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

Back to Articles Welcome Muse Spark Open: open-weight frontier multimodal mixture-of-experts model from Meta Published October 15, 2026 Update on GitHub Upvote 6 Pedro Cuenca pcuenq Follow merve merve Follow ben burtenshaw burtenshaw Follow Guarin Flück guarin Follow Quentin Gallouédec qgallouedec Follow Alvaro Bartolome alvarobartt Follow Serveurperso Serveurperso Follow Jun Kim Jundot Follow 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. Overall Capabilities and Architecture 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. Feature Muse Spark Open Model size ~816B total parameters; ~42B active per token Expert routing 256 experts per MoE layer; 8 selected per token Inputs Text, images, audio, and video Output Text Context window Up to 1M tokens Checkpoint formats BF16 and NVFP4 The checkpoint repositories are Muse-Spark-Open-816B-A42B and Muse-Spark-Open-816B-A42B-NVFP4. 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. Mixture of Experts 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. 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.

Perception Encoder 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.

Muse Glimmer 30B uses an encoder with the same architecture. The design follows the Perception Encoder work from Meta. Video input goes through the same encoder as images.

Audio Encoder Muse Spark Open accepts audio as input through its audio encoder. An audio tokenizer converts the audio into tokens, and the chat template marks the audio position with tokens. The encoder emits one audio token every 80 ms, which yields 12.5 tokens per second.

So let’s say, you have an audio of 10 seconds, the number of input audio tokens would be 125. The encoder itself is a convolutional pyramid where each stage consists of a patch embedder of different patch sizes and 2 convolutional blocks of 1d kernels of size 7 with stride of 1. These stages are repeated for patch sizes of [16, 6, 5, 4].

At the end of the final stage, the temporal frames are fed to a final block of RMSNorm → linear projection → RMSNorm. Long Context The context window is 1M tokens at inference. The configuration sets max_position_embeddings to 1,048,576.

This includes image, video, and audio tokens in the same window as text. Grouped-query attention keeps the KV cache small for a model of this size. Each token adds 2 × 16 heads × 64 dimensions × 62 layers = 126,976 values to the cache.

In BF16, this is about 250 KB per token and about 266 GB for a full 1M-token context. Inference Support Transformers The Muse Spark Open implementation provides MuseSparkForConditionalGeneration for text generation from multimodal inputs and MuseSparkProcessor for preparing conversations. The processor combines the tokenizer, image and video processors, audio feature extractor, and chat template.

Use a DistributedConfig to configure the parallelism settings. tp_size is the number of devices across which the dense layers are sharded, and ep_size is the number of devices across which the experts are distributed. You can initialize the model and processor as follows. import os from transformers import AutoModelForMultimodalLM, AutoTokenizer, DistributedConfig distributed_config = DistributedConfig( tp_size=int(os.environ["WORLD_SIZE"]), ep_size=int(os.environ["WORLD_SIZE"]), ) tokenizer = AutoTokenizer.from_pretrained("meta-models/Muse-Spark-Open-816B-A42B ") model = AutoModelForMultimodalLM.from_pretrained( "meta-models/Muse-Spark-Open-816B-A42B", distributed_config=distributed_config, ) messages = [ {"role": "user", "content": "Explain in one sentence why the sky is usually blue."}, ] inputs = tokenizer.apply_chat_template( messages, reasoning_effort="medium", # default: xhigh, options: low/medium/high/xhigh add_generation_prompt=True, tokenize=True, return_dict=True, return_tensors="pt", ).to(model.device).to(model.dtype) input_len = inputs["input_ids"].shape[-1] outputs = model.generate(**inputs, max_new_tokens=2000) Text-only Inference Prepare the conversation, then pass the messages to processor.apply_chat_template, enabling add_generation_prompt=True, tokenize=True, return_dict=True, and return_tensors="pt". Pass the processor output to model.generate and decode the newly generated tokens, excluding the input prefix.

