StepFun Releases Step 5 Preview Open Weights: 600B MoE, 27B Active Parameters, 1M Context

StepFun has released open weights for Step 5 Preview, a 600B total parameter Mixture-of-Experts model with 27B active parameters per forward pass and a 1 million token context window. The MoE architecture means inference costs scale with active parameters rather than total model size, making a frontier-class model substantially more accessible to self-hosters and fine-tuners. The 1M context window places it among the longest-context open-weights models available, enabling applications like full-codebase analysis, long-document reasoning, and extended agentic task execution without chunking. Developers working on retrieval-free long-context applications or looking for a large open-weights base for fine-tuning will find this a significant new option. StepFun's release adds to a growing roster of massive open-weights MoE models pushing the frontier of what can be self-hosted.