Most prediction market arbitrage examples use a single yes-or-no question. Many Polymarket events are not like that: "Who will win?" has several outcomes, and only one can win. Polymarket handles these with negative risk markets. This article explains the mechanism as documented, and what to watch for when you pair one of these outcomes with a Kalshi contract. It is not trading advice, and the details can change, so check the current documentation.
The problem negative risk solves
Polymarket's documentation notes that in a standard multi-outcome event each market is independent. If you want to bet against one outcome you buy that outcome's No, but that No has no relationship to the other outcomes. In a negative risk event, there is a link.
Conversion
In a negative risk event, a No share in any market can be converted into one Yes share in every other market. The documentation's example is an event with three outcomes (Trump, Harris, Other). One No on "Other" can be converted into one Yes on Trump and one Yes on Harris. The point it makes is that betting against one outcome is economically equivalent to betting for all the others, and conversion makes that equivalence real, which is capital-efficient. Conversion is atomic: it either completes or it does not. Neg risk markets use different contracts from standard markets.
This is related to, but not the same as, the split and merge of a single market described in complete-set arbitrage. Conversion links outcomes across an event; split and merge link the two sides of one market.
Augmented negative risk
Standard negative risk needs the full list of outcomes at creation. For events where a new outcome can appear later, such as a new candidate entering a race, Polymarket documents augmented negative risk, with three kinds of outcome:
- Named outcomes: known outcomes.
- Additional outcomes: placeholders that can become available after the event starts.
- Explicit Other: catches any outcome not explicitly named.
The documentation's trading rules are plain: trade only named outcomes, ignore placeholders until they are named or the event resolves, and avoid trading Other directly because its definition changes as placeholders are clarified. If the correct outcome at resolution is not named, the market resolves to Other.
What it means when you pair with Kalshi
- One outcome, one pair. Each outcome is a separate market. The Kalshi contract you match it with must cover exactly the same outcome under the same rules. See resolution mismatch.
- "Other" is the riskiest leg. Its meaning can shift, so a pair built on it is the hardest to check.
- Depth varies by outcome. Each outcome has its own book, and the thinner leg sets your size, as in the liquidity guide.
- Count fees and the other costs. See Polymarket fees and net edge after fees.
Where Arbitrage Agent fits
Arbitrage Agent uses AI matching with every pair validated, including a check of the resolution rules on both sides, and shows the result in a live feed with a step-by-step guide for both legs. You place the orders yourself, and the final check of the rules is still yours. See the plans or start with the Starter plan.
Related articles
- Complete-Set Arbitrage on Polymarket vs Cross-Venue Arbitrage
- Polymarket UMA Disputes and Resolution Mismatch
- Event Contracts Explained
- How to Arbitrage Polymarket vs Kalshi
Key terms: Binary Event Market · Resolution Risk · Complement Arbitrage