The processor has a lever for the reasoning effort, mapping symbolic levels (low/medium/high/xhigh) to the integers the model itself accepts (16, 64, 256, 512). Note that turning off reasoning is not possible with Muse Spark Open, so keep the model reasoning at “low” if you want to consume less tokens to solve an easier task. You can also set a numeric effort by including a line like Reasoning strength: 1 in the system prompt. messages = [ {"role": "system", "content": "You should only answer with a number."}, {"role": "user", "content": "What is 17 * 23?"}, ] inputs = processor.apply_chat_template( messages, add_generation_prompt=True, tokenize=True, return_dict=True, return_tensors="pt", reasoning_effort="high", # options:low/medium/high/xhigh ).to(model.device).to(model.dtype) output = model.generate(**inputs, max_new_tokens=2000) generated_tokens = output[0][inputs["input_ids"].shape[1] :] print(processor.decode(generated_tokens, skip_special_tokens=False)) Images and Text The Muse Spark Open processor expands each image marker into the number of patch tokens required by the processed image, surrounded by image boundary tokens.

This expansion keeps the text positions aligned with the visual features passed to the model. The processor also handles multiple images within a single prompt. image_url = ( "https://huggingface.co/datasets/merve/vl-test-suite/" "resolve/main/pills.jpg" ) messages = [ { "role": "user", "content": [ { "type": "image", "image": image_url, }, { "type": "text", "text": "Do any of the components in this supplement interact?", }, ], }, ] inputs = processor.apply_chat_template( messages, tokenize=True, add_generation_prompt=True, reasoning_effort="medium", return_dict=True, return_tensors="pt", ).to(model.device).to(model.dtype) input_len = inputs["input_ids"].shape[-1] outputs = model.generate(**inputs, max_new_tokens=2000) response = processor.decode(outputs[0][input_len:], skip_special_tokens=False) processor.parse_response(response) Video Inference Video prompts include frame timing as well as visual content. The processor handles videos with audios in them (which is an upgrade from Muse Glimmer thanks to the audio encoder).

The processor downsamples video frames and audio to align frames together. By default, videos are sampled to 2 FPS, capped at 96 frames. You can modify this by setting the fps of the video processor: processor.video_processor.fps = 4.0 As a general rule of thumb, videos are frames in temporal sequence, and the more you downsample, the more you will lose frames in between specific events, so you need to keep fps high if you want more accurate generations.

However, there’s a trade-off because you will require more memory and increase latency for prefill. The frames are interleaved with text input that states timestamps (e.g. “Time: 3.5s”). To enable passing in videos with audio, pass load_audio_from_video=True to apply_chat_template.

Preserve the video's metadata when preparing inputs with return_metadata=True so the processor can assign the correct framerate. import torch messages = [ { "role": "user", "content": [ { "type": "text", "text": ( "Describe what happens in this video. " "Use timestamps and include relevant speech or sounds." ), } {"type": "video", "url": "https://huggingface.co/datasets/merve/vlm_test_images/resolve/main/ramen.mp4"}, , ], } ] inputs = processor.apply_chat_template( messages, add_generation_prompt=True, tokenize=True, return_dict=True, return_tensors="pt", load_audio_from_video=True, return_metadata=True, ).to(model.device).to(model.dtype) with torch.inference_mode(): output_ids = model.generate( **inputs, max_new_tokens=256, do_sample=False, ) generated_ids = output_ids[:, inputs["input_ids"].shape[1] :] response = processor.batch_decode( generated_ids, skip_special_tokens=True, )[0] print(response) Audio Inference The Muse Spark Open audio tower consumes a raw waveform. The processor expects mono audio at 24 kHz and pads the waveform to a multiple of 1,920 samples, corresponding to one audio embedding per 80 milliseconds.

