Safe deployment of black-box AI (BBAI) systems such as foundation model agents requires methods for evaluating their capabilities in novel settings. We define an agent's capability as its ability to achieve a short term objective and formalize the problem of learning models that predict whether, with what effects, and under what conditions, an agent can perform a capability. We introduce Monte Carlo Query Search (MCQS), an active query-synthesis method for learning symbolic stochastic capability models of BBAIs. MCQS models capabilities as conditional probability distributions over outcomes and formulates capability evaluation as an active learning problem over policies. We use Monte Carlo tree search to synthesize queries that maximally distinguish between extremal capability hypotheses: the lattice meet and join corresponding to the most pessimistic and optimistic models consistent with observed behavior. Executing these queries yields trajectories that prune inconsistent hypotheses. We prove soundness, completeness, and convergence properties under standard realizability and sampling assumptions. Experiments with multiple BBAI systems show that MCQS learns accurate capability models more efficiently than baseline query strategies, enabling systematic characterization of agent capability boundaries with fewer interactions.

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