Prediction Market Arbitrage Trading Bot

An open-source arbitrage strategy for Kalshi and Polymarket: it finds arbitrage opportunities, runs the strategy on live prices as paper trading, and backtests it on history. No orders are placed.

Powered by Synpath: one API for every prediction market. Kalshi, Polymarket, Polymarket US and Opinion through one open-source Python SDK:

• Smart order routing across both order books, at the cheapest price after fees
• Advanced order types: stop-limit, trailing stop, OCO, TWAP and more, on every venue
• Cross-venue market matching, with settlement rules compared
• Unified data: order books, fees, live streams and history

Open source (MIT): github.com/Synpath-ai/synpath · pip install synpath

Kalshi and Polymarket often list the same question at different prices. When that happens, buying YES on one platform and NO on the other can cost less than the $1 the pair always pays out. That difference is the arbitrage profit.

Profit per pair = $1.00 − (YES price + NO price) − trading fees

This bot:

• Matches markets across both platforms and checks that they settle under the same rules
• Detects arbitrage opportunities from live order books, after fees
• Signals entries and exits and tracks paper positions and P&L at the quoted prices
• Backtests the same strategy on historical prices and charts the results

Example backtest on three markets listed on both Kalshi and Polymarket: 60 days of 1-minute prices (Jul 29 – Sep 27, 2026), 1,000 contracts per leg. +$1,771.40 total P&L, a +60.4% return on the $2,934 peak capital tied up at once, across 44 trades, every one closed at a profit.

Placing real orders is done with Synpath, the library this bot is built on: limit, IOC and FOK orders on Kalshi, Polymarket and Opinion, plus the live fill streams a two-legged trade needs. This bot stops at the signal.

Polymarket and Kalshi sometimes price the same event differently.

The bot looks for cases where it can buy YES on one platform and NO on the other for a combined cost of less than $1.

Because exactly one side will pay $1 at settlement, buying both sides below $1 creates a built-in profit.

Suppose:

• YES on Kalshi = 39¢
• NO on Polymarket = 55¢

Buying both costs 94¢. After fees, the total cost is about 96.7¢.

At settlement, one of the two positions will pay $1, so the trade locks in roughly 3.3¢ profit per contract pair. On 1,000 contracts, that's about $33.

If the price difference disappears before the event settles, the bot can close both positions early and take the profit.

For example, if the two positions can later be sold for a combined $1.02, that's about 2.6¢ profit per pair after fees (about $26 on 1,000 contracts). The bot exits and frees up the capital for the next opportunity.

If prices don't converge, it simply holds until settlement and collects the spread it locked in at entry.

Find the same event priced differently → buy both sides for less than $1 → exit when profitable or hold to settlement.

The bot automates this process across Polymarket and Kalshi.

Three simple rules:

1. Buy when YES on one platform plus NO on the other costs at least 2¢ less than $1, after fees.
2. Sell both sides as soon as selling them makes at least 2¢ profit per pair, after fees.
3. Otherwise hold until settlement, where the pair pays $1 and the profit locked in at entry is collected.

So the strategy never sells at a loss. YES and NO are always bought in the same quantity, so the position stays balanced, and the size is limited to what's available at the best price on each platform.

Both 2¢ thresholds can be changed in config.py (entry_edge and take_profit).

Requires Python 3.10+.

git clone https://github.com/Synpath-ai/prediction-market-arbitrage-trading-bot
cd prediction-market-arbitrage-trading-bot
python3 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt

cp .env.example .env        # then set SYNPATH_API_KEY

Get a key at synpath.dev. It's used for market matching, live prices and history. No exchange accounts or trading keys are needed.

python -m src.discover --save

This finds the 10 markets with the best arbitrage profit available right now (after fees), among markets listed on both Kalshi and Polymarket that settle under the same rules, and saves them to markets.json. The bot and the backtest use them automatically. It reads live prices for every candidate, so it can take a minute.

Change what it picks with --limit 20, --sort gap (largest price difference right now) or --sort volume (most traded), and narrow the search with --query "senate" or --domain election.

For live Kalshi prices over WebSocket, also add a Kalshi API key (KALSHI_KEY_ID and KALSHI_PRIVATE_KEY_PATH): Kalshi signs every WebSocket connection, even for market data. Without one, the bot reads Kalshi's public order book every 2 seconds instead. Polymarket's WebSocket needs no key.

