Heroku app runs locally but gets H12 timeout error (uses a package)

The issue is that run.py unguardedly calls app.run - this actually calls werkzeug.serving.run_simple which starts a sub-process to handle incoming requests ... which you don't want to do when running under gunicorn (since gunicorn will handle the process management for you).

Simply add an if __name__ == "__main__" guard before your app.run call and everything should work:

# run.py
if __name__ == "__main__":
    app.run()