Meta’s 816B-Parameter Muse Spark Open Weights Are Now Public

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.

Muse Spark Open is a Mixture-of-Experts Transformer paired with a Perception Encoder, according to the model card published by Meta Superintelligence Labs. The language model has 62 layers, a hidden dimension of 8192, and 256 experts with eight activated per token through top-k routing. Its attention block uses gated attention with QK-norm, 128 query heads and 16 key-value heads in a grouped-query configuration, and a head dimension of 64.

The vocabulary size is 202,048 with untied input and output embeddings. The Perception Encoder carries 1.8B parameters across 50 layers, with a hidden dimension of 1536, 16 heads, a 14×14 patch size, a 4096 output dimension, and sparse attention with a factor of four, under which every fourth layer plus the final…