Polymarket and Kalshi Arbitrage Tools (2026)

Polymarket and Kalshi are the pair almost everyone starts with, and for good reason: they list the same events, both have real depth, and the price difference between them is the most visible arbitrage in the category. This guide covers the tools that watch both venues at once, what each one costs, and the two things about this specific pair that decide whether a gap is real.

Every figure below was checked against each tool's own site, documentation or repository in August 2026. Where a free tier excludes the arbitrage feature, that is stated rather than left for you to discover after signing up.

Tools That Watch Both Venues

1. Predictefy: Both Venues, Plus the Ones Nobody Watches

Predictefy's arbitrage aggregator is free and covers more than fifteen venues, Polymarket and Kalshi among them. On a page about this specific pair, the wider coverage is the point rather than a distraction.

Here is why. Polymarket against Kalshi is the first pair every new arbitrage trader watches, which means it is also the most competitively priced. Gaps there close fastest and run thinnest. The same event listed on Opinion, Limitless, Predict.fun or Myriad often carries a materially wider spread simply because fewer people are quoting it. A tool that watches only the crowded pair will show you the hardest version of the trade.

Two mechanics matter more here than raw coverage. Predictefy maps differently worded contracts to the same underlying event, then checks deadlines and resolution rules before treating two markets as equivalent. And prices come from the best executable bid and ask on each venue's live order book rather than the last traded price, which on a thin market can be hours stale and will show you a spread that closed long ago.

A Pro subscription unlocks more advanced arbitrage. The aggregator itself, which is what this page is about, costs nothing.

2. Prediction Hunt: Free, and Wider Than the Pair

Prediction Hunt runs an arbitrage scanner in the browser with no login wall, covering seven platforms per its own documentation: Polymarket, Polymarket US, Kalshi, PredictIt, ProphetX, Opinion and Predict.fun.

Worth noting that it treats Polymarket and Polymarket US as separate venues, which is correct. They are distinct exchanges with separate order books and different fee schedules, and a price on one tells you little about the other. Any tool that collapses them into a single entry is hiding a real trading distinction from you.

The browser scanner is free. A separate API has a free tier of a thousand requests a month with paid plans from forty-nine dollars, which you only need if you are building.

3. PolyRouter: One API Key Across Both

PolyRouter puts Kalshi, Polymarket, Manifold, Limitless, SX Bet, Novig and ProphetX behind a single API key with one normalised schema, free while it is in open beta and rate limited to a hundred requests a minute.

The caveat that decides its usefulness: seven venues are documented but full order book data is available on four. Arbitrage needs the book rather than the headline price, because a five percent gap with forty dollars of depth behind it is not a five percent opportunity. It is also not a scanner. It is the plumbing you would build one on.

4. Adjacent and Eventarb

Adjacent documents Kalshi and Polymarket, with a free tier giving a public REST API and an MCP server, and Pro at fifty dollars a month. Read the free tier carefully: the data is delayed fifteen minutes. For indices, research or letting an AI agent query prediction market data that is fine. For live arbitrage it is useless, because any gap you can see on a fifteen minute delay closed before you saw it.

Eventarb is a free calculator covering both venues. You bring the two prices, it tells you whether the pair clears a dollar once costs are counted. Not a scanner, but genuinely useful while you are learning to judge whether a spread survives fees.

The Fee Difference That Decides the Trade

Polymarket and Kalshi do not charge the same, and on a thin gap that difference is the whole trade.

Both use the same shape: a coefficient multiplied by contracts, price, and one minus price. That curve means a contract at fifty cents costs the most to trade and both tails cost almost nothing. Kalshi's schedule effective 7 July 2026 uses a coefficient of 0.07, and Polymarket US's effective 1 July 2026 uses 0.06. Polymarket's global platform runs from zero to 0.07 depending on category, so some categories are free and others are not.

The practical consequence: a gross gap that looks tradeable can be net negative once you subtract the Kalshi side. Most scanners display gross spreads. Check whether yours subtracts fees before you treat the number as profit, and if it does not, do that arithmetic yourself every time.

The Risk That Actually Costs People Money

The expensive mistake on this pair is not fees. It is two markets that read the same and settle differently.

Polymarket and Kalshi write their own resolution criteria, and on the same real-world event those can diverge in ways that only matter once. One market may require a specific source to call a result while the other accepts a different one. One may resolve on a date the other treats as provisional. Two markets that sound identical but settle on different criteria are not an arbitrage at all. They are two independent bets that happen to look related, and if they disagree you lose on both legs rather than netting out.

So before you size anything: open both resolution descriptions and read them side by side. Ask any tool you are considering how it decides two markets are equivalent. If the answer amounts to matching titles, be careful.

How to Check a Gap by Hand

Whatever tool surfaces it, the check is the same four steps.

One. Confirm both markets resolve on identical criteria, in the same wording, on the same date.

Two. Take the executable prices, meaning the best bid and ask on the live book, not the last trade.

Three. Add both sides. Buying yes on one venue and no on the other should cost less than a dollar combined for a gap to exist at all.

Four. Subtract both venues' fees, then look at the depth behind each quote. A gap you cannot fill at size is a gap you cannot trade.

Conclusion

Polymarket against Kalshi is the right pair to learn on and the wrong pair to stop at. It is the most watched matchup in prediction markets, which makes the gaps easiest to find and hardest to profit from. Start with a free scanner covering both, learn to read resolution criteria and fee maths properly, then widen your coverage once you can tell a real gap from an apparent one. Our guide to how prediction market arbitrage works covers the mechanics, and Polymarket vs Kalshi compares the two venues themselves.