You’re using Roomy to collaborate with your community. Someone created an event on OpenMeet and it shows up in Roomy’s calendar view.

You click the event, and now you’re on OpenMeet, staring at a login page. You already proved who you are when you logged into Roomy. Why do you have to do it again?

This is the classic federated identity problem. How do you carry your authenticated session from one app to another without a centralized identity provider like “Sign in with Google”?

Tom Scanlan's answer isn't a million miles from OpenWebAuth, but using ATProto service auth instead of Fediverse-style signed requests.