What
Deploy a subdirectory (e.g., server/) from a monorepo to Heroku.
Why
Heroku's buildpack expects the app at the repository root. git subtree push --prefix server creates a synthetic commit containing only that folder's history, so Heroku sees a standalone app without restructuring your repo.
How
Add Heroku remote:
heroku git:remote -a your-app-name
Deploy subdirectory:
git subtree push --prefix server heroku main