Substack is a popular platform for writers to publish newsletters, employing machine learning (ML) for various purposes such as spam detection and sentiment analysis. The company has moved its ML training and deployment from AWS SageMaker to Modal, a shift that has significantly improved their developer experience, collaboration, and deployment speed. Previously, Substack's pipelines were built on SageMaker and Airflow, but this process was slow and painful due to issues with the developer experience, collaboration, and container startup times. By migrating to Modal, Substack can now iterate quickly, share components across projects, and deploy ML workflows with greater speed and flexibility than ever before.