Zhengye Han

Zhengye Han, Quanyan Zhu

Mobile robots can fail before they collide: a velocity that is safe now may commit the robot to a passage that moving obstacles will soon close. We study this predictive near-miss commitment problem and propose Risk-Sensitive Conjectural Scenario Planning (RCSP), a planning layer that evaluates candidate commands against plausible short-horizon obstacle futures. RCSP maintains a lightweight belief over local motion conjectures, samples future interactions, penalizes high-risk tails, and executes through a local safety check. In controlled MuJoCo bottleneck tasks, the RCSP planner reaches the goal without collisions and yields higher secondary safety and path-quality point estimates than a non-adaptive predictor, with additional latency. In ROS2/Gazebo, adding the local safety layer to a standard Nav2 stack reduces dynamic near-miss failures. On official DynaBARN/Jackal transfer, tuned DWA and TEB remain stronger on strict benchmark success, revealing the boundary of the approach. These simulation results position RCSP as a predictive-risk module that complements existing navigation stacks in dynamic bottleneck regimes.

Quanyan Zhu, Zhengye Han

Coalition formation is often modeled as a static equilibrium problem, neglecting the dynamic processes governing how agents self-organize. This paper proposes a dynamic split-and-merge framework that balances two conflicting economic forces: individual fairness and collective efficiency. We introduce a control-theoretic mechanism where topological operations are driven by distinct signals: splits are triggered by fairness violations (specifically, negative Shapley values representing "agent-responsible inefficiency"), while merges are driven by strict surplus improvements (superadditivity). We prove that these dynamics converge in finite time to a specific class of steady states termed Shapley-Fair and Merge-Stable (SFMS) partitions. Convergence is established via a vector Lyapunov function tracking aggregate fairness deficits and system surplus, leveraging a discrete-time LaSalle invariance principle. Numerical case studies on a 10-player game demonstrate the algorithm's ability to resolve fairness tensions and reach stable configurations, providing a rigorous foundation for endogenous coalition formation in dynamic environments.

Quanyan Zhu, Zhengye Han

We study strategic interaction in linear-quadratic network games where agents act on subjective, misspecified models of their environment. Agents observe noisy aggregate signals generated by local network externalities and interpret them through simplified conjectures, such as constant or mean-field representations. We characterize the long-run behavior using the Berk-Nash equilibrium (BNE) concept, establishing conditions under which BNE diverges from the Nash equilibrium of the perfectly specified game. We quantify this divergence using a Value of Misspecification (VoM) metric. Building on this framework, we introduce "cognitive arbitrage" -- a design paradigm where a system designer strategically shapes agents' conjectures via minimal observation distortions to steer equilibrium outcomes. We formulate the cognitive arbitrage problem as a Stackelberg optimization with closed-form solutions and prove the convergence of a two-time-scale learning algorithm to the optimal BNE. Our results provide a principled framework for influencing behavior in networked systems with bounded rationality, offering a new perspective on mechanism design that operates on agents' representations rather than their incentives.

Quanyan Zhu, Zhengye Han

We study sequential decision-making when the agent's internal model class is misspecified. Within the infinite-horizon Berk-Nash framework, stable behavior arises as a fixed point: the agent acts optimally relative to a subjective model, while that model is statistically consistent with the long-run data endogenously generated by the policy itself. We provide a rigorous characterization of this equilibrium via coupled linear programs and a bilevel optimization formulation. To address the intrinsic non-smoothness of standard best-response correspondences, we introduce entropy regularization, establishing the existence of a unique soft Bellman fixed point and a smooth objective. Exploiting this regularity, we develop an online learning scheme that casts model selection as an adversarial bandit problem using an EXP3-type update, augmented by a novel conjecture-set zooming mechanism that adaptively refines the parameter space. Numerical results demonstrate effective exploration-exploitation trade-offs, convergence to the KL-minimizing model, and sublinear regret.

Zhengye Han, Quanyan Zhu

As large language models (LLMs) are increasingly deployed in critical applications, the challenge of jailbreaking, where adversaries manipulate the models to bypass safety mechanisms, has become a significant concern. This paper presents a dynamic Stackelberg game framework to model the interactions between attackers and defenders in the context of LLM jailbreaking. The framework treats the prompt-response dynamics as a sequential extensive-form game, where the defender, as the leader, commits to a strategy while anticipating the attacker's optimal responses. We propose a novel agentic AI solution, the "Purple Agent," which integrates adversarial exploration and defensive strategies using Rapidly-exploring Random Trees (RRT). The Purple Agent actively simulates potential attack trajectories and intervenes proactively to prevent harmful outputs. This approach offers a principled method for analyzing adversarial dynamics and provides a foundation for mitigating the risk of jailbreaking.

Quanyan Zhu, Zhengye Han

This paper introduces a performative scenario optimization framework for decision-dependent chance-constrained problems. Unlike classical stochastic optimization, we account for the feedback loop where decisions actively shape the underlying data-generating process. We define performative solutions as self-consistent equilibria and establish their existence using Kakutani's fixed-point theorem. To ensure computational tractability without requiring an explicit model of the environment, we propose a model-free, scenario-based approximation that alternates between data generation and optimization. Under mild regularity conditions, we prove that a stochastic fixed-point iteration, equipped with a logarithmic sample size schedule, converges almost surely to the unique performative solution. The effectiveness of the proposed framework is demonstrated through an emerging AI safety application: deploying performative guardrails against Large Language Model (LLM) jailbreaks. Numerical results confirm the co-evolution and convergence of the guardrail classifier and the induced adversarial prompt distribution to a stable equilibrium.