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

Platform comparison

Market context

Polymarket is pricing this close into a **100% YES** outcome, which means the market is effectively treating the S&P 500 as certain to finish **up** versus the prior trading day’s close for the July 31 settlement check. On Polymarket, the position is expressed in **USDC** and settled through **conditional tokens** on **Polygon**, so the contract’s value depends only on the official SPX close used by the market rules, not on intraday swings or headline sentiment.

The comparison point matters because the benchmark close is the **most recent prior trading day**. Here, the reference appears to be the 30 July close at **7,437.63**, after the index rose **1.7%** on Thursday following the Federal Reserve’s decision to leave rates unchanged[2][4]. Friday’s cash close was reported at **7,489.72**, up **0.7%** on the day, extending the index’s year-to-date gain to about **9.4%**[1]. That makes the contract’s current 100% implied price consistent with a straightforward higher close already having printed.

For traders watching the mechanics rather than the narrative, the key catalyst was the Fed timetable and any late-session reversal risk into the cash close; Thursday’s rebound was led by **tech and consumer discretionary** shares, which tend to carry the index most on a point basis[2]. Since the settlement uses the official SPX close, the practical watchlist is the final index print, any closing auction imbalance, and whether late volatility from the rates backdrop or mega-cap earnings can alter the day’s net change before the bell[1][2].

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

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

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 PolyGram. 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.

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.