tl;dr: DeepSeek's production sandbox platform, DSec, provides an elastic execution environment for large-scale LLM agentic training and RL workloads, exposing FnCall, container, microVM, and full-VM backends through a unified SDK backed by their 3FS distributed filesystem. It's co-designed with their RL framework to decouple stateful rollouts from preemptible GPU training. A single ~160-node deployment handles ~3 million sandboxes/day, 380,000+ concurrent sandboxes, and sustains 5,000+ sandbox creations per second.