Matched Betting Risks with Polymarket and Kalshi Props

Polymarket and Kalshi do not always settle player props the way most sportsbooks do. When a player does not play, most books void the bet and return your stake. Polymarket and Kalshi each do something different, and if one of them is part of your matched bet, that difference can leave you at risk of losing money.

This guide explains how each one settles a player who does not play, and how it impacts matched betting. If you are not sure what house rules are, start with our House Rules guide.

Note: House rules change, and this is only an example. The rules described here may change. Confirm the current settlement rules on both platforms before placing any trade.

Why Kalshi and Polymarket are Different

When you hedge a player prop across two platforms, you are counting on both legs settling the same way. The standard behavior across most sportsbooks is simple: if the player does not play, the bet is voided, and your stake is returned. Both legs void, no harm done. For MLB and soccer, many sportsbooks void if a player is subbed in after the game starts, but not all of them. Learn more about these risks in our MLB and Soccer guides.

Polymarket and Kalshi break that assumption. Neither voids a player who does not start. Each resolves it in its own way, and both of those ways are unusual enough that you need to treat them as special cases when they appear in a matched bet.

Polymarket: Grades the bet. If the player is inactive or does not play, the market resolves No (or Under). It does not void. Grading a non-participant rather than canceling the bet is not common practice and something you need to be aware of about Polymarket.

Kalshi: Settles to "fair market price". If the player does not play, Kalshi settles the contract to the last traded price before the game starts, which is the market's "fair price" at that point. It does not void and does not grade a flat No/Under. When wagering on Kalshi, you need to be aware of this to avoid risk.

In both cases, if the player is subbed in and does play, the market grades on actual performance, the same as a normal prop.

Where the Risk Comes From

The danger is not Polymarket or Kalshi on their own. It is pairing one of them against a book that voids.

Matched betting relies on a pair of bets where one is graded as a win and the other a loss. Polymarket grades a bet if a player never plays and many sportsbook will void it.  If your matched bet is Polymarket vs a sportsbook, and the player does not play, you are at risk of losing money. Polymarket grades your bet as a loss and the sportsbook voids your hedge bet.

With Kalshi. Same setup, but Kalshi settles to the last traded price (odds) instead of grading. Your risk is the gap between your odds and the closing odds . If the odds improved, you would come out slightly ahead; if the odds got worse, you lose the difference. Either way, you are no longer hedged, and the outcome is set by where the price landed rather than by your bet.

Polymarket Example

You place an arb on a baseball player "to record a hit".

• Primary Bet: On a sportsbook (voids if the player does not start): $100 on No Hit at -200.
• Hedge: On Polymarket, $47 on Yes (player records a hit) at +220.

If both legs grade, whichever side hits returns ~$150, and you lock in a $3 profit.

Now, let's look at what could happen if the player is not in the starting lineup and does not play.

• The sportsbook voids the No Hit leg. Your $100 is returned.
• Polymarket does not void. It grades your bet as a loss, because the player did not play. That leg loses. You are out the full $47.

The matched bet designed to profit, became a $47 loss, entirely due to the settlement mismatch. If Leg 2 had been on Kalshi instead, your loss would be the gap between your entry and the last traded price rather than the full $47, but you would still be holding unwanted exposure.

Kalshi Example

You place an arb on a baseball player "to record a hit".

• Primary Bet: On a sportsbook (voids if the player does not start): $100 on No Hit at -200.
• Hedge: On Kalshi, $47 on Yes (player records a hit) at +220.

If both legs grade, whichever side hits returns ~$150 and you lock in a $3 profit.

Now, let's look at what could happen if the player is not in the starting lineup and does not play.

• The sportsbook voids the No Hit leg. Your $100 is returned.
• Kalshi does not void. It settles your Yes contract to the last traded price before the game starts, and if that price closed slightly worse than your entry. That leg settles at a small loss instead of returning your stake. You are left with a small loss and no hedge to cover it.

The Subbed-In Wrinkle

If the player is subbed in and plays, both Polymarket and Kalshi grade on performance. In sports with limited substitutions, such as soccer and baseball, some books void bets on a player who does not start even if they later enter, while others let the bet stand and grade it. So a sub-in does not always grade on the opposing side. Whether you are exposed depends on that sportsbook's house rules.

Always Double Check

Our Bet Finders include a warning when a player prop from Kalshi or Polymarket appears in your Bet Finder results.

Use the warning as a prompt to check, not as a guarantee. Review both platforms' rules for the specific market and confirm how each handles it, keeping in mind that house rules may change over time.