How Polymarket pricing works: CLOB, AMM, and real liquidity

When you see “Trump wins: 65¢” on Polymarket, most people assume there's a single price. There isn't. Underneath is a CLOB (Central Limit Order Book) with bids and asks, plus an AMM (Automated Market Maker)that fills small orders when the book is thin. Understanding this tells you why “market price” is a simplification — and where the real costs hide.

The CLOB (the real market)

A CLOB is a list of buy orders (bids) and sell orders (asks). If someone wants to sell YES at 66¢ and someone else wants to buy at 64¢, the market has a 2¢ spread. Your order fills wherever there's matching liquidity. Large orders walk up the book, eating through asks at 66, 67, 68… Your effective price is the volume-weighted average, not the top-of-book price.

The AMM (backstop for thin books)

When the CLOB is thin, Polymarket's AMM provides synthetic liquidity using a formula (essentially constant-product with an outcome-pair twist). The AMM always offers a price, but at a cost: the wider the imbalance, the worse the fill. Betting “at market” on a tiny market means taking AMM prices, which can be 3-5% worse than the last trade.

Why the spread matters

If the mid-price is 65¢ but the book is 63¢/67¢, placing a market buy costs 67¢. You're paying 2¢ extra vs the quote you saw. That's a 3% frictional cost before any AI analysis even starts. On low-liquidity markets this can be 5-10%.

The liquidity rule of thumb

• > $100k liquidity — tight spreads, professional-grade fills.
• $10k - $100k — 1-3% frictional cost on moderate bets.
• < $10k — avoid unless you accept 5-10% cost. Your bet moves the price.

How PolyGuru uses this

We filter daily picks to markets with ≥ $10k liquidity and rank by net EV(after 2% slippage + $0.20 gas), not naive gross edge. This means a 10% gross edge on a $500-liquidity market doesn't make the cut — the spread eats it.