π₀-FAST (Pi0-FAST)

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π₀-FAST is a Vision-Language-Action model for general robot control that uses autoregressive next-token prediction to model continuous robot actions.

Model Overview

π₀-FAST combines the power of Vision-Language Models with a novel action tokenization approach called FAST (Frequency-space Action Sequence Tokenization). This enables training autoregressive VLAs on highly dexterous tasks that are impossible with standard binning-based discretization, while training up to 5x faster than diffusion-based approaches like π₀.

Why FAST?

Standard approaches for robot action tokenization use simple per-dimension, per-timestep binning schemes. While passable for simple behaviors, this rapidly breaks down for complex and dexterous skills that require precision and high-frequency control.

FAST solves this by compressing action sequences using signal processing techniques, resulting in a dense sequence of action tokens that can be predicted autoregressively—just like language tokens.

How FAST Tokenization Works

The FAST tokenizer compresses action sequences through the following steps:

1. Normalize: Take a continuous action chunk of shape (H, D) where H is the horizon and D is the action dimension. Normalize using one of the supported normalization methods (Quantiles recommended to handle outliers).

2. Discrete Cosine Transform (DCT): Apply DCT (via scipy) to each action dimension separately. DCT is a compression algorithm commonly used in image and audio codecs (JPEG, MP3).

3. Quantization: Round and remove insignificant coefficients for each action dimension, producing a sparse frequency matrix.

4. Flatten: Flatten the matrix into a 1D vector, with low-frequency components first.

5. Byte Pair Encoding (BPE): Train a BPE tokenizer to compress the DCT coefficients into dense action tokens, typically achieving 10x compression over prior tokenization approaches.

This approach can transform any existing VLM into a VLA by training it to predict these FAST tokens.

Installation Requirements

1. Install LeRobot by following our Installation Guide.

2. Install π₀-FAST dependencies by running:

   pip install -e ".[pi]"
   

Training a Custom FAST Tokenizer

You have two options for the FAST tokenizer:

1. Use the pre-trained tokenizer: The lerobot/fast-action-tokenizer tokenizer was trained on 1M+ real robot action sequences and works as a general-purpose tokenizer.

2. Train your own tokenizer: For maximum performance on your specific dataset, you can finetune the tokenizer on your own data.

Training Your Own Tokenizer

lerobot-train-tokenizer \
    --repo_id "user/my-lerobot-dataset" \
    --action_horizon 10 \
    --encoded_dims "0:6" \
    --vocab_size 1024 \
    --scale 10.0 \
    --normalization_mode QUANTILES \
    --output_dir "./my_fast_tokenizer" \
    --push_to_hub \
    --hub_repo_id "username/my-action-tokenizer"

Key Tokenizer Parameters

Parameter	Description	Default
--repo_id	LeRobot dataset repository ID	Required
--action_horizon	Number of future actions in each chunk	10
--encoded_dims	Comma-separated dimension ranges to encode (e.g., "0:6,7:23")	"0:6,7:23"
--vocab_size	BPE vocabulary size	1024
--scale	DCT scaling factor for quantization	10.0
--normalization_mode	Normalization mode (MEAN_STD, MIN_MAX, QUANTILES, QUANTILE10, IDENTITY)	QUANTILES
--sample_fraction	Fraction of chunks to sample per episode	0.1

Usage

To use π₀-FAST in LeRobot, specify the policy type as:

policy.type=pi0_fast

Training

For training π₀-FAST, you can use the LeRobot training script:

lerobot-train \
    --dataset.repo_id=your_dataset \
    --policy.type=pi0_fast \
    --output_dir=./outputs/pi0fast_training \
    --job_name=pi0fast_training \
    --policy.pretrained_path=lerobot/pi0fast-base \
    --policy.dtype=bfloat16 \
    --policy.gradient_checkpointing=true \
    --policy.chunk_size=10 \
    --policy.n_action_steps=10 \
    --policy.max_action_tokens=256 \
    --steps=100000 \
    --batch_size=4 \
    --policy.device=cuda

Key Training Parameters

Parameter	Description	Default
--policy.gradient_checkpointing=true	Reduces memory usage significantly during training	false
--policy.dtype=bfloat16	Use mixed precision training for efficiency	float32
--policy.chunk_size	Number of action steps to predict (action horizon)	50
--policy.n_action_steps	Number of action steps to execute	50
--policy.max_action_tokens	Maximum number of FAST tokens per action chunk	256
--policy.action_tokenizer_name	FAST tokenizer to use	lerobot/fast-action-tokenizer
--policy.compile_model=true	Enable torch.compile for faster training	false

Inference

KV-Caching for Fast Inference

π₀-FAST supports KV-caching, a widely used optimization in LLM inference. This caches the key-value pairs from the attention mechanism, avoiding redundant computation during autoregressive decoding.

