Meta Releases Muse Spark Open, an 816-Billion-Parameter Model for Coding and AI Agents

Meta has released Muse Spark Open, an open-weight AI model with 816 billion parameters and a 1 million token context window, marking what the company describes as the most capable US open-weight model to date for coding and agentic work. The model, released on October 15, 2026, uses a Mixture-of-Experts Transformer architecture with 62 layers and 8192 hidden dimensions, activating 42 billion parameters at a time across 256 experts through top-k routing. It accepts multimodal inputs—text, image, video, and audio—while producing text output, positioning it for complex software engineering, multi-step reasoning, and long-horizon agentic tasks.