Build a Real-Time Polymarket Order Book Monitor with Python
Build a Real-Time Polymarket Order Book Monitor A trading bot should not make decisions from stale snapshots. If you want to understand liquidity, spread, depth, or changes in market structure, you need a continuously updated view of the Polymarket order book. This tutorial builds a lightweight Polymarket order book Python monitor using the public CLOB Market WebSocket. Polymarket documents this channel as a real-time feed for order-book, price, and market lifecycle updates. The implementation intentionally focuses on market data—not order execution—so it can be used as the foundation for research, dashboards, alerts, or an automated trading system. What You'll Learn How Polymarket token IDs relate to order-book subscriptions How to connect to the CLOB Market WebSocket How to process book and price_change events How to calculate best bid, best ask, and spread How to handle reconnects and heartbeats How to detect stale market data How to turn raw WebSocket events into trading signals Architecture flowchart LR A[Polymarket CLOB] --> B[Market WebSocket] B --> C[Python Async Client] C --> D[Order Book State] D --> E[Spread / Depth Metrics] D --> F[Trading Signal Engine] D --> G[Logging / Monitoring] The important design decision is separating transport from state . The WebSocket delivers events; your application maintains the current book. 1. Install the Dependencies For this monitor, authentication is not required because the Market WebSocket is public. pip install websockets You need a Polymarket asset ID/token ID for the outcome you want to monitor. The Market Channel subscribes using assets_ids . For example: TOKEN_ID = " YOUR_TOKEN_ID " Do not hard-code credentials into a market-data monitor. In this example, there are no credentials at all. 2. Connect to the Market WebSocket The documented Market Channel endpoint is: wss://ws-subscriptions-clob.polymarket.com/ws/market The subscription message contains type: "market" and one or more asset IDs. A minimal monitor lo