FastH3 4-step Preview (VSA, data-free) — MiniMax-H3 in four forward passes

FastVideo/FastVideo-FastH3-4-step-Preview-v1-VSA-DataFree is a data-free DMD2 distillation of MiniMaxAI/MiniMax-H3, the 33B dual-modality DiT that emits a video and its synchronized soundtrack from one denoising pass. The student keeps the teacher's architecture exactly and only replaces transformer/, so it is a drop-in for the same diffusers modular pipeline — but it needs four DiT forwards instead of thirty, and it was distilled with Video Sparse Attention on.

Everything runs unquantized at bfloat16.

The sampling contract: five grid points, four forwards

MiniMaxH3Scheduler.set_timesteps(n) builds linspace(1, 0, n), applies the shift σ' = s·σ / (1 + (s−1)·σ), and then drops the trailing zero when it forms the timesteps — so n sigma grid points drive n − 1 model evaluations. The distilled ladder is t = 999, 749, 500, 250 → 0: five points, four forwards.

This Space therefore fixes num_inference_steps = 5, matching the checkpoint's own fastvideo_inference.json (num_inference_steps: 5, transformer_forwards: 4, dmd_denoising_steps: [999, 749, 500, 250], guidance_scale: 1.0). It is not a knob. There is no CFG and no negative prompt — the teacher is guidance-distilled and the student inherits that.

Video Sparse Attention is not optional here

This checkpoint is the VSA variant. Its fastvideo_inference.json pins attention_backend: VIDEO_SPARSE_ATTN_H3, vsa_sparsity: 0.9, vsa_tile_size: 64, and the transformer ships 50 trained attn.to_gate_compress tensors (~3.6 GiB) that only the sparse path consumes. FastVideo publishes a separate …-Dense-DataFree checkpoint for people who want dense — running this one dense is running it off-distribution.

The published kernel is vsa_kernel: sm100a, which is GB200-only, and the ZeroGPU pool is sm120 (RTX PRO 6000 Blackwell). FastVideo's other officially supported route is --vsa-kernel triton, which is pure Triton and architecture-agnostic — so this Space vendors those two files verbatim from FastVideo (Apache-2.0) into vsa_kernel/ and ports the H3 backend on top of them:

vsa_h3.py follows FastVideo's video_sparse_attn_h3.py step for step: 64-token (4, 4, 4) tiles over the post-patchify video grid, segment-pure prefix tiles, per-head fp32 pooled tile scores, topk = max(1, min(⌈(1 − sparsity)·n_video_tiles⌉, n_video_tiles)), prefix keys exempt (always selected) and prefix queries always dense, plus the compression branch softmax(scores) @ pool(v) broadcast back over each tile row and scaled by the trained gate with no activation. The tile geometry is derived per-forward from the pipeline's own token_tags / position_ids, so the [text | cond | audio | video] packing stays authoritative.

diffusers 0.40.0 has no to_gate_compress, so vsa_h3.add_gate_compress_modules() patches MiniMaxH3TransformerBlock.__init__ before the pipeline loads; otherwise the 50 gate tensors load as "unexpected keys" and are silently dropped. Blocks whose gate is all-zero have it removed again after load, exactly as FastVideo does.

A hidden /selftest API endpoint runs the ported kernel at sparsity 0 against F.scaled_dot_product_attention on a real packed layout, so a wrong tile order or transpose is caught without spending a generation.

Split across two Spaces

MiniMax-H3 is ~196 GiB in bfloat16 and a ZeroGPU Space is evicted at 150 GB of storage, so no single unquantized Space can hold it. Cutting MiniMaxH3Blocks at its text_encoder step splits it in two, and both halves fit:

The conditioner is a public Space and is unchanged by the distillation — the student's modular_model_index.json points its text_encoder at the same Qwen3-VL weights — so this Space reuses it as-is and calls it over the gradio API for every request.

h3_split_blocks.py subclasses the pipeline's blocks with the text_encoder step removed. Dropping the step drops the components it declares, so load_components resolves only transformer / vae / audio_vae / the two schedulers, and prompt_embeds + text_token_tags become ordinary required pipeline inputs. The wire format is those two tensors — (1, num_text_tokens, 5120) bfloat16 and (num_text_tokens,) int64 — in one safetensors file with the resolved height / width / num_frames in its metadata header.

