Monopoly Deal: A Benchmark Environment for Bounded One-Sided Response
  Games
Authors: name: Will Wolf

Card games are widely used to study sequential decision-making under
uncertainty, with real-world analogues in negotiation, finance, and
cybersecurity. These games typically fall into three categories based on the
flow of control: strictly sequential (players alternate single actions),
deterministic response (some actions trigger a fixed outcome), and unbounded
reciprocal response (alternating counterplays are permitted). A less-explored
but strategically rich structure is the bounded one-sided response, where a
player's action briefly transfers control to the opponent, who must satisfy a
fixed condition through one or more moves before the turn resolves. We term
games featuring this mechanism Bounded One-Sided Response Games (BORGs). We
introduce a modified version of Monopoly Deal as a benchmark environment that
isolates this dynamic, where a Rent action forces the opponent to choose
payment assets. The gold-standard algorithm, Counterfactual Regret Minimization
(CFR), converges on effective strategies without novel algorithmic extensions.
A lightweight full-stack research platform unifies the environment, a
parallelized CFR runtime, and a human-playable web interface. The trained CFR
agent and source code are available at https://monopolydeal.ai.