Audio supplied directly to the processor at another sample rate must first be resampled. You can do this by using the load_audio util of transformers. load_audio uses torchcodec, librosa or torchaudio backends, so make sure to have one of these installed locally before running inference. If you want to simply use apply_chat_template with messages though, this is all handled for you under the hood: import torch from transformers import AutoProcessor, AutoModelForMultimodalLM model_id = "meta-models/Muse-Spark-Open-816B-A42B" " processor = AutoProcessor.from_pretrained(model_id) model = AutoModelForMultimodalLM.from_pretrained( model_id, ).eval() messages = [ { "role": "user", "content": [ { "type": "audio", "url": "https://huggingface.co/buckets/huggingface/audio-samples/resolve/bbaudio_question_39.mp3", }, { "type": "text", "text": "Answer the question.", }, ], } ] # Downloads the audio, decodes it as mono, resamples it to 24 kHz, pads it, creates input_values_mask, and expands the audio placeholders. inputs = processor.apply_chat_template( messages, add_generation_prompt=True, tokenize=True, return_dict=True, return_tensors="pt", load_audio_backend="auto", ).to(model.device).to(model.dtype) with torch.inference_mode(): output_ids = model.generate( **inputs, max_new_tokens=128, do_sample=False, ) # Decode only newly generated tokens prompt_length = inputs["input_ids"].shape[1] generated_text = processor.batch_decode( output_ids[:, prompt_length:], skip_special_tokens=True, ) print(generated_text[0]) The processor inserts the matching number of audio tokens between audio boundary markers.

It also returns a mask identifying real samples, allowing the model to exclude padded audio embeddings. Tool Calling Muse Spark Open can do text-only and multimodal tool calling. You can make the model call tools as follows. from transformers import AutoProcessor, AutoModelForMultimodalLM MODEL_ID = "meta-models/Muse-Spark-Open-816B-A42B" # Load model processor = AutoProcessor.from_pretrained(MODEL_ID) model = AutoModelForMultimodalLM.from_pretrained( MODEL_ID, dtype="auto", device_map="auto" ) tools = [ { "type": "function", "function": { "name": "weather.get", "description": "Get the current weather for a city.", "parameters": { "type": "object", "properties": { "city": {"type": "string"}, }, "required": ["city"], }, }, } ] messages = [ { "role": "user", "content": [ {"type": "image", "image": "https://huggingface.co/datasets/merve/vl-test-suite/resolve/main/SF.png"}, {"type": "text", "text": "I'm going to the city in this picture.

What clothes should I wear?"}, ], }, ] inputs = processor.apply_chat_template( messages, tools=tools, tokenize=True, return_dict=True, return_tensors="pt", add_generation_prompt=True, reasoning_strength="high" ).to(model.device).to(model.dtype) input_len = inputs["input_ids"].shape[-1] outputs = model.generate(**inputs) response = processor.parse_response( completion, prefix=inputs["input_ids"][0], tools=tools, ) print(response) Muse Spark Open uses a channel-based chat template (Harmony, first introduced by gpt-oss). Note that you don’t have to know about the internals, but it’s useful to check the channels the model routes to, in case you get low quality outputs. In the input message below: system You are a helpful AI assistant.

Knowledge cutoff: 2026-01-04. Current date: ... ... user... assistant assistant is intentionally left open, it’s the model that chooses recipient with to= . The model has different recipients, e.g. self asks the model to reason, to=selfthe model reasons here # reasoning assistant to=userfinal answer # final answer self is for reasoning, commentary are comments before and between tool calls visible to the end user, user is the final response.Tool-call routing is parsed as follows (assuming you have passed tools to apply_chat_template: assistant to=weather.get San Francisco Llama.cpp Muse Spark 2-bit running on Llama Server Muse Spark Open is integrated in llama.cpp and comes in the prebuilt binaries.

You can install this with following command: curl -LsSf https://llama.app/install.sh | sh You can start the server as follows with quant suffix attached to model ID: llama serve -hf ggml-org/Muse-Spark-Open-816B-A42B-GGUF:Q2_K The repository containing quants is here. Once the server has started, you can head to localhost:9931 to chat with the built-in WebUI. From the web UI you can add audio and images for inference and adjust the reasoning lever.Note that turning off reasoning is not possible with Muse Spark Open, so keep the model reasoning at “low” if you want to consume less tokens to solve an easier task.We tested quants on Mac M3 Ultra (512GB).