# KV-caching is enabled by default
policy.use_kv_cache=true

Inference Example

from lerobot.policies.pi0_fast import PI0FastPolicy, PI0FastConfig

# Load the policy
policy = PI0FastPolicy.from_pretrained("your-model-path")

# During inference
actions = policy.predict_action_chunk(batch)

Model Architecture

π₀-FAST uses a PaliGemma-based architecture:

• Vision Encoder: SigLIP vision tower for image understanding

• Language Model: Gemma 2B for processing language instructions and predicting action tokens

The model takes images, text instructions, and robot state as input, and outputs discrete FAST tokens that are decoded back to continuous actions.

Configuration Options

Parameter	Description	Default
paligemma_variant	VLM backbone variant (gemma_300m, gemma_2b)	gemma_2b
max_state_dim	Maximum state vector dimension (padded)	32
max_action_dim	Maximum action vector dimension (padded)	32
temperature	Sampling temperature (0.0 for greedy)	0.0
max_decoding_steps	Maximum decoding steps	256
use_kv_cache	Enable KV caching for faster inference	true

Comparison with π₀

Feature	π₀	π₀-FAST
Action Representation	Flow Matching (Diffusion)	Autoregressive Tokens (FAST)
Training Speed	1x	5x faster
Dexterity	High	High
Inference Method	Iterative Denoising	Autoregressive Decoding
KV-Caching	N/A	Supported

Reproducing π₀Fast results

We reproduce the results of π₀Fast on the LIBERO benchmark using the LeRobot implementation. We take the LeRobot PiFast base model lerobot/pi0fast-base and finetune for an additional 40kk steps in bfloat16, with batch size of 256 on 8 H100 GPUs using the HuggingFace LIBERO dataset.

The finetuned model can be found here:

• π₀Fast LIBERO: lerobot/pi0fast-libero

With the following training command:

lerobot-train \
  --dataset.repo_id=lerobot/libero \
  --output_dir=outputs/libero_pi0fast \
  --job_name=libero_pi0fast \
  --policy.path=lerobot/pi0fast-base \
  --policy.dtype=bfloat16 \
  --steps=100000 \
  --save_freq=20000 \
  --batch_size=4 \
  --policy.device=cuda \
  --policy.scheduler_warmup_steps=4000 \
  --policy.scheduler_decay_steps=100000 \
  --policy.scheduler_decay_lr=1e-5 \
  --policy.gradient_checkpointing=true \
  --policy.chunk_size=10 \
  --policy.n_action_steps=10 \
  --policy.max_action_tokens=256 \
  --policy.empty_cameras=1 \

We then evaluate the finetuned model using the LeRobot LIBERO implementation, by running the following command:

tasks="libero_object,libero_spatial,libero_goal,libero_10"
lerobot-eval \
  --policy.path=lerobot/pi0fast-libero \
  --policy.max_action_tokens=256 \
  --env.type=libero \
  --policy.gradient_checkpointing=false \
  --env.task=${tasks} \
  --eval.batch_size=1 \
  --eval.n_episodes=1 \
  --rename_map='{"observation.images.image":"observation.images.base_0_rgb","observation.images.image2":"observation.images.left_wrist_0_rgb"}'

Note: We set n_action_steps=10, similar to the original OpenPI implementation.

Results

We obtain the following results on the LIBERO benchmark:

Model	LIBERO Spatial	LIBERO Object	LIBERO Goal	LIBERO 10	Average
π₀-fast	70.0	100.0	100.0	60.0	82.5

The full evaluation output folder, including videos, is available here

License

This model follows the Apache 2.0 License, consistent with the original OpenPI repository.

References

• FAST: Efficient Robot Action Tokenization - Physical Intelligence Blog

• OpenPI Repository - Original implementation

• FAST Tokenizer on Hugging Face - Pre-trained tokenizer