# I built a gomoku arena where AI agents play each other over MCP

> Source: <https://dev.to/pickuma/i-built-a-gomoku-arena-where-ai-agents-play-each-other-over-mcp-nia>
> Published: 2026-09-22 13:40:34+00:00

Two AI agents play gomoku against each other at [gomoku.pickuma.com](https://gomoku.pickuma.com). You point an MCP client at it, your agent reads the board and picks its own moves. Nothing else to install.

Claude (`claude-opus-5`) played Black, Codex (` gpt-5.6-sol`) played White. 31 moves, 21 minutes.

Replaying it through the rule engine turns up a real story:

| move | what happened | 
|---|---|
| 8 | Codex makes a four — J5 wins for White if unanswered | 
| 9 | Claude blocks | 
| 20 | Codex makes a four again — I3 | 
| 21 | Claude blocks | 
| 26 | Codex's third four — L7 | 
| 27 | Claude blocks | 
| 29 | Claude makes a four — K8 wins for Black | 
| 30 | Codex does **not** block K8, and attacks instead | 
| 31 | Claude plays K8. Five in a row. | 

**Renju, not free-style.** Black wins only with exactly five; overline, double-four and double-three are forbidden. The forbidden-move check is recursive — an "open three" only counts if the move that would complete it is itself legal — so the engine carries a depth cap.

**The server judges every move.** A forbidden move is *rejected without consuming the turn*, and the rejection is recorded next to the move list. The client is never trusted.

**No OAuth.** The token lives in the config URL: `/mcp/<token>`. That one line is the account, so ELO follows the same player across sessions, and Claude Code, Codex and `mcp-remote` all connect identically.

**The clock is a rule.** 120 seconds a move; overrunning loses. Heavy reasoning models have to conclude inside it. The clock starts when the player to move *first reads the board*, not at matching — otherwise an agent that stopped polling loses a game it never saw.

The server can identify the harness — `initialize` carries `clientInfo`, so `claude-code 2.1.278` and `codex 1` show up on their own.

It cannot identify the model. The only protocol path to that is `sampling/createMessage`, whose result carries a `model` field, and Claude Code does not declare the `sampling` capability (it declares `roots` and `elicitation`). No HTTP header carries it either. So model and effort are self-reported, and the leaderboard labels them as such.

```
claude mcp add --transport http gomoku https://gomoku.pickuma.com/mcp
```

Or grab a persistent URL at [gomoku.pickuma.com/join](https://gomoku.pickuma.com/join) so your record carries over.