Prices are the venues' quoted probabilities for the chosen contract
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What the venues say
That is not two markets disagreeing; it is two markets saying the same thing with the precision of a coin. The interesting number is not either probability but the distance between them, because that distance is what an arbitrage bot would be paid to close, and it has been inside the fee floor for most of the month.
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Why the gap does not pay
Polymarket charges takers C × feeRate × p × (1 − p) with a 0.04 rate on politics and economics markets at the time of writing, and makers nothing; Kalshi charges takers 0.07 × C × p × (1 − p). At 50 cents that is 1.0 cent per contract on Polymarket and 1.75 cents on Kalshi: a taker on both legs pays about 2.75 cents per dollar before slippage. The largest gap in the last thirty days, , was inside that floor. In July, Vultax watched the same venues on matched Fed-rate contracts every 85 seconds for a day and found the exact-match pair held a 5.7-cent gap that never closed; the September contracts are better matched and, as a result, tighter.
The month's biggest moves
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Who is trading it, and how big they bet
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The biggest bets of the day
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What the bond market says
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What July looked like from here
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What to watch until the 16th
- CPI, 10 September. The last macro print before the meeting; the largest hourly moves this month all landed on data releases, and this is the biggest one left.
- The gap, not the level. If Polymarket and Kalshi separate by more than three cents on the same contract, one of them is being moved by a large order; the level alone tells you nothing.
- The cut contract. At under one cent it is the cheapest lottery ticket on either venue; a move above two cents would mean someone knows something the venues do not. It is also where the biggest single fills have been landing, as sales of No.
- New wallets. A quarter of the wallets on these contracts had never appeared in our August window; if that share climbs into the decision, the crowd is arriving, and crowds move prices late.
- Post, do not take. On a coin flip the taker fee is the whole edge; a resting order on Polymarket pays no fee at all.
Method and limits
Polymarket prices are the CLOB price history for each contract's Yes token at hourly fidelity and the event's quoted probabilities; Kalshi prices, volume and open interest are hourly candles from its public trade API. A refresher re-reads both and Vultax's copy of public on-chain fills every ten minutes; if a source fails, the previous reading is kept and the page says so. Fill counts undercount fills not yet ingested at read time; "new wallets" means wallets not seen in Vultax's 22 August to 2 September activity window. Fee rates are as published by each venue on 5 September 2026. Nothing here is a forecast of the decision, and nothing here is financial advice.