We've got a number of loosely coupled stacks. Today a change in one broke a stack reference from another upon the invocation of that dependent stack. This, obviously, is suboptimal. Does Pulumi have or plan to add ways to communicate changes between stacks? (Honestly, for our purposes, a publish hook of some flavor that allows us to ping an SNS topic would be fine, but if there's a better pattern I'd love to hear about it.)

Yes, this is definitely an area we plan on refining. As it’s a request we hear a lot from people. For today, would our webhooks feature be a viable solution? https://www.pulumi.com/docs/intro/console/extensions/webhooks/

We don't and for various reasons can't use the Pulumi Cloud stuff, and webhooks are really suboptimal anyway given that we then have to build infrastructure to respond to them (when SNS is right there already)