Codex vs. Claude Code at Liar's Dice: the Winning Bluff Was the Truth

One authoritative engine, two seat-locked MCP servers, three best-of-threes, and a 3-millisecond whodunit The matches are real: Codex CLI ( gpt-5.6-sol ) against Claude Code (Claude Opus 5), both playing through the same rules engine. Every number below was recomputed from the raw run.json and both session logs, and every game replays deterministically from its seed. Quotes from the agents are verbatim from decision-time records. None of this is a general model ranking. I wired Codex CLI and Claude Code into the same Liar's Dice engine over MCP and had them play three best-of-3 series. Claude won all three, 2–0 each time. Its challenge calls hit 8 out of 11; Codex's hit 4 out of 26. The score takes two sentences. The parts worth writing down took longer: how to build a table that two closed-source agents can't cheat at, two numbers that surprised me, and an incident where I almost blamed a model for something its CLI did. The table Liar's Dice in sixty seconds: five dice each, and you only see your own. Players alternate bids of the form "there are at least N dice showing X across the whole table." On your turn you either raise the bid or challenge it. On a challenge everyone reveals; if the bid stands, the challenger loses a die, otherwise the bidder does. Run out of dice and you lose the match. Ones are wild by default. The rules are the easy part. The hard part is making the result trustworthy. Codex and Claude Code ship with their own system prompts and tool loops, so the referee has to guarantee three things by construction: neither side can see the other's dice, the referee has no side channel that favors anyone, and the "what it was thinking" quotes you read afterward were actually written at decision time. The setup is one in-process rules engine behind a localhost-only HTTP coordinator, with two stdio MCP servers doing nothing but forwarding: Codex CLI (gpt-5.6-sol) Claude Code (Opus 5) | stdio MCP | stdio MCP v v [seat-mcp A] --token A--+ +--token B-- [sea