Welcome Muse Spark Open: open-weight frontier multimodal mixture-of-experts model from Meta

Muse Spark Open is an open-weight model from Meta. The mixture of experts model brings text, images, audio, and video into a model with approximately 816 billion parameters, around 42 billion active per token, and a context window of up to one million tokens. It generates text, giving developers a common interface for asking questions about different kinds of input.

Following Muse Glimmer, Muse Spark Open introduces a mixture-of-experts architecture at a much larger scale, and the most capable US open-weights model to date.

Meta trained Muse Spark Open for agentic coding: reading a codebase, writing and debugging code, and calling tools. It also takes images, audio, and video as input, so the same model can read a screenshot, listen to a voice note, or watch a screen recording while it works on your code. This post shows you how to run it, how to fine-tune it, and what we built with it. Find all the models here .

Muse Spark Open is the full-size model of the Muse series of models. Muse Glimmer, which Meta released in August, is a small dense model from the same series, ideal for running locally. Both models use the same tokenizer and vocabulary. The chat template follows the Muse Glimmer template, with additions for the new modalities. In Muse Spark Open the input embedding and the output layer are separate matrices of about 1.66B parameters each.

Muse Glimmer uses one dense feed-forward network in each layer. Muse Spark Open replaces this dense network with a Mixture-of-Experts (MoE) block. Each MoE block has 256 experts, and a router selects 8 experts for each token. Each expert is a feed-forward network that projects from 8192 to 4096 dimensions and back to 8192, then applies its own normalization to its output. The experts hold ~97% of the total model parameters.

Only 8 experts run per token, so the model computes each token with about 42B parameters. All 256 experts stay in memory, so deployment needs the memory of an 816B-parameter model, but runs at the much faster speed of a 42B model. The BF16 checkpoint is about 1.5 TB in safetensors format. The NVFP4 checkpoint stores the parameters in 4-bit precision with negligible loss in accuracy, specifically for NVIDIA Blackwell GPUs. In transformers, the BF16 checkpoint runs across 4 nodes of 8 H200 GPUs with tensor parallelism (TP=32) and the NVFP4 runs on a single B200 node. If you want to deploy Muse Spark Open, check out the llama.cpp , vLLM, and SGLang recipes below.

A perception encoder converts images and video frames into tokens for the language model, just like Muse Glimmer. The encoder has 1.8B parameters, 50 layers, a hidden dimension of 1536, and 16 attention heads. It splits each image into 14×14 patches and produces features with 4096 dimensions. Position information comes from learned absolute embeddings and 2D RoPE. The feed-forward network in each layer uses LayerNorm and GELU. Every fourth layer and the last layer use full attention, and the other layers use sparse attention.

Meta describes Muse Spark Open as a coding-optimized model trained on real developer workflows such as multi-file refactors and long debugging sessions. Muse Spark Open is an agentic focused model that plans, acts, observes, and self-corrects over long tasks, including browsing the web with images.

The checkpoint repositories are Muse-Spark-Open-816B-A42B and Muse-Spark-Open-816B-A42B-NVFP4.

The language model has 62 layers and a hidden dimension of 8192. Each layer has 128 query heads and 16 key-value heads (grouped-query attention). Each head has 64 dimensions. The attention block has an output gate and an output normalization layer. The model also applies normalization to the queries and the keys (QK-norm). Each block combines its input and its output through a learned residual gate, not through a plain sum.