BFL (fp8r / nvfp4)

Black Forest Labs ships quantized variants of its Flux3 transformers in two schemes, stored as single-file checkpoints next to the original bf16 weights:

• fp8r: rowwise FP8. Each quantized linear stores an E4M3 weight and an fp32 per-row weight_scale. Activations are quantized to E4M3 per row at runtime.
• nvfp4: NVFP4 in the ModelOpt layout. Each quantized linear stores a packed E2M1 weight (two values per byte), E4M3 block scales of 16 (weight_scale), a global fp32 weight_scale_2, and a calibrated fp32 activation scale input_scale.

Diffusers loads both through ~FromSingleFileMixin.from_single_file with a BFLQuantizationConfig. The scheme is read from the checkpoint, so the same config works for either file. Only loading prequantized checkpoints is supported; pipeline-level loading is not.

The example below loads the fp8r checkpoint of FLUX 3 Image Klein Base, which lives at variants/fp8r/model.safetensors in the model repository. Pass config so that the model config is read from the Diffusers folder of that repository, then load the other components by name (see Single-file loading).

import torch

from diffusers import BFLQuantizationConfig, Flux3Transformer2DModel, ModularPipeline

ckpt_path = "https://huggingface.co/black-forest-labs/flux-3-image-klein-base/blob/main/variants/fp8r/model.safetensors"
transformer = Flux3Transformer2DModel.from_single_file(
    ckpt_path,
    config="black-forest-labs/flux-3-image-klein-base",
    subfolder="transformer",
    quantization_config=BFLQuantizationConfig(compute_dtype=torch.bfloat16),
    dtype=torch.bfloat16,
)
pipe = ModularPipeline.from_pretrained("black-forest-labs/flux-3-image-klein-base")
pipe.load_components(dtype=torch.bfloat16)
pipe.update_components(transformer=transformer)
pipe.to("cuda")

prompt = "A wide-angle wildlife photograph of a red fox standing alert in a snow-covered forest clearing at golden hour."
image = pipe(prompt=prompt, generator=torch.Generator().manual_seed(0), output="images")[0]
image.save("flux3-klein-base-fp8r.png")

Replace variants/fp8r with variants/nvfp4 to load the nvfp4 variant. The step-distilled FLUX 3 Image Klein ships the same two variants; load them with its repository id as config and pipeline.

Kernels

The quantized weights stay in their low-precision storage dtype. Which GEMM runs depends on the device:

Install the two FlashInfer packages at the same version, for example pip install flashinfer-python==0.6.13 flashinfer-cubin==0.6.13. The flashinfer-cubin release can lag behind flashinfer-python, and FlashInfer refuses to import when the versions differ.

FlashInfer compiles its NVFP4 kernels on first use with the CUDA toolkit of the environment and picks the target architecture from the visible GPU (or from FLASHINFER_CUDA_ARCH_LIST when set). Consumer and workstation Blackwell GPUs (compute capability 12.0) need a CUDA toolkit of 12.9 or newer. With an older toolkit FlashInfer finds no eligible architecture and the first NVFP4 call fails with FlashInfer requires GPUs with sm75 or higher.

On any other device the weights are dequantized to compute_dtype in each forward pass and the layer runs as a regular linear. In that case the activations are not quantized, so outputs differ slightly from the fused kernels.