u/YukiChuuu

A few days ago, I posted here about the arbitrage spreads I found between Polymarket and Kalshi. A lot of the discussion came down to execution — finding an edge is one thing, actually getting a good fill is another. That got me looking at a different execution problem: if the same contract trades on both venues at different prices, why send the entire order to just one?

Say you want to buy 1000 contracts:

• Polymarket: 200 @ 51¢, 400 @ 53¢, 400 @ 55¢
• Kalshi: 300 @ 52¢, 500 @ 52.5¢, 200 @ 54¢

Neither venue has the best price for the full 1000. But if you merge both order books, you can take the cheapest liquidity from each. So I built a smart order router. It combines both books, adjusts each level for fees, and routes the order from the cheapest executable price outward across venues.

I tested it across ~200 markets listed on both Kalshi and Polymarket. For a 1000-contract order, routing across both venues saved an average of 2.3¢ per contract versus sending the full order to Kalshi alone.

The router is fully open source here: https://github.com/Synpath-ai/synpath

Anyone else executing across venues? What’s your experience with fill rates and partial fills?

Been scanning Polymarket vs Kalshi and there are still consistent arb opportunities sitting in plain sight. I’m regularly seeing the same events priced differently across platforms, with spreads of around 4–6 cents after fees and many of them lasting ~1 hour.

The inefficiency comes down to fragmentation — most traders only stick to one platform, so the gaps just sit there. Thinner liquidity on certain events makes it even better, though position limits can be restrictive, and running accounts across multiple platforms means dealing with KYC and funding friction on each one.

I built synpath to aggregate real-time data and match same markets across platforms for exactly this. It's open-source if anyone wants to run their own scans: https://github.com/Synpath-ai/synpath

Anyone else trading prediction market arb? What kind of slippage and fill rates are you seeing on smaller events?