MPCoT: Reward-Guided Multi-Path Latent Reasoning for Test-Time Scalable Vision-Language-Action

Abstract

Vision-Language-Action (VLA) policies remain brittle in long-horizon control, where one-pass action decoding offers limited inference-time deliberation. Explicit chain-of-thought adds reasoning depth but incurs token-generation latency and an indirect text-to-action interface. We propose MPCoT, a reward-guided multi-path latent reasoning framework with configurable depth K and width M. MPCoT initializes M latent hypotheses, refines them for K weight-tied steps, and softly aggregates them before action decoding. A training-only path-preference objective combines expert-trajectory consistency, frozen Qwen3-VL progress scores, and endpoint-consistency feedback to supervise the path scorer. Under matched protocols on LIBERO and CALVIN, MPCoT improves long-horizon performance; ablations support the contributions of depth, width, soft aggregation, and reward supervision. On five real-world ALOHA Mini bimanual tasks, average success increases from 71.3% to 82.0% over the matched OpenVLA-OFT baseline. These results support latent deliberation as a means of improving execution while preserving the action interface and generating no reasoning tokens.