This is very neat - my favorite part so far is being able to synchronize local .env across all developers instantly. We currently use 1Password as a hacky solution for this, which is a bit of a pain.

I saw the demo video which looks great - one question though, how does this work with Heroku add-ons? If you configure Heroku Postgres for example, a DATABASE_URL env var gets automatically added. This variable can change (e.g. when Heroku applies a patch to your DB and restarts it). Is the sync two way, or do you expect applications to have two sets of environment variables (split across Doppler and Heroku)?