Large Language Model (LLM) agents exhibit powerful autonomous capabilities across domains such as robotics, virtual assistants, and web automation. However, their stochastic behavior introduces significant safety risks that are difficult to anticipate. Existing rule-based enforcement systems, such as AgentSpec, focus on developing reactive safety rules, which typically respond only when unsafe behavior is imminent or has already occurred. These systems lack foresight and struggle with long-horizon dependencies and distribution shifts. To address these limitations, we propose Pro2Guard, a proactive runtime enforcement framework grounded in probabilistic reachability analysis. Pro2Guard abstracts agent behaviors into symbolic states and learns a Discrete-Time Markov Chain (DTMC) from execution traces. At runtime, it anticipates future risks by estimating the probability of reaching unsafe states, triggering interventions before violations occur when the predicted risk exceeds a user-defined threshold. By incorporating semantic validity checks and leveraging PAC bounds, Pro2Guard ensures statistical reliability while approximating the underlying ground-truth model. We evaluate Pro2Guard extensively across two safety-critical domains: embodied household agents and autonomous vehicles. In embodied agent tasks, Pro2Guard enforces safety early on up to 93.6% of unsafe tasks using low thresholds, while configurable modes (e.g., reflect) allow balancing safety with task success, maintaining up to 80.4% task completion. In autonomous driving scenarios, Pro2Guard achieves 100% prediction of traffic law violations and collisions, anticipating risks up to 38.66 seconds ahead.