An imatrix-calibrated 2-bit quant runs at 18 tokens/s, with plenty of room free for the KV cache (304 GB used by the model). You can also query the server as follows. Feel free to check out the API reference for multimodality, tool calls and more. ## text-only inference curl http://localhost:8080/v1/chat/completions \ -H "Content-Type: application/json" \ -d '{ "messages": [ {"role": "system", "content": "You are a helpful assistant."}, {"role": "user", "content": "Write a limerick about python exceptions"} ] }' ## image-text inference curl -s http://127.0.0.1:8080/v1/chat/completions \ -H "Content-Type: application/json" -H "Authorization: Bearer " \ -d '{"model":"muse-spark-open","messages":[{"role":"user","content":[ {"type":"text","text":"What is in this image?"}, {"type":"image_url","image_url":{"url":"data:image/png;base64,..."}} ]}],"temperature":0}' ## audio-text inference curl -s http://127.0.0.1:8080/v1/chat/completions \ -H "Content-Type: application/json" -H "Authorization: Bearer " \ -d '{"model":"muse-spark-open","messages":[{"role":"user","content":[ {"type":"text","text":"Transcribe this audio."}, {"type":"input_audio","input_audio":{"data":"data:audio/wav;base64,..."}} ]}],"temperature":0}' llama.cpp team ships different quantized versions.

We have tested Q2_K, Q4_K_S and Q_8 on Apple Silicon chips. You can run each quant by adding the quant id as suffix, llama serve -hf ggml-org/Muse-Spark-Open-816B-A42B:Q2_K. SGLang The Muse Spark Open integration for SGLang supports text, image, audio, and video inputs.

The NVFP4 reference configuration runs across four GB300 GPUs. From the SGLang integration environment, download the checkpoint from the Hugging Face Hub and start the server: export MODEL_ID=meta-models/Muse-Spark-Open-816B-A42B-NVFP4 export CKPT="$(hf download "$MODEL_ID")" SGLANG_FLASHINFER_NVFP4_PER_TOKEN_ACTIVATION=1 \ uv run sglang_meta serve \ --model-path "$CKPT" \ --served-model-name "$MODEL_ID" \ --quantization modelopt_fp4 \ --moe-runner-backend flashinfer_trtllm \ --tp-size 4 \ --ep-size 4 \ --chat-template "$CKPT/muse_spark_1_2_oss.jinja" \ --port 30000 The checkpoint includes the chat template, which we pass explicitly. The launcher configures the Muse reasoning and tool-call parsers.

Send a request through the OpenAI-compatible API: curl http://localhost:30000/v1/chat/completions \ -H "Content-Type: application/json" \ -d '{ "model": "meta-models/Muse-Spark-Open-816B-A42B-NVFP4", "messages": [{ "role": "user", "content": "Explain how a mixture-of-experts model works." }], "max_tokens": 2048 }' The response separates the reasoning trace in reasoning_content from the final answer in content. vLLM The Muse Spark Open reference integration for vLLM supports text and image inputs with the NVFP4 checkpoint that require blackwell series GPUs. After installing the reference plugin and its dependencies, download the checkpoint from the Hugging Face Hub and launch the server: export MODEL_ID=meta-models/Muse-Spark-Open-816B-A42B-NVFP4 export CKPT="$(hf download "$MODEL_ID")" VLLM_ALLOW_LONG_MAX_MODEL_LEN=1 \ VLLM_PLUGINS=musespark1x_models,ptq_nvfp4 \ vllm serve "$CKPT" \ --served-model-name "$MODEL_ID" \ --tokenizer-mode musespark1x \ --config-format auto \ --load-format internal_safetensor \ --tensor-parallel-size 4 \ --enable-expert-parallel \ --max-model-len 131072 \ --gpu-memory-utilization 0.92 \ --reasoning-parser musespark1x \ --tool-call-parser musespark1x \ --enable-auto-tool-choice \ --port 8000 This configuration sets a 128K-token serving context. The plugin supplies the model implementat