Text-to-video+audio only

The preview distills the T2VA path only, so this Space exposes no keyframe / reference inputs — the student's transformer_ref tower is not packaged with the checkpoint. Image conditioning is what multimodalart/minimax-h3 (the undistilled 30-step teacher) is for.

Prompt format

MiniMax-H3 is trained on a structured multimodal caption, not a bare sentence:

integrated_multimodal_description: <shots, camera, subjects, action, lighting>
overall_soundscape: <diegetic sound>
non_diegetic_music: <score>

Expand prompt (on by default) sends a short prompt through the conditioner's Qwen3-VL prompt rewriter, which writes that structure for you and returns it. Turn it off when you have already written a full-format prompt — the examples that carry MiniMax's own official prompts do exactly that.

Examples

The two long examples are MiniMax's own published prompts, taken verbatim from the base model's repo (Apache-2.0 code / docs):

• the starship-bridge two-shot from scripts/readme/reproducible-768p-t2va-request.sh
• the bakery two-shot, Case 1 of docs/VIDEO_PROMPT_WRITING_GUIDE_base_en.md

Generation constraints

Fixed by the checkpoint: 24 fps, a 768 pixel short edge at the training canvas, num_frames snapped up to the next 17·n + 5. The distillation's operating point is 1344×768 × 124 frames (≈5 s) — that is the default, and it is exactly the layout VSA was tuned on (grid = 37×24×42, 37 296 video rows, 672 tiles, 66 selected). The duration slider reaches 8 s and the canvas dropdown offers smaller/faster grids, both outside the distilled operating point, so quality degrades gracefully rather than being guaranteed. The 8 s ceiling is a VRAM limit, not a model limit: 75.7 GiB of weights sit resident on a 95.0 GiB card and the sparse working set grows with sequence length.

The checkpoint itself is a preview.

Measured

On this Space, over gradio_client. Startup — the ~85 GB download plus the load, with no placement and therefore no pack — is 78 s.

Sparse attention makes the cost linear in the packed rows rather than quadratic, so get_duration drops the quadratic term a dense path needs. The spread on the 5 s point is how fast a slice of the pool the request lands on; the fit is taken at the slow end (1.95e-3 · rows) so a slow slice is not aborted mid-video. That books 98 s at the 5 s default and 143 s at the 8 s maximum.

A cold worker pays a one-time 68 s: 75.7 GiB across PCIe (~11 s) plus the Triton JIT of the vendored kernels (~57 s). The same 544×544 request measures 74 s cold against 6 s warm. That is booked only on the first request a process serves rather than padded onto every request.

At 88.00 GiB on a 95.0 GiB card the 8 s ceiling leaves ~7 GiB — which is why the slider stops at 8 s and not 10 s.

The vendored kernels are checked against F.scaled_dot_product_attention on the live GPU by the hidden /selftest endpoint: at sparsity = 0 the ported path reproduces dense attention to 4.1e-03 relative / 0.999996 cosine, which is bf16 rounding.

Placement

H3_PLACEMENT=lazy: the weights move onto the card on the first GPU call and stay there. spaces' startup torch.pack() would write a second on-disk copy of every resident CUDA tensor, and 85 + 75 GB exceeds the 150 GB quota, so packing is not an option here. The one-time .to("cuda") plus the Triton JIT of the block-sparse kernels lands inside the first request of a cold worker; after that there is no offloading in the request path at all.

Space variables

Secrets

None. Every weight this Space downloads is public, and the conditioner is a public Space called on the requesting user's own ZeroGPU token.

License

The weights are under the MiniMax H3 Community License, inherited from the base model — it carries territory and acceptable-use restrictions. Read the base model's license before using outputs. The vendored kernels in vsa_kernel/ are Apache-2.0, from hao-ai-lab/FastVideo.