Why Substack moved its AI and ML pipelines to Modal

Substack has moved training and deployment for nearly all of its ML models—including spam detection, newsletter recommendations, audio transcription, sentiment analysis and image generation—from AWS SageMaker to Modal. Head of AI & ML Engineering Mike Cohen cited SageMaker's convoluted developer experience, hard-to-share code across notebooks and 5+ minute container startup times; on Modal, an inference endpoint took an hour to implement. Substack now runs a full fine-tune, validation and deployment workflow on Modal's storage primitives.