SolitaryThinker 255311cf25 [bugfix] cosmos3: fix black-video UniPC config; wire E2E inference
The first real-weights Cosmos3-Nano T2V run produced an all-black video.
Root cause: the checkpoint's diffusers-style scheduler_config.json sets
use_karras_sigmas=true, and FastVideo's vendored UniPC checks karras
*before* use_flow_sigmas, so it built diffusion (beta) sigmas instead of
flow-matching sigmas -> scheduler.step diverged to NaN latents -> black
frames. The DiT/CFG velocity itself was clean.

The framework samples with FlowUniPCMultistepScheduler (pure flow:
shift + num_train_timesteps only). FastVideo's vendored UniPC is the same
algorithm, so the fix is config, not a port:

- Coerce the loaded scheduler to the flow setup in
  Cosmos3OmniDiffusersPipeline.initialize_pipeline (kill karras/exp/beta
  sigmas, force use_flow_sigmas + flow_prediction + final_sigmas_type=zero)
  and rebuild the runtime scheduler from it.
- Use FastVideo's native UniPC (not diffusers) in the pipeline and tests,
  honoring the "no diffusers at runtime" constraint.

Parity:
- Add test_cosmos3_scheduler_parity.py: FastVideo vendored UniPC (flow
  config) vs the framework FlowUniPCMultistepScheduler. timesteps bit-exact,
  sigmas within fp32 epsilon (~1e-8), full multi-step trajectory < ~1e-6
  across shift in {10 (video), 3 (t2i)} and steps in {4,10,35}.
- Fix test_cosmos3_denoise_cfg_parity to use the framework scheduler as the
  oracle (it previously compared diffusers-vs-diffusers, so the scheduler
  was never actually checked against the framework).

Integration fixes to run end-to-end on real weights:
- registry: map checkpoint DiT class Cosmos3OmniTransformer ->
  Cosmos3VFMTransformer.
- component_loader: route the "text_tokenizer" module to TokenizerLoader;
  filter scheduler config to the class's __init__ params (robust to
  diffusers schema drift like shift_terminal/sigma_min/sigma_max).
- DiT: materialize_non_persistent_buffers (recompute rotary_emb.inv_freq
  after meta-device FSDP load) + cast vision latents / timestep embeds to
  the compute dtype (no-ops in the fp32 parity tests).
- sequence_packing: move packed tensors to the model device in to_dit_kwargs.
- config: drop the unused identity text-preprocess (Cosmos3 has no text
  encoder; the DiT tokenizes in the denoise stage).

Adds examples/inference/basic/basic_cosmos3_new_api.py (new VideoGenerator
API). Verified: 1280x704, 29 frames, 35 steps on B200 -> coherent video
matching the prompt (no NaNs, real pixel variance). Full cosmos3 parity
suite green.
2026-06-07 01:31:19 -07:00
2026-02-23 15:14:42 -08:00
2026-04-02 16:07:24 -07:00
2026-04-02 16:07:24 -07:00
2026-02-10 01:11:17 -08:00
2025-02-20 11:33:58 -08:00
2025-11-27 00:37:03 -08:00

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FastVideo is a unified post-training and real-time inference framework for accelerated video generation.

NEWS

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Key Features

FastVideo has the following features:

  • End-to-end post-training support for bidirectional and autoregressive models:
    • Support full finetuning and LoRA finetuning for state-of-the-art open video DiTs
    • Data preprocessing pipeline for video, image, and text data
    • Distribution Matching Distillation (DMD2) stepwise distillation.
    • Sparse attention with Video Sparse Attention
    • Sparse distillation to achieve >50x denoising speedup
    • Scalable training with FSDP2, sequence parallelism, and selective activation checkpointing.
    • Causal distillation through Self-Forcing
    • See this page for full list of supported models and recipes.
  • State-of-the-art performance optimizations for inference
    • Sequence Parallelism for distributed inference
    • Multiple state-of-the-art attention backends
    • User-friendly CLI and Python API
    • See this page for full list of supported optimizations.
  • Diverse hardware and OS support
    • Support H100, A100, 4090
    • Support Linux, Windows, MacOS
    • See this page for full list of supported models, hardware assumptions, and optimization compatibility.
  • Realtime video generation & editing
    • Dreamverse: stream and "vibe direct" video in realtime (live demo), deployable on local GPU, a self-hosted B200 server, Docker, or serverless Modal

Getting Started

We recommend using uv to create a clean environment. If you previously used Conda, switching to uv generally gives faster and more stable installs.

