Polymarket CLOB vs Last Price: Why Most Tools Get It Wrong

What is the CLOB?

Polymarket uses a Central Limit Order Book (CLOB) for trading — the same mechanism used by stock exchanges. Traders post bids (prices they're willing to buy at) and asks (prices they're willing to sell at). When a bid meets an ask, a trade executes.

The best ask is the lowest price at which someone is willing to sell you a share right now. This is the price you actually pay when you click Buy.

What is the "last price"?

The "last price" is the price of the most recent completed trade. It's historical data — it tells you what someone paid, at some point in the past.

In a liquid market that trades every few seconds, last price ≈ current price. But Polymarket has hundreds of markets with highly variable liquidity. Some markets trade once an hour. Some trade once a day.

A market that last traded at 35c three hours ago might have a current best ask of 52c. If you're making a trading decision based on the 35c last price, you're working with wrong data.

How to spot stale prices

The difference between the CLOB best ask and the last price tells you how stale the market is. PolyEdge flags markets where the gap exceeds 2.5 cents as "STALE LAST" — this is a signal to use extra caution.

A large gap between bid and ask (the spread) can also indicate low liquidity — the market maker is demanding a premium to take the other side of your trade.

Crossed books: when something is very wrong

Occasionally, Polymarket markets develop "crossed books" where the best bid is higher than the best ask. This is theoretically impossible in a functioning market — it would allow instant risk-free profit — but it happens due to data feed issues or market microstructure problems.

Crossed book markets have unreliable prices and should generally be avoided. PolyEdge automatically flags and skips these when calculating edges.

How to access CLOB data yourself

Polymarket exposes its CLOB data via a public API:

GET https://clob.polymarket.com/book?token_id=YOUR_TOKEN_ID

Response:
{
  "bids": [{"price": "0.48", "size": "150"}, ...],
  "asks": [{"price": "0.52", "size": "200"}, ...]
}

The token ID for each outcome is available in the market data from the Gamma API. The best ask is asks[0].price when sorted ascending.