Prediction Market Odds: Kalshi, Polymarket and PredictIt

The same real-world question is often listed on several prediction venues at once, and they frequently price the YES outcome differently. This actor pulls live binary Yes/No markets from Kalshi (the CFTC-regulated US event-contract exchange), Polymarket (the crypto order book) and PredictIt (the US real-money political market), matches equivalent questions across venues, and reports the cross-venue YES-price gap so you can see at a glance where the same bet is cheaper.

• Keyless. No account, no API key, no browser. Straight from each venue's public API.
• Binary only. Only Yes/No markets are compared, so the YES probability is directly comparable.
• Tradeability check. Every gap is compared against the venues' own bid/ask, so you can tell a real edge from a wide spread.

With three venues a single question can appear as up to three pairs (Kalshi vs Polymarket, Kalshi vs PredictIt, Polymarket vs PredictIt), which lets you see every leg of the same market side by side.

The gap that is not a gap

A mid-price difference between two venues is only worth acting on if it survives crossing both order books. Thin contracts routinely quote bid/ask tens of points wide, and a mid-to-mid "gap" inside that width is not tradeable at any size.

Every row therefore carries each venue's own quoted width and a verdict:

Field	Meaning
widestBookWidthPct	The wider of the two venues' YES bid/ask spreads, in points
gapExceedsBookWidth	true = the gap clears both books, false = it sits inside the spread, null = a venue did not quote both sides

In a representative run, 17 of 25 matched pairs had gaps sitting inside the quoted bid/ask. Filter on gapExceedsBookWidth: true to keep only the pairs where the edge is real.

What you get

One row per matched cross-venue pair:

Field	Meaning
question	The matched question
venueA / venueB	Which two venues this row compares
venueAYesPct / venueBYesPct	Implied YES probability on each side of this pair (0-100)
spreadPct	Absolute gap between the two YES prices, the headline number
cheaperYesVenue	Which venue prices YES lower
gapExceedsBookWidth	Whether the gap clears both venues' bid/ask (see above)
buySignal	Plain-language read, including when the gap is not tradeable
matchScore	Match confidence (0-1) between the two questions
kalshiYesPct / polymarketYesPct / predictitYesPct	Per-venue YES price. The venue not in this pair reads null.
kalshiBookWidthPct / polymarketBookWidthPct / predictitBookWidthPct	Each venue's own quoted bid/ask width
kalshiVolume / polymarketVolume24h / predictitVolume	Liquidity. predictitVolume is always null: PredictIt publishes no volume field, and null means not reported rather than zero traded.
predictitContract	Which contract inside the PredictIt market this leg is
predictitBestBuyYesCost / predictitBestSellYesCost	The raw YES ask and bid, null when that side of the book is empty
kalshiCloseTime / polymarketEndDate / predictitDateEnd	Resolution timing. PredictIt leaves this unset on most contracts, so it reads null.
kalshiUrl / polymarketUrl / predictitUrl	Links to the underlying markets

Column blocks for all three venues are present on every row, so the shape stays stable no matter which pair produced it.

Input

Field	Description
venues	Which venues to compare. Pick at least two; a shorter selection falls back to all three.
searchQueries	Topics to compare (e.g. president, minister, trump, fed). Empty = most-traded markets on every selected venue.
minSpreadPct	Only return pairs whose YES prices differ by at least this many points.
minMatchScore	How strict the cross-venue match must be (0-1, default 0.5). Raise for precision, lower for recall.
minKalshiVolume	Skip thin Kalshi markets (lifetime contracts).
minPolymarketVolume24h	Skip thin Polymarket markets (24h USD).
poolPerVenue	Top-by-volume markets scanned per venue before matching (default 600).
maxPairs	Cap on pairs returned.

Volume filters do not apply to PredictIt, which reports no volume.

How matching works

Each question is reduced to a token set (stopwords dropped, common synonyms and month names normalised, $150,000 becomes 150000). A pair is scored by the overlap coefficient (shared tokens divided by the smaller set) and only kept when it shares at least one strong token (a number or a word of 4+ letters), so pairs never form on filler words alone.

The venues structure the same race differently, and the matcher is built for that. Kalshi lists one binary market per candidate ("Will Marine Le Pen win the 2027 French presidential election?"); PredictIt nests a contract per candidate inside one market ("Who will win the next French presidential election?" plus "Marine Le Pen"). Both are expanded into individual Yes/No legs before matching, so the candidate name carries the match even when the wording differs.

Matches are assigned greedily, best score first, one market per side per venue combination, so the Kalshi leg of a question can still pair with both Polymarket and PredictIt. Use matchScore to filter and eyeball the per-venue titles before trading.

Pricing

Pay per event: $0.005 per matched pair pushed. The first 2 rows of every run are free, and a run that finds no matches emits a single free note row. Nothing else is charged.

Notes & limits

• PredictIt is US politics only and is a small market with per-contract position limits, so a gap against it may not be fillable at size even when gapExceedsBookWidth is true.
• PredictIt contracts are independent Yes/No bets. The Yes prices inside one PredictIt market do not sum to 1, because the contracts are not mutually exclusive. Each is treated as its own binary question, which is what makes it comparable to Kalshi and Polymarket at all.
• Works best on named-entity event markets (elections, geopolitics, courts, awards) where the venues list the identical yes/no question. Crypto price levels are structured differently on each venue (Kalshi uses price ranges, Polymarket uses thresholds) and rarely match cleanly.
• Prices are live snapshots at scrape time; all three venues move continuously.
• A high spreadPct can also mean the two questions differ in a detail (date, threshold) the matcher missed. Confirm the titles and resolution terms before acting.
• Fees, slippage, and settlement rules differ between the venues; the gap is a signal, not guaranteed profit.