# Create and activate a new uv environment
uv venv --python 3.12 --seed
source .venv/bin/activate

# Install FastVideo
uv pip install fastvideo

Please see our docs for more detailed installation instructions.

Sparse Distillation

For our sparse distillation techniques, please see our distillation docs and check out our blog.

See below for recipes and datasets:

Model Sparse Distillation Dataset
FastWan2.1-T2V-1.3B Recipe FastVideo Synthetic Wan2.1 480P
FastWan2.2-TI2V-5B Recipe FastVideo Synthetic Wan2.2 720P

Dreamverse — Realtime Video Generation & Editing

Dreamverse is FastVideo's realtime video generation and editing platform — "vibe directing" a video as it streams. It lives in the monorepo under apps/dreamverse/ and ships its own backend (dreamverse-server) plus a web UI.

Try the live demo, read the blog, or run it yourself. Dreamverse deploys on a local GPU, a self-hosted B200 server over SSH, Docker, or serverless Modal — see the Dreamverse README.

Inference

Generating Your First Video

Here's a minimal example to generate a video using the default settings. Make sure VSA kernels are installed. Create a file called example.py with the following code:

import os
from fastvideo import VideoGenerator

def main():
    os.environ["FASTVIDEO_ATTENTION_BACKEND"] = "VIDEO_SPARSE_ATTN"

    # Create a video generator with a pre-trained model
    generator = VideoGenerator.from_pretrained(
        "FastVideo/FastWan2.1-T2V-1.3B-Diffusers",
        num_gpus=1,  # Adjust based on your hardware
    )

    # Define a prompt for your video
    prompt = "A curious raccoon peers through a vibrant field of yellow sunflowers, its eyes wide with interest."

    # Generate the video
    video = generator.generate_video(
        prompt,
        output_path="my_videos/",  # Controls where videos are saved
        save_video=True
    )

if __name__ == '__main__':
    main()

Run the script with:

python example.py

For a more detailed guide, please see our inference quick start.

Awesome work using FastVideo or our research projects

  • SGLang: SGLang's diffusion inference functionality is based on a fork of FastVideo on Sept. 24, 2025.
  • DanceGRPO: A unified framework to adapt Group Relative Policy Optimization (GRPO) to visual generation paradigms. Code based on FastVideo.
  • SRPO: A method to directly align the full diffusion trajectory with fine-grained human preference. Code based on FastVideo.
  • DCM: Dual-expert consistency model for efficient and high-quality video generation. Code based on FastVideo.
  • HY-WorldPlay: An action-conditioned world model model trained using FastVideo framework.
  • Hunyuan Video 1.5: A leading lightweight video generation model, where they proposed SSTA based on Sliding Tile Attention.
  • Kandinsky-5.0: A family of diffusion models for video & image generation, where their NABLA attention includes a Sliding Tile Attention branch.
  • LongCat Video: A foundational video generation model with 13.6B parameters with block-sparse attention similar to Video Sparse Attention.

🤝 Contributing

We welcome all contributions. Please check out our guide here. See details in development roadmap.

Acknowledgement

We learned the design and reused code from the following projects: Wan-Video, ThunderKittens, DMD2, diffusers, xDiT, vLLM, SGLang. We thank MBZUAI, Anyscale, and GMI Cloud for their support throughout this project.

Citation

If you find FastVideo useful, please consider citing our research work:

@article{zhang2025vsa,
  title={Vsa: Faster video diffusion with trainable sparse attention},
  author={Zhang, Peiyuan and Chen, Yongqi and Huang, Haofeng and Lin, Will and Liu, Zhengzhong and Stoica, Ion and Xing, Eric and Zhang, Hao},
  journal={arXiv preprint arXiv:2505.13389},
  year={2025}
}

@article{zhang2025fast,
  title={Fast video generation with sliding tile attention},
  author={Zhang, Peiyuan and Chen, Yongqi and Su, Runlong and Ding, Hangliang and Stoica, Ion and Liu, Zhengzhong and Zhang, Hao},
  journal={arXiv preprint arXiv:2502.04507},
  year={2025}
}
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