Polymarket's regulated US exchange has begun beta testing combo trades that bundle several outcomes into one position. The first one settled on 5 August, about 10 weeks after the company self-certified the product with the CFTC in May as 'Combinatorial Athletic Outcome Contracts' (CAOCs).
Mechanics
Pricing runs through request-for-quote rather than Polymarket's standard central limit order book. A trader asks to combine several outcomes into a single position, market makers quote within 400 milliseconds, and the trader has 5 seconds to take the best offer. A 'Last Look' window gives market makers one more second to confirm or refuse. A combo pays only if every leg lands the trader's way. One miss voids the whole ticket.
Because pricing leans on the visible book, users can usually take only the 'yes' side unless they trade through the API. Casual bettors end up on the wrong end of that price.
Volume and competition
In the days after 5 August, combo volume on Polymarket US reached $7.4 million, with $928,093 in taker bets across 16,173 trades, most of it since last Thursday. Over that weekend the volume beat what Kalshi managed in the first days after it opened parlays to its whole base.
Kalshi has grown parlay volume from $4.77 billion in May to $13.78 billion in July, with August tracking similar, and collected $25 million in taker fees from parlays in the first 16 days of August.
Polymarket has not decided how fees will be charged across the legs of one combo, or how settlement runs if one leg closes before the others. Fewer market makers than on single markets means thinner liquidity and wider spreads. The beta caps combos at 10 legs, the build option is not yet visible in the Polymarket US app, and the desktop site has not launched widely. Combo data sits behind a public API, with no authorisation tied to the order book.
Kalshi is preparing maker fees on parlays at roughly a quarter of taker fees, tied to odds. Separately, Polymarket is examining whether Kalshi is copying its model. An internal 12-point document called 'Copycat' lists similar products in sports, weather and the Iran conflict. Head of marketing Matthew Modabber confirmed the review, citing 'too many coincidences'.
