To monitor Polymarket prices without repeatedly fetching snapshots, connect to its public CLOB market WebSocket, subscribe to the outcome token IDs you care about, and maintain local order-book state from snapshots and incremental updates. The market feed is separate from the authenticated user feed: a read-only price monitor needs the former, not account credentials.
Connect to the public market feed
Polymarket’s maintained WebSocket guide documents the public market endpoint as wss://ws-subscriptions-clob.polymarket.com/ws/market. The channel is described as unauthenticated and subscribes by asset ID: the outcome token ID, not the broader market’s condition ID.
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After the socket opens, send a JSON subscription immediately. Keep the field name assets_ids exactly as documented:
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"assets_ids": ["TOKEN_ID_1", "TOKEN_ID_2"],
"type": "market",
"custom_feature_enabled": true
}Replace the example strings with the outcome token IDs to monitor. The custom feature flag requests best_bid_ask, new_market, and market_resolved events. It is not documented as necessary for the core book and trade events.
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Choose the channel and events for your use case
| Channel or event set | What it provides | Subscription key or setting |
|---|---|---|
| Public market channel | Market order-book and trade information for subscribed outcome tokens | Asset IDs in assets_ids; no authentication for this channel |
| Authenticated user channel | Account-specific order and trade lifecycle events | Requires API credentials and subscribes by condition IDs |
| Core market events | Book snapshots, price-level changes, last trades, and tick-size changes | Basic market subscription; the guide does not require the custom feature flag for these |
| Optional market events | Best bid/ask, new-market, and market-resolved notifications | Set custom_feature_enabled to true |
For a public price monitor, use the market channel. Use the user channel only if account activity is within scope; it is not a substitute for the public market subscription.
Process snapshots and incremental updates
The feed mixes full book information with messages that change part of that information. Keep state per asset so a new snapshot establishes the book and later updates can modify the correct price level. Rendering only the latest message can leave a display incomplete because an incremental update is not a replacement for the whole book.
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book: initialize or refresh the book
A book event contains bid and ask arrays, along with a hash and timestamp. Treat it as a snapshot for that subscribed asset and use it to initialize or refresh the displayed levels.
price_change: update a price level
A price_change event contains a price_changes array with price, size, side, best bid, and best ask fields. Apply each change to the matching asset and side at the specified price. A size of "0" means that price level was removed; delete it from local state rather than displaying a zero-sized level.
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last_trade_price: show an executed trade
This event reports a trade and includes price, side, size, and fee-rate fields. Keep it distinct from a quoted bid or ask: it indicates an execution, not merely a resting order-book level.
tick_size_change: surface changed price increments
The guide says this event occurs when a price crosses above 0.96 or below 0.04. A monitor can log or display the change as context. The guide’s order-placement warning is that a bot using the old tick size can have an order rejected; that concern applies to trading systems, not to a read-only display.
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Optional events
When custom_feature_enabled is true, the feed can also provide best_bid_ask, new_market, and market_resolved. Enable the option when your monitor needs those notifications; otherwise, the core event set may be sufficient.
Keep the WebSocket alive
- When the socket opens, send the market subscription immediately.
- Send the literal text
PINGevery 10 seconds on the market channel. - Handle the documented
PONGresponse separately from JSON event messages. Do not try to parse that plain-text heartbeat response as an event object. - Continue processing incoming event messages into the per-asset state used by your display.
Polymarket’s guide recommends this heartbeat on both market and user channels. It identifies a connection that drops after roughly 10 seconds as a possible sign that heartbeats are missing. The guide publishes no latency, uptime, or market-coverage guarantee, so a live socket should not be treated as a promise of any particular delivery speed or availability.
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Troubleshoot a quiet or disconnected monitor
- The connection closes quickly: check that the client sends the literal
PINGevery 10 seconds and handlesPONGas plain text. - No market messages arrive: verify the asset IDs are the intended outcome token IDs, that the subscription was sent after the socket opened, and that the markets are active.
- The displayed book looks incomplete or stale: confirm that
booksnapshots initialize the local book and that eachprice_changeupdates or removes the correct level. A zero size removes the level. - Account events are missing: the public market feed does not report private account order and trade lifecycle activity. That requires the separate authenticated user channel and condition-ID subscription.
The endpoint, payload, available events, and heartbeat details here follow Polymarket’s maintained WebSocket guide. API details can change; consult that guide when implementing or updating a client.
Quick Recap
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