Designing an MCP Arena Where AI-Agent Actions Are Replayable

AI agents are easy to demo and surprisingly hard to evaluate. A polished chat transcript can hide stale state, invalid actions, accidental retries, and private information leaking into the model's observation. I built WagerCall as a bounded environment for studying those problems. Agents play casino-style simulations through the Model Context Protocol (MCP), but every balance is made of synthetic, non-transferable points with zero monetary value. There are no deposits, purchases, prizes, withdrawals, or redemption paths. The games are useful because they compress several agent-engineering problems into short, inspectable loops: partial information, strict legal actions, versioned state, risk decisions, and irreversible transitions. Here are the design choices that made the environment auditable instead of merely entertaining. 1. Bound the world before evaluating the agent An evaluation environment should say exactly what an agent can observe and change. WagerCall's MCP tools set openWorldHint to false and operate only on arena state. The agent cannot call a generic SQL, admin, execute, or debug tool. That boundary matters. If an agent can quietly reach unrelated systems, it becomes difficult to tell whether a result came from reasoning inside the task or from an accidental side channel. The same rule applies to the economy. Integer synthetic points make trade-offs visible without introducing payments, transferable assets, or anything redeemable for value. 2. Let pure game logic propose; let the database decide The game engine is deterministic and side-effect free. Given a state and an action, it produces a proposal containing the next state, ledger entries, events, presentation frames, and an optional outcome. A proposal is not yet a fact. PostgreSQL commits the transition in one transaction after rechecking the current round version, account balance, session ownership, and terminal state. It either writes the action, balance change, new round state, and audit event