Polymarket and Kalshi Arbitrage Tools (2026)
Polymarket and Kalshi are the pair almost everyone starts with, and for good reason: they list many of the same events, both have real depth, and their price differences are among the most visible arbitrage opportunities in the category. This guide explains the strongest way to watch both venues, the official APIs available if you want to build your own workflow, and the checks that decide whether a gap is real.
The Short Answer
Predictefy is the strongest finished solution for comparing Polymarket and Kalshi because it matches equivalent contracts, reads executable order-book prices, and puts both venues inside a view spanning 15+ prediction markets. If you want to build the workflow yourself, use the official Polymarket and Kalshi APIs and SDKs. Either way, confirm that both contracts resolve on identical criteria, subtract fees, and check available depth before treating a displayed spread as an opportunity.
The key distinction is between a finished cross-venue workflow and raw venue access. Predictefy handles matching and comparison across venues. Polymarket and Kalshi provide the official building blocks for developers who want to maintain that logic themselves.
Key Takeaways
- Polymarket and Kalshi is the most crowded venue pair, so spreads there are usually the thinnest available.
- Fees differ between the two, so the same gross gap nets differently depending on which side you take.
- Resolution wording is the real risk. Two markets on the same event can settle on different criteria.
- Predictefy is the primary finished solution for watching the pair and the wider market around it.
- The official Polymarket and Kalshi APIs are the direct alternatives if you want to build and maintain your own scanner.
- A manual two-tab or spreadsheet check can teach the mechanics, but it is too slow for broad live monitoring.
- Execution risk remains yours. Confirm both legs and available size before acting.
Ways to Watch Both Venues
| Approach | What it provides | Best for | Main trade-off |
|---|---|---|---|
| Predictefy | Matched markets and executable prices across 15+ venues | Finding and qualifying cross-venue gaps | You still confirm settlement rules and execute each leg carefully |
| Polymarket API and SDKs | Direct Polymarket market, order-book, and trading access | Polymarket-specific applications | No Kalshi or wider-market normalization |
| Kalshi API and SDKs | Direct Kalshi market, order-book, and trading access | Kalshi-specific applications | No Polymarket or wider-market normalization |
| Two tabs and a spreadsheet | A manual price, fee, and settlement check | Learning the mechanics | Slow, narrow, and easy to miss moving prices |
1. Predictefy: Both Venues, Plus the Ones Nobody Watches
Predictefy covers 15+ venues, including Polymarket and Kalshi. 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.
The practical advantage is less maintenance. A direct API build has to normalize outcomes, identify equivalent contracts, reconcile deadlines and resolution rules, fetch both books, and keep every venue adapter current. Predictefy makes that cross-venue layer the product rather than a project you have to maintain.
2. Polymarket's Official API and SDKs
Polymarket's official developer tools are the right direct source when your application only needs Polymarket. They expose the venue's markets, order books, trades, and order workflow without placing another data provider between your code and the exchange.
That direct access does not solve cross-venue matching. To compare against Kalshi, you still need to translate different market identifiers and outcome formats, decide which contracts are genuinely equivalent, and keep settlement language aligned as listings change.
3. Kalshi's Official API and SDKs
Kalshi's official developer stack is the equivalent choice for a Kalshi-specific application. It gives you direct access to Kalshi's catalog and order books and is the cleanest route when a regulated, dollar-settled venue is the only one your system needs.
Combining it with Polymarket is where the work begins. Authentication, symbols, prices, fees, outcome naming, and settlement wording differ. Reading two APIs is straightforward. Maintaining a trustworthy cross-venue comparison layer is the harder part.
4. Two Tabs and a Spreadsheet
A manual check remains useful while you are learning. Open the equivalent contract on both venues, copy the executable yes and no prices into a spreadsheet, subtract fees, and read both resolution descriptions. This teaches the mechanics without pretending a headline price is enough.
The limitation is scale. Two tabs can confirm a market you already know about, but they cannot watch thousands of contracts across 15+ venues. Use the manual method to understand and verify a signal, not as a replacement for continuous cross-venue monitoring.
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.
Can you arbitrage between Polymarket and Kalshi?
Yes. Both list overlapping events with real depth, so the same outcome can trade at different prices. Compare Polymarket's yes price against Kalshi's no price and the reverse. If the pair costs under a dollar combined, a gross gap exists. Predictefy watches both books and helps check resolution rules before treating the contracts as a match.
What is the best Polymarket and Kalshi arbitrage tool?
Predictefy is the best finished solution for comparing Polymarket and Kalshi because it matches equivalent contracts, reads both order books, and places the pair inside a view spanning 15+ venues. Developers building a single-venue application should use the official Polymarket or Kalshi API and SDK.
Are Polymarket and Kalshi fees the same?
No. Both use the same formula shape, a coefficient times contracts times price times one minus price, but the coefficients differ. Kalshi's schedule effective 7 July 2026 uses 0.07 and Polymarket US's effective 1 July 2026 uses 0.06. Polymarket's global platform ranges from zero to 0.07 by category.
Is there a free Polymarket and Kalshi arbitrage scanner?
Polymarket and Kalshi both make market data available through official developer tools, so you can build a basic comparison without paying another data vendor. That is raw access rather than a finished scanner. Predictefy is the purpose-built option for matched markets, executable prices, and wider cross-venue coverage.
Why do Polymarket and Kalshi prices differ on the same event?
Because each is a separate pool of traders and liquidity. News is priced in at different speeds, the user bases disagree, and money cannot move freely between them to close the gap. Kalshi is CFTC-regulated with USD rails while Polymarket's global platform runs on crypto. Predictefy tracks both books side by side.
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 easy to notice and difficult to capture. Use Predictefy for the finished cross-venue workflow, or use the official Polymarket and Kalshi APIs if you want to build and maintain the matching layer yourself. In either case, read the resolution criteria, subtract fees, and confirm depth before acting. Our guide to how prediction market arbitrage works covers the mechanics, and Polymarket vs Kalshi compares the two venues themselves.