Meta released the weights for Muse Spark Open on October 15, 2026, a natively multimodal open-weight model with 816B total parameters, 42B active parameters, and a 1M-token context window. Meta AI Research describes it as the most capable US open-weight model to date, built for coding and agentic work such as long-horizon tasks, complex software engineering, and multi-step reasoning. Architecture and Training Data Muse Spark Open is a Mixture-of-Experts Transformer paired with a Perception…
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