What a combo contract is, and who is on the other side

Most contracts on a prediction-market venue have a price because people are leaving orders on a book. A combo usually does not. It is a contract written on top of other contracts, the number of possible combinations is too large for anyone to keep a book in each, and so the price is made the other way round: you ask, somebody answers, and the answer is shown to you alone.

That changes three things a reader of single-leg markets takes for granted: where the price comes from, who is on the other side, and what "void" means.

How it works

One contract built from others

Polymarket US filed its combo product with the CFTC on 20 May 2026 as a Combinatorial Athletic Outcome Contract, self-certified under 17 C.F.R. 40.2(a) for listing no earlier than 21 May 2026; the CFTC's filings index lists it as filing 60854, certified. The underlying is "the joint Settlement Amount of two or more underlying Contracts", and the payout condition is joint: the contract resolves to $1.00 "if and only if every leg is satisfied", and to $0.00 if any single leg is not, "regardless of the outcomes of any remaining unsettled legs". The exchange names itself Source Agency for the combo and takes each leg's source from that leg's own terms.

Kalshi's version is written the same way. Its FOOTBALLSTATS terms define each leg as a component whose resolution is dictated by "the active Kalshi certification" for that kind of outcome, incorporated by reference, and say the market resolves to "No" immediately if any outcome becomes impossible or does not occur.

How many legs you may combine depends on which page you read. Polymarket US's combos FAQ says 2 to 10 markets; its API overview and trader guide say 2 to 50 legs. OG's help centre allows up to 20 legs, sports only, with futures such as championship winners not permitted; it lists selected live games, and same-game combos in selected leagues, as permitted.

Why there is no price on the screen

Every distinct set of legs is a distinct instrument. Polymarket US's API overview says a canonical set of legs always maps to the same symbol, that existing combos carry the caoc- prefix of the filed product, and that each participant may create up to 1,000 new combo instruments a week. Its rulebook requires a combination contract to be "listed and opened on the Trading System in the same manner as any other Contract" before anyone can ask for a quote in it.

So a book exists in principle. In practice the trader guide says that combo instruments trade through normal order entry, "but most combo price discovery starts with a request for quote (RFQ)". A combo somebody built for the first time a minute ago has no resting orders, and nothing to show you until somebody quotes.

OG's help article describes what that looks like from the app: when a price is unavailable, it "usually means there is currently not enough trading activity (liquidity) to offer a confirmed price for this combo, or the quoted price has expired."

Who answers: the request for quote

Kalshi and Polymarket US have filed nearly the same procedure, and it is worth reading in the order it runs. Kalshi's is Rule 5.3, as amended effective 14 February 2026; Polymarket US's is Rule 5.3(n) of the rulebook dated 30 September 2026.

1. You ask, in public. The request names the contract, the size in contracts or in total cost, and a pseudonymous ID that stays the same for you across requests (Kalshi lets you hide it until execution). Every other member can see it. It does not say whether you want to buy or sell.
2. Quoters answer, in private. A quote is two-sided, for the full size, and visible only to you (and, on Polymarket US, the exchange). A quoter may only quote what it has the collateral for and what would not take it past a position limit.
3. You may accept only the best price. Kalshi's rule says so in terms; Polymarket US's guide ranks quotes by price, then by time.
4. The quoter gets a last look. After you accept, the quoter must confirm, and if it does not, the acceptance is void. Both rulebooks set 30 seconds by default and allow one second in markets the exchange designates. Polymarket US's guide gives the current configuration as a 200 millisecond target for quotes, a 3-second last look and a 1-second delay before the paired orders are submitted.
5. The trade happens on the book. Both orders are entered with time priority behind everything already resting, and if a resting order offers a better price, it fills first.

The party on the other side of your combo is therefore not the venue and not the crowd. It is one liquidity provider who chose to answer, and chose again to confirm. Fanatics Markets states the same chain in a notice repeated across its legal pages: "Upon submission of a Combo order, the order is transmitted to the exchange, which transmits a RFQ to liquidity providers who may respond with executable price quotations." Its important risk disclosures add that an affiliate under common control, Morton St. Market Maker, LLC, participates on one or more designated contract markets and may take part in an exchange's liquidity provider programme. The document does not say whether that affiliate quotes Fanatics Markets' own combos.

What happens when a leg cannot resolve

This is where single-leg intuition fails hardest. On a single contract, a cancelled event is handled by that contract's own rule. In a combo, the venues have published three different answers, and none of them removes the leg.

Polymarket US multiplies by the leg's last fair market price. Its combos FAQ says a leg that cannot resolve, because a match is walked over or cancelled, a listed player does not take part, or an event is postponed and not replayed, "is not removed from your combo. It is assigned a fair price, and your payout is multiplied by that price." The price is the prevailing fair market price at the moment of the official announcement, set by the Settlement Committee, whose decisions are final. Its own example: a $10 combo quoted to pay $80, two legs win and the third goes to a last fair market price of $0.60, pays $48. A leg at a fair price "never rescues a combo that already has a losing leg".