Adjust the rest of config.py to taste:

The bot doesn't ship with any markets preselected. Use the discovery tool to list markets that trade on both platforms:

python -m src.discover                        # 10 markets with the best profit right now
python -m src.discover --sort volume --limit 5  # most traded
python -m src.discover --query "senate" --save  # search by keyword and save

Market matching is done by Synpath, not by comparing titles. Each matched market comes with a rule check:

• same: both platforms settle the market the same way. These are the only markets used by default.
• insufficient: one platform's rules don't state every detail.
• not_same: the markets can settle differently, so it is not an arbitrage. Never used.

Add --save to keep the same markets in markets.json, which the bot and backtest then use. To pick markets by hand instead, list them in config.py or pass --market EVENT_ID KALSHI_ID.

python -m trading                                      # every saved market
python -m trading --market <event_id> kalshi:<TICKER>  # a single market

The bot subscribes to both platforms' order books over WebSocket (Polymarket's always; Kalshi's when a Kalshi key is set, otherwise it reads Kalshi every 2 seconds) and re-prices a market the moment either book moves. It prints what the strategy does: an entry signal opens a paper position at the quoted prices, an exit signal closes it at the bids and records the profit, and otherwise it reports the position being held, at most once a minute per market. Paper positions are saved to state/positions.json, so they carry over after a restart, and every completed trade is recorded in state/trades.csv.

No orders are sent. To trade for real, place the two legs with Synpath. Two exchanges can't fill one atomic trade, so one leg can fill while the other doesn't: the usual answer is to rest a limit order on the thinner side and, the moment it fills, take the other side with a fill-or-kill order, with an unwind ready if the hedge misses.

python -m backtest.backtest --days 60                     # 1-minute bars
python -m backtest.backtest --days 60 --timeframe 1h      # hourly bars
python -m backtest.backtest --until "2026-09-27 11:00"    # a fixed window, to reproduce a run
python -m backtest.charts
python -m backtest.charts --market kalshi:<TICKER> --start YYYY-MM-DD --end YYYY-MM-DD

The backtest runs the same strategy code on 1-minute price history (hourly or 5-minute with --timeframe) and reports the trades and P&L for each market. The charts show, for each market:

• Prices: both platforms' prices, with the spread shaded and every entry and exit marked
• P&L: cumulative P&L, with the profit of each trade

Options: --by-close counts trades by the date they closed, --best shows each market's best run of consecutive trades, and --generic-names labels markets as Market A, B, C for sharing.

prediction-market-arbitrage-trading-bot/
├── config.py            # Markets, strategy parameters, position size
├── trading/
│   ├── __main__.py      # Entry point: python -m trading
│   ├── bot.py           # Live strategy loop, paper positions and trades
│   └── feed.py          # Live order books over the venues' WebSockets
├── src/
│   ├── strategy.py      # Entry and exit rules
│   ├── arbitrage.py     # Arbitrage pricing, fees, profit calculation
│   ├── matcher.py       # Cross-platform market matching and rule check
│   └── discover.py      # Finds markets listed on both platforms
├── backtest/
│   ├── data.py          # Historical 1-minute (or hourly) prices
│   ├── backtest.py      # Strategy simulation
│   └── charts.py        # Entry/exit and P&L charts
└── tests/               # Offline unit tests

This bot is built on Synpath, one API for every prediction market. It uses Synpath's cross-venue market matching, live order books, fee schedules and price history.

Synpath does much more than this bot needs:

Get an API key at synpath.dev. The Synpath SDK is open source (MIT) at github.com/Synpath-ai/synpath.

Read this before using this project.

• Not financial advice. Nothing in this repository is investment, financial, legal or tax advice, or a recommendation to buy or sell anything.
• Educational and research use only. The code is provided to demonstrate a strategy, not as a trading product.
• No trading. The bot only paper trades: it records simulated positions at quoted prices and never places real orders.
• No guarantee of profit. "Locked-in" profit depends on both platforms settling the market the same way. Rules can differ, markets can be disputed or voided, and you can lose money.
• Not affiliated. This project is not affiliated with, endorsed by, or sponsored by Kalshi or Polymarket.
• Use at your own risk. The software is provided "as is", without warranty of any kind (see the MIT license). The authors are not liable for any loss arising from its use.

MIT