots it's only caught as IOException — CastMediaRouteController.java:57 and CastDeviceControllerImpl.java:108 / 146 / 169. Adding GeneralSecurityException to those catches clears it.
• After that I hit framework errors I couldn't quickly resolve — SessionManagerImpl.java:145/:165 (IObjectWrapper not convertible to ISession) and MediaRouterCallbackImpl.java:141 (cannot find symbol). These might be a base/rebase mismatch on my end, so worth double-checking the branch builds standalone for you.
• Minor: looks like full_structure1.txt got committed by accident.

Worth noting microG doesn't run a build check on PRs, so nothing flags this automatically.

One protocol detail I wanted to compare notes on: when I reverse-engineered play-services-cast 21.4.0, the listener method at transaction 14 came out as onConnectedWithResult(int statusCode) rather than onConnected(String sessionId). I went with the int form and it works end-to-end against the real Amazon Prime Video app (launches Prime's own receiver and plays a DRM title). I couldn't runtime-check yours since it didn't build for me — did you test the String form against a first-party app like Prime/YouTube, and what did you see?

Happy to run this through my hardware rig once it builds — and given how much our two attempts overlap, I'd be glad to compare notes and reconcile them into whatever's cleanest so this finally lands. Thanks again for pushing on it.