PA-FAS: Towards Interpretable and Generalizable Multimodal Face Anti-Spoofing via Path-Augmented Reinforcement Learning

In recent years, face anti-spoofing (FAS) has made notable progress in multimodal fusion, cross-domain generalization, and interpretability. With the development of large language models and reinforcement learning (RL), strategy-based training paradigms offer new opportunities for jointly modeling multimodality, generalization, and interpretability. However, compared to unimodal reasoning, multimodal reasoning introduces more complex logic, such as accurate feature representation and cross-modal verification, which significantly increases reasoning complexity and labeling difficulty. Due to the lack of high-quality annotations in existing multimodal FAS datasets, directly applying RL strategies is sub-optimal, hindering robust multimodal reasoning. In this paper, we find two key issues of supervised fine-tuning combined with reinforcement learning (SFT+RL) paradigms in multimodal FAS reasoning: 1) limited multimodal reasoning paths not only hinder the full utilization of multimodal information but also constrain the model’s exploration space after SFT, thereby affecting the effectiveness of subsequent RL; and 2) the mismatch between single-task supervision and the diversity of multimodal reasoning paths leads to reasoning confusion, where models may exploit shortcuts by directly mapping input images to answers, bypassing the intended reasoning process. These issues further increase the complexity of multimodal reasoning and hinder the effective application of RL strategies. To address these challenges, we propose the PA-FAS framework with a reasoning path enhancement strategy for high-quality extended reasoning sequences construction based on limited annotated data to enrich the reasoning paths and alleviate exploration constraints. Additionally, we introduce an answer shuffling mechanism during SFT for comprehensive multimodal analysis rather than mining superficial cues, thus encouraging deeper reasoning and avoiding shortcut learning. Our method significantly improves multimodal reasoning accuracy and generalization, and successfully unifies multimodal fusion, cross-domain generalization, and interpretability towards trustworthy multimodal FAS.