OG multiplies by the leg's VWAP price. Its help centre says a cancelled leg, or one that ends in a tie, leaves the combo active unless another leg loses, and at final settlement "the payout is calculated by multiplying the settlement prices of all legs together — including the VWAP price of the canceled leg". A tie is not necessarily half: the article notes that a four-way tie settles at $0.25 a leg.

Kalshi settles the whole contract at a fair price. The FOOTBALLSTATS terms in Kalshi's contract-terms store, read on 23 October 2026, say "Yes" pays "the product of the payouts for each component", rounded down to the cent. Kalshi's amendment of 5 October 2026 replaces that for the case that matters here: if any component resolves to a value other than zero or one, "including where a component settles at a fair price, or is voided", the contract settles at a Fair Price. The exchange sets it by reference to the combo's traded price before the problem became known, quotes, related contracts and "the Exchange's assessment of the reasonable probability of the outcomes", and it "may, but need not, equal the product" of the legs' values. Only if the exchange determines nothing does the product apply, rounded to a hundredth of a cent. Determinations "are final and not subject to review". The same filing cuts the minimum tick from $0.001 to $0.0001.

Read together: on none of these venues does a void leg turn a four-leg combo into a three-leg one.

What it costs

A separate, steeper taker curve on Polymarket US. Under the fee schedule effective 19 October 2026, the taker side of a combo pays

Fee = C × p × [0.0695 × (1 − p) + 0.06 × (1 − p)^4]

on the combo's own execution price p, not on each leg. The schedule's table puts 100 combo contracts at 10 cents at $1.02, and at 50 cents at $1.92. The standard single-market formula, 0.0695 × C × p × (1 − p), gives about 63 cents and $1.74 for the same two orders. In relative terms the gap widens as the price falls: about 10% more at 50 cents, about 60% more at 10 cents. Combo fills carry no maker rebate.

On Kalshi, the quoter pays maker and the requester pays taker. A fee filing of 12 July 2026, effective no earlier than the close of business on 24 July 2026, adjusts fees in the KXMVE series, the prefix on the multi-leg markets listed in Kalshi's own featured-combos data file: for trades not involving an order that has rested on the book for more than 5 seconds, the trader executing against a quoter pays the equivalent of a taker fee and the quoter the equivalent of a maker fee. Normal fees are charged first and adjusted within 5 seconds of execution.

Part of the fee goes to the people pricing the legs. Kalshi's sports-prop combo programme, effective no earlier than 16 September 2026 and running until 1 January 2027 at the latest, distributes up to 25% of the fees collected on each sports-prop combo market to members who made markets in its prop legs (a player's points, say, rather than the result, spread or total), scaled down when some legs are not props and shared by a score built from each maker's share of volume in every leg, which pays more for quoting more of them. The filing gives the reason: liquidity in the legs is what provides "more valuable information for the purposes of pricing" the combo.

The spread is invisible. On a single market you can see the bid and the ask before you trade. On a combo the quote is the price and there is no second quote on the screen to compare it with.

Size is capped by accountability levels, not by the payout. Polymarket US's filed terms set a position accountability level of $25,000 of notional exposure, no position limits, and margin at 100% of the amount at risk; Kalshi's FOOTBALLSTATS terms set $25,000 per strike, per member. What those words mean is on how much one account may hold.

What you can do about it

Price the legs before you ask. Each leg trades on its own book. The product of the legs' prices is the price a combo would carry if the legs were independent, and the gap between that product and the quote you receive is what you are paying the quoter, before the fee. It is a reference, not a fair value: legs from the same game move together, and the product ignores that. Stream an order book shows how to read a book from code.

Read the void rule for every leg, before you build the combo. A combo inherits each leg's settlement source, and then adds the venue's own rule for what an unresolvable leg is worth. What a sports contract settles on covers the first half; the section above covers the second. A walkover, a postponed match and a player who does not take part are all covered, and on none of these venues do they give your stake back.

Check the fee at the price you will actually pay. A combo at 10 cents is a different fee bracket from a single leg at 60. On Polymarket US the curve is published; on Kalshi the adjustment depends on whether a resting order was involved. The fee calculator covers the single-market curves.

Assume a quote can disappear. A last look lets the quoter decline after you accept. A quote also expires, and closing a combo early needs another quote; OG's help centre says early closing is "subject to price availability".

Count your legs against the venue's own limit, and if two of its pages disagree, as Polymarket US's do, expect the trading system to enforce one of them without telling you which.

The venues that list combos are in the venues category; Polymarket US and Kalshi carry the fee detail on their cards.