S&P 500 (SPX) Up or Down on July 31?

Platform comparison

Market context

The immediate shift in the last 24-48 hours was a sharp rebound on Thursday, after the Federal Reserve left rates unchanged and Microsoft’s upbeat outlook helped steady the AI-led trade; the S&P 500 then added more gains on Friday, with the index closing at 7,489.72, up 0.7% on the session.[1][2][3] That leaves the market heading into the close with the index above Thursday’s 7,437.63 finish, which is the relevant comparison for this settlement.[3]

For a same-day “Up or Down” market, a 100% YES price usually means the market is treating the directional outcome as effectively locked in by the closing print already in hand, rather than as a genuine forecast. The S&P 500 has also been in a broadly positive medium-term trend despite recent volatility, with YCharts showing a 2025 year-to-date gain and TradingEconomics recording a June 2026 record high, so an up day on a late-month session fits the broader pattern.[15][16][17]

The remaining catalysts are mostly after-hours or calendar-driven rather than market-moving for the settlement itself: traders were watching the ripple effects of the Fed decision, the post-earnings response in large-cap tech, and whether energy and inflation-sensitive names would keep pressuring sentiment into the bell.[1][11][14] For anything that can still change the official close, the only live dependency is the final SPX print versus the prior trading day’s close, because the market resolves strictly on the official closing levels.[3][13]

Sources: 1 · 2 · 3 · 4 · 5

Methodology

Methodologically we separate two layers: the live probability (Polymarket mid-price) and the platform attributes (fee, KYC, settlement currency, payment rails). That keeps the comparison honest — a single canonical probability across the row, with the venue-by-venue trade-offs spelt out in the columns next to it.

Resolution & payout

At resolution the UMA oracle takes over: a proposer posts the outcome with a bond, any token holder can dispute within two hours. Without dispute the result is accepted and the smart contract distributes USDC instantly.

On Kalshi (CFTC-regulated) resolution runs through their in-house clearing engine in USD. Betfair Exchange settles after match end in the account's local currency. Manifold pays no cash — only its in-platform "mana" currency.

FAQ

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is Prediction Today. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

Is this market available outside the US?

Polymarket itself is geo-blocked in the US/UK/EU. Always check the legal status of prediction markets in your jurisdiction before trading.

How does resolution work?

Through the UMA Optimistic Oracle on Polygon: a proposer submits the outcome, a two-hour challenge window opens, and USDC payouts settle automatically once the result is final.

What does Polymarket cost to trade?

Polymarket itself charges 0% — the only cost is the Polygon network fee, typically under $0.01 per transaction. Off-chain venues like Kalshi or Betfair charge 2-7% commission.

Do I need to KYC for this market?

On Polymarket directly, no — it's wallet-based. Intermediary brokers like Prediction Today trigger KYC only above $1,500 of lifetime trading volume; under that you trade pseudonymously with a single wallet address.