Causal-Graph-Guided Reinforcement Learning for LLM Security Testing: An Empirical Study

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This preprint investigates whether structural causal information derived from adversarial interaction trajectories can improve reinforcement-learning-based black-box security testing of large language models. The study combines PPO-based adaptive prompt search with an interpretable six-factor representation derived from red-teaming trajectories. Structural relationships among these factors are estimated using Fast Causal Inference (FCI) and represented through a Partial Ancestral Graph (PAG). A Structural Causal Model (SCM) is subsequently used to provide model-based structural guidance during reinforcement learning through intermediate feedback and causal-graph-guided action selection. Under the reported experimental configuration, the causally guided approach achieved a mean attack success rate of 61.54%, compared with 19.33% for the archived RL-based baseline, while reducing API and token usage. Because the primary evaluations were not fully prompt-aligned, this headline comparison is treated as descriptive rather than as a paired statistical comparison. The work contributes an empirical investigation of how causal structure can be incorporated into adaptive LLM security testing while maintaining an interpretable representation of adversarial prompt characteristics.