CSSA: A Cross-modal Semantic-Structural Alignment Framework via LLMs and Graph Contrastive Learning for Fraud Detection of Online Payment

Graph Neural Networks (GNNs) have demonstrated exceptional performance in modeling structural dependencies within networked data. However, in complex decision-making environments, structural information alone often fails to capture the latent semantic logic and domain-specific heuristics. While Large Language Models (LLMs) excel in semantic reasoning, their integration with graph-structured data remains loosely coupled in existing literature. This paper proposes CSSA, a novel Cross-modal Semantic-Structural Alignment framework that synergizes the zero-shot reasoning of LLMs with the topological aggregation of GNNs through a contrastive learning objective. Specifically, we treat node attributes as semantic prompts for LLMs to distill high-level "risk indicators," while a GNN branch encodes the local neighborhood topology. A cross-modal alignment layer is then introduced to minimize the representational gap between semantic intent and structural behavior. We evaluate CSSA on a massive dataset of 2.84 million online transaction records. Experimental results demonstrate that CSSA achieves a superior F1-score and AUC compared to state-of-the-art GNNs, particularly in scenarios characterized by extreme class imbalance and covert adversarial patterns.