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Polymarket’s order book is a central limit order book (CLOB): it shows resting buy bids and sell asks for an outcome. Developers can retrieve a snapshot through the CLOB API or SDK, or subscribe to the market WebSocket stream for updates. The key implementation details are choosing the right outcome identifier, interpreting bid/ask prices rather than assuming the displayed price is executable, and rebuilding stream state after connection interruptions.
How does the Polymarket order book work?
A book records open offers to buy and sell an outcome. Bids are buyer offers; asks are seller offers. The best bid is the highest bid, the best ask is the lowest ask, and their difference is the spread. A narrower spread generally indicates a more liquid market, though it does not guarantee that a particular order will fill.
Polymarket outcome shares are priced from $0.00 to $1.00, with the price interpreted as an implied probability. Polymarket describes its prices as emerging from users’ supply and demand rather than being set by the platform. The displayed price is normally the midpoint between the best bid and ask. If the spread is wider than $0.10, the displayed price uses the last traded price instead. These are display conventions, not execution guarantees. Polymarket’s Prices & Orderbook documentation explains the convention.
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Displayed price versus executable price
Suppose the best bid is $0.34 and the best ask is $0.40. The midpoint is $0.37, but a buyer who wants to trade immediately crosses the spread and pays the ask, while an immediate seller receives the bid. The example illustrates why a dashboard should label midpoint, bid, ask, and last trade distinctly instead of presenting one number as the price at which every user can transact.
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How do I get a Polymarket order book snapshot?
Each outcome has a trading identifier. Select the identifier that matches the market version and the outcome data you are using: CTF markets use an outcome token ID, while Polymarket Protocol V2 markets use a position ID. The unified SDK calls this value an outcome assetId; verify the market version before passing an ID to an endpoint or SDK method. See the CLOB client documentation and Polymarket quickstart.
Fetch one outcome
The documented HTTP endpoint is GET https://clob.polymarket.com/book?token_id=<TOKEN_ID>. In TypeScript, the SDK provides fetchOrderBook; in Python, it provides get_order_book. The HTTP example uses a token ID, so for Protocol V2 use the SDK’s version-aware identifier handling rather than assuming that every market’s identifier is interchangeable.
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A response contains bid and ask price levels with sizes, plus metadata such as timestamp, minimum order size, tick size, negative-risk flag, last trade price, and a hash representing the book state. Consult the current API response and SDK documentation when implementing a parser, since the documentation pages and payload conventions can change.
Read the price-level arrays carefully
The documented snapshot ordering puts bids in ascending price order and asks in descending order, so the best price is the last item in each array. Do not assume that the first element is best simply because that convention is common elsewhere. Confirm ordering against the current documented response and validate it in code before using the result for pricing or display.
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Fetch several books
For multiple outcomes, the documentation describes a batch method and a maximum of 500 books per request. The raw API example posts a list of outcome IDs to /books. Keep batches within that documented maximum, and use the correct identifiers for the relevant market version. See the prices and books API documentation.
How do I stream Polymarket order-book updates?
The market WebSocket stream accepts one or more outcome assetId values. Standard event types include book, price_change, last_trade_price, and tick_size_change. A book event includes bids, asks, condition ID, and asset ID; optional metadata can include timestamp, minimum order size, tick size, negative-risk status, and last trade price. With customFeatureEnabled, the stream can also include best_bid_ask, new_market, and market_resolved events. Event details and subscription examples are in the market-channel documentation.
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A stream avoids repeatedly requesting snapshots just to learn about changes, but it adds client-side state and recovery work. Track event timestamps where provided, handle disconnects, and rebuild or reconcile local state after reconnecting according to your application’s needs. The reviewed documentation does not specify a numeric latency service-level agreement, delivery guarantee, or universal update frequency, so do not design around an assumed instant or fixed-rate feed.
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| Approach | Best suited to | What to account for |
|---|---|---|
| Snapshot API or SDK | One-off reads, periodic polling, and initializing a local view | Each read is a point-in-time snapshot. Compare its book hash with a prior snapshot to detect whether the state changed between reads; the hash does not deliver the changes themselves. |
| Market WebSocket stream | Applications that need event-driven updates instead of repeated polling | Maintain local state and plan for connection recovery and state rebuilding. The documentation does not state a numeric latency or delivery guarantee. |
The snapshot hash is useful for checking whether two reads represent different book states. It is not a substitute for subscribing to events when the application needs to learn about updates as they occur.
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How do market and limit orders interact with the book?
Polymarket’s documentation distinguishes market-order behavior from limit-order behavior while noting that technically all Polymarket orders are limit orders. A market order is a limit order priced to execute immediately against available resting liquidity. A limit order executes only at its specified price or better; it can rest in the book, fill partially, or be canceled while unfilled. See the order-book concepts documentation.
| Order behavior | Price control | Execution trade-off |
|---|---|---|
| Immediate, market-style execution | Less control over the final execution price; the order takes available opposing liquidity | Can execute promptly when liquidity is available, but crossing the spread means a buyer pays the ask or a seller receives the bid. |
| Resting limit order | Sets the price threshold at which the order can execute | May wait in the book, fill only in part, or remain unfilled until canceled. |
Polymarket describes its CLOB as hybrid-decentralized: an operator performs matching offchain, and matched trades settle onchain through smart contracts. Settlement routing depends on the outcome-token system. The quickstart’s high-level flow is to authenticate, fetch a market and select a version-appropriate outcome ID, submit an order, wait for asynchronous settlement, and check the resulting position. This distinction matters when building execution flows: an accepted or matched order should not be treated as proof that settlement and the resulting position are already final.
Quick Recap
Implementation checks for a reliable integration
- Use the right identifier: distinguish CTF token IDs from Protocol V2 position IDs when selecting an outcome’s
assetId. - Respect the documented array ordering: the best bid and ask are the last entries in their respective arrays under the documented ordering.
- Keep display and execution prices separate: midpoint or last-trade display values do not replace the ask a buyer crosses or the bid a seller receives.
- Preserve book metadata: timestamp, tick size, minimum order size, negative-risk flag, last trade, and hash can affect interpretation and downstream handling.
- Design for mutable documentation: Polymarket’s technical pages do not show publication dates; recheck method names, payload shapes, and market-version handling against the current docs when shipping changes.
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