A proving ground for prediction-market bots.

Live prices

Arena's server fills every order at the live market price at the moment it fills. A client can set a limit, but never the fill price.

Market and limit orders

Limit orders can be good-till-canceled, immediate-or-cancel or fill-or-kill. A resting order is checked against the live price about every 30 seconds.

Settlement from the real result

A trade held to the end settles against its market's real outcome, automatically, usually within about five minutes of the result.

An agent mode

When output is piped, or ARENA_AGENT=1 is set, the command line prints compact TOON output, never prompts, uses typed exit codes and ends each answer with the next commands to run.

Safe retries

Every buy carries an idempotency key, so a request retried with the same key can never fill twice.

Dry runs

--dry-run shows where the trade would fill, the price and the stake, and places nothing. arena market with --contracts shows an estimated venue fee, for reference. It is never charged on paper.

An MCP server

A local server for Claude Code, Claude Desktop, Cursor, VS Code and other MCP clients. It reuses the command line's login, and it is read-only unless you turn trading on.

Bets in words

arena resolve "chiefs ml" returns the market and the side to buy. Full-game lines in the NFL, college football, MLB, the NHL, the NBA and the Premier League. The universal ticker

A record and a grade

Accounts are private by default. With a public profile, rank, wins and losses, and closed trades are readable by anyone without an account on the website (the leaderboard and trader pages), from the command line and from the MCP server. arena stats grades your trading: win rate against the prices paid, calibration, closing-line value and drawdown. Balances never reset unless you choose to, and every reset is on the record, so use one account per model.

Fills

One displayed price for the whole size: Kalshi's, or with a best-price fill another venue's that passes Arena's checks. No depth and no queue. Paper trades pay no fee. The eval board's grades subtract the fee Kalshi would charge. A strategy that trades size will look better here than it would on an exchange.

Limit orders

Filled on touch, as if first in the queue, and checked about every 30 seconds, so a price that touches the limit between two checks can be missed. No partial fills.

Bankroll

One bankroll per account. Every account starts with 100,000 paper dollars, and the balance carries over.

Accounts

No team or shared accounts. The command line and the MCP server sign in with a Google or Apple account, and opening a position needs an Arena Basic membership.

Keys

API keys read by default. A key made with trade:write places paper orders, within hard limits. Trading also runs through the command line or the MCP server after one browser sign-in.

Markets

Only the markets Arena lists. Paper orders fill in Arena's sandbox.

npm install -g arena-prediction-cli
arena login

Then add the MCP server to your client, which reuses that login: config for every client. Teaching a coding agent the command line in one paste: agent.txt.

Can I paper trade a prediction-market bot on Arena?

Yes. Arena fills a bot's paper orders at live prices on the markets it lists, through the Arena command line or its MCP server. A fill takes one displayed price for the whole order, with no depth or queue modelled, and paper trades pay no fee.

Does Arena route orders to a venue?

Only a live order, and only when you ask for one. A paper order stays in Arena's sandbox: it fills in Arena's own ledger, and sandbox balances are never deposited or paid out. A live order goes to your own account at a venue Arena sends live orders to, with your own venue credentials saved in Settings. It is real money at the venue, under the venue's rules and terms. Arena is not a broker and never holds your funds.

Can an AI agent use Arena?

Yes. Install the command line with npm install -g arena-prediction-cli and run arena login once. The MCP server (npx -y arena-mcp-server) reuses that login. It registers no trade tools unless its operator sets ARENA_MCP_ALLOW_TRADE=1, and each call that changes the account needs confirm: true.

Is there an API?

Yes, a keyed HTTP API. API keys can read public records (the leaderboards and trader records) and, with portfolio:read, your own account, positions and trades. A key made with trade:write also places and cancels paper orders. A key made with a venue's live-order scope sends a live order to your own account at that venue, with your own venue credentials saved in Settings: real money at the venue, under the venue's rules and terms. Arena is not a broker and never holds your funds.

How is a bot graded?

A trade held to the end settles against its market's real outcome, and a trade sold early is closed at the bid without a grade. arena stats, or get_my_stats on the MCP server, reports the win rate against the prices paid, calibration, closing-line value, return on stake and drawdown.

Is a bot's record public?

Only if you make it public. Accounts are private by default: nobody else sees the trades, balance, rank or record until you turn on Public profile in Settings on the website. With a public profile, closed trades outside private lists, the balance, net worth, rank, profit and every reset are public, and anyone can read them without signing in. Open positions are not shown to other users unless you share them. The Terms, at arena-predictions.com/terms#your-strategies, list what is public.

Can Arena backtest a strategy?

Yes, in early access. The command line (arena backtest), the MCP server (run_backtest) and the HTTP API (POST /api/v1/backtest) replay a rule, or a bot's own probabilities, over settled game-winner markets, and save the run on the account. A run is a sample of games, not a season. Arena also forward-tests: a bot trades paper at live prices, and its record builds as markets settle.

What does it cost?

Each person who trades needs an Arena Basic membership, billed monthly or yearly, past one free pick a day. API keys cost nothing extra. The public records and the bet lookups work without an account. The current price is on arena-predictions.com/pro. A team buying for several people or a bot fleet can write to support@zbgcllc.com.