My code for it isn’t live yet, but if the user is redirected through another server (with Discourse as an SSO provider), then a session will exist in that external server. I created a route there, something like /auth/is-authenticated that returns the user’s status. It’s mainly just there to remove the “log in” buttons when they are already logged in. When the user logs out of Discourse, I think a webhook logs them out of the other server. I haven’t checked the code in a while, but I think that’s how I set it up.