ActionPiece: Rethinking Action Tokenization for Autoregressive Vision-Language-Action Models
ActionPiece improves action tokenization for vision-language-action models via physical rank consistency, reaching 94.8% on LIBERO with Qwen3-VL-4B.
The paper introduces physical rank consistency (PRC), a metric measuring whether tokenization preserves local physical distance rankings of actions after reconstruction. ActionPiece preserves physical action relationships through joint supervision of representation learning and quantization, alongside reconstruction losses. Under the same Qwen3-VL-4B policy training setup, ActionPiece achieves 94.8% on LIBERO, 68.8% on unseen LIBERO-Plus, 71.9% on SimplerEnv, and 51.5% across VLA-Arena L0-L2.
- Pointwise MSE fails to capture distortion of relative action adjustments.
- PRC supervises near-far ordering in encoder and quantized feature distances.
- Quantization regularization applies the same ordering to codeword assignments.
- Ablations show both objectives jointly improve PRC and policy success.
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Action tokenizers play a central role in autoregressive vision-language-action (VLA) models, determining both the targets for policy training and the executable commands recovered from predicted tokens. Their fidelity is commonly evaluated using pointwise reconstruction metrics such as mean squared error (MSE), yet small individual errors do not fully characterize how faithfully action adjustments across demonstrations are preserved. After compression, similar actions may still cluster around a representative motion, while the adjustments needed for different contexts are diminished, distorted, or even reversed. We introduce physical rank consistency (PRC) to measure how well tokenization preserves local physical distance rankings after reconstruction. Evaluating decoded actions provides a common reference across token vocabularies and decoder architectures, complementing pointwise accuracy with a measure of relational fidelity. We further present ActionPiece, which preserves physical action relationships through joint supervision of representation learning and quantization. Physical rank preservation supervises near-far ordering in encoder and quantized feature distances, while quantization regularization applies the same ordering to codeword assignment distributions. Both objectives augment reconstruction, producing discrete action tokens for standard autoregressive policy learning and execution through a frozen decoder. Under the same Qwen3-VL-4B policy training setup, ActionPiece achieves 94.8% on LIBERO and 68.8% on unseen LIBERO-Plus, with additional evaluations reaching 71.9% on SimplerEnv and 51.5% across VLA-Arena L0-L2. Component ablations show that the two objectives jointly improve PRC and policy success, demonstrating the value of physical relationship supervision for action tokenization.
Text extracted automatically; images, tables and formatting may be missing. Original: https://huggingface.co/papers/2609.18487