URGENT: Persistent "Can't link to Nest" Error After Exhaustive Troubleshooting (Nabu Casa, New GCP Client, Billing Verified)

I am facing a persistent and frustrating issue with the official Nest integration. I have successfully created the Device Access Project and OAuth credentials, but the final authorization step fails with “Can’t link to Nest”.
I have gone through every troubleshooting step possible, including correcting configuration errors, manually clearing files, and verifying my Google Cloud account. I am hoping the community can help me identify if there is a known server-side or account-level issue I am missing.

  1. Final State of Configuration (Confirmed Correct)
  2. The Current Problem
    The process fails at the very last step:
    I start the integration with the new, correct credentials.
    I successfully log into Google and arrive at the Google Nest Permissions page.
    I grant all permissions and click “Next”.
    The page attempts to redirect back to Home Assistant but immediately shows: “Can’t link to Nest. Please contact Nest if the issue persists”.
    Crucially: I have waited 30 minutes and performed the retry using Incognito mode and confirmed that the local redirect URL is not being used in the background.
    My Question to the Community
    Given that all configuration, client secrets, and account verification steps are complete and correct, what is the most likely non-configuration reason for the “Can’t link to Nest” error?
    Is there a known issue with Google’s servers syncing new clients this week?
    Could my specific Google/Nest account have an internal ban or restriction that prevents final token exchange, despite granting permissions?
    Any insight into this specific failure point (after permission grant but before final redirect) would be greatly appreciated. Thank you!

Is this for the Nest thermostat? I thought Google dropped support for this. Even when it did used to work, the latency was horrible, and it often wouldn’t work because of Google servers being down. I got rid of my nest thermostat and bought an Ecobee Advanced at a deep discount through my utility’s online store and use it with full local control in HA via the homekit integration.

Good luck