Inside Replit’s New Sandbox Engine, Part 1: The Path to MicroVMs

Replit has completed a full rewrite of its sandbox engine, shifting to a microVM-based isolation model and migrating hundreds of millions of projects without data loss. The company said the new engine improves security, capabilities and portability, and underpins its Bring Your Own Cloud offering across AWS, Google Cloud and Azure. Replit began the rebuild in November 2025, was using it internally by February and spent subsequent months hardening and rolling it out. The move matters as autonomous AI agents increasingly reach beyond intended boundaries, a concern raised by OpenAI's Hugging Face incident, Anthropic's evaluation incidents and NIST's agent standards work. Replit is positioning the infrastructure against a wave of sandbox startups—Daytona, E2B, Runloop and Blaxel—that have collectively raised $59.3 million since 2025. Watch for Part 2 on orchestrating tens of thousands of microVMs on Kubernetes, and for how the engine performs under higher agent concurrency.