GPT Game Arena

GPT Game Arena is a ChatGPT plugin/MCP app with a standalone preview. It offers seven presets: Chess, Tic-Tac-Toe, Connect Four, Reversi, and Go on 9×9, 13×13, or 19×19 boards. The React widget is a self-contained web/dist/index.html; the Node MCP server serves it at ui://gpt-game-arena/v6/widget.html, while /preview provides the same UI locally.

Architecture

The server is authoritative for all rules and sessions through five MCP tools: create_game({ game, playerColor, boardSize?, difficulty? }), get_game_state({ gameId }), play_game_move({ gameId, actor, move, expectedVersion }), reset_game({ gameId }), and render_game({ gameId }). Go accepts boardSize 9, 13, or 19 and defaults to 9 when omitted. Difficulty accepts easy, medium, or hard and defaults to medium. Each successful tool output has structuredContent containing the complete game snapshot.

Moves must be exact legalMoves entries: Chess uses long algebraic coordinates such as e2e4; Tic-Tac-Toe and Reversi use uppercase squares such as A1; Connect Four uses an uppercase column A–G; and Go uses uppercase coordinates such as D4 (skipping I) or pass. Tic-Tac-Toe ends at three in a row; Connect Four applies gravity and ends at four in a row. Reversi flips bracketed discs, automatically skips a side with no legal move, and finishes with the higher disc score. Go uses positional superko and ends after two consecutive passes.

After a player move in ChatGPT, the widget asks GPT via ui/message to call get_game_state, follow the selected fixed difficulty guidance, choose one exact legalMoves entry, then call play_game_move as gpt using the version from that same fetched state. It polls for the result. The standalone preview uses deterministic difficulty-aware move selection: Easy is casual, Medium preserves the stable baseline, and Hard uses game-aware heuristics for chess, Tic-Tac-Toe, Connect Four, Reversi, and Go. Hard favors tactical or positional moves without claiming calibrated engine strength; Reversi can make consecutive GPT turns when an automatic skip leaves GPT to move again. No OPENAI_API_KEY is needed: GPT chooses moves through the host tool loop and the server never calls OpenAI.

reset_game is destructive and returns stateVersion: 0; treat it as a new epoch/ABA boundary, not a monotonic continuation. Reset preserves the selected game kind, player color, difficulty, and Go board size.

The Go engine uses positional superko, two-pass completion, and simplified Chinese-area scoring with 6.5 komi on every board size. The 19×19 preset is the full standard board, but this demo does not include a tournament dead-stone agreement phase after passing.

Local commands

npm install       # or npm ci (Node >=20.19 for this workspace build)
npm test
npm run typecheck
npm run build
npm run dev       # Node server, then visit /preview
npm run dev:web   # Vite widget development; run npm run dev:server alongside it
npm run preview   # Vite built-widget preview; run npm run dev:server alongside it

For MCP Inspector, build first, start npm run dev, and connect the inspector to http://localhost:8000/mcp using Streamable HTTP.

ChatGPT developer setup

Enable Developer mode at Settings → Security and login → Developer mode, then open ChatGPT Plugins → plus to add the server. Serve or tunnel a public HTTPS endpoint ending in /mcp, add the plugin/MCP server in ChatGPT, and refresh metadata whenever tools change. See the official Plugins quickstart and Connect ChatGPT deployment guide. The interaction design was inspired by the OpenAI Apps SDK examples, especially Cards Against AI.

Production and demo limits

This demo uses in-memory sessions (lost on restart), has no authentication or hosted deployment, and has not completed actual ChatGPT host acceptance. Difficulty is guidance for ChatGPT and deterministic heuristics in standalone mode, not calibrated engines or Elo ratings. The web workspace build requires Node >=20.19; the server remains Node 18.18-compatible, and an actual Node 18 built-server smoke is a separate gate. Production needs persistent authenticated storage, distributed quotas, HTTPS, observability, abuse controls, and explicit trusted-proxy configuration before honoring forwarded IP headers.