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The Orderbook WebSocket streams real-time orderbook updates for prediction markets. Subscribe to multiple markets at once to build live trading interfaces with accurate bid/ask spreads and depth visualization. Endpoint: wss://synthesis.trade/ws/orderbook

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Use cases

Use CaseDescription
Live price displayShow real-time best bid/ask prices derived from the top of the orderbook
Event pagesSubscribe to all markets in an event to show live prices for each outcome
Depth chartsBuild orderbook depth visualizations showing liquidity at each price level
Spread monitoringTrack bid-ask spreads to identify trading opportunities or market inefficiencies

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Subscribe

Subscribe using token IDs (for Polymarket) or market IDs (for Kalshi). You can subscribe to up to 5,000 markets at once: 
const ws = new WebSocket("wss://synthesis.trade/ws/orderbook");

ws.onopen = () => {
    ws.send(JSON.stringify({
        type: "subscribe",
        venue: "polymarket",
        markets: [
            "21742633143463906290569050155826241533067272736897614950488156847949938836455",
            "48572956138472615938471625938471659384716593847165938471659384716593847165938"
        ]
    }));
};

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Initial response

Upon subscribing, you receive the full orderbook for each market: 
{
    "success": true,
    "response": {
        "orderbooks": [
            {
                "venue": "polymarket",
                "orderbook": {
                    "condition_id": "0xabcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890",
                    "token_id": "21742633143463906290569050155826241533067272736897614950488156847949938836455",
                    "bids": {
                        "0.65": "1500",
                        "0.64": "3200",
                        "0.63": "5000"
                    },
                    "asks": {
                        "0.66": "1200",
                        "0.67": "2800",
                        "0.68": "4500"
                    },
                    "best_bid": "0.65",
                    "best_ask": "0.66",
                    "hash": "a1b2c3d4e5f6",
                    "created_at": "2026-01-01T12:00:00"
                }
            }
        ]
    }
}

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Real-time updates

After the initial snapshot, you receive delta updates whenever the orderbook changes: 
{
    "success": true,
    "response": {
        "venue": "polymarket",
        "delta": {
            "condition_id": "0xabcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890",
            "token_id": "21742633143463906290569050155826241533067272736897614950488156847949938836455",
            "amount": "800",
            "price": "0.66",
            "side": "BUY",
            "best_bid": "0.66",
            "best_ask": "0.67",
            "hash": "f6e5d4c3b2a1",
            "created_at": "2026-01-01T12:00:05"
        }
    }
}

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Complete example: Event page

Build a live event page that shows prices for all markets in an event: 
const eventResponse = await fetch(
    "https://synthesis.trade/api/v1/polymarket/market/event/democratic-presidential-nominee-2028"
);
const { response: event } = await eventResponse.json();

const tokenIds = event.markets.flatMap(market => [
    market.left_token_id,
    market.right_token_id
]);

const ws = new WebSocket("wss://synthesis.trade/ws/orderbook");
const orderbooks = new Map();

ws.onopen = () => {
    ws.send(JSON.stringify({
        type: "subscribe",
        venue: "polymarket",
        markets: tokenIds
    }));
};

ws.onmessage = (event) => {
    const message = JSON.parse(event.data);
    if (!message.success) return;
    
    const data = message.response;
    
    if (data.orderbooks) {
        data.orderbooks.forEach(({ orderbook }) => {
            orderbooks.set(orderbook.token_id, orderbook);
        });
        updateUI();
        return;
    }
    
    if (data.delta) {
        const existing = orderbooks.get(data.delta.token_id);
        if (existing) {
            existing.best_bid = data.delta.best_bid;
            existing.best_ask = data.delta.best_ask;
        }
        updateUI();
    }
};

function updateUI() {
    event.markets.forEach(market => {
        const yesBook = orderbooks.get(market.left_token_id);
        if (yesBook) {
            const yesPrice = yesBook.best_ask || "0";
            document.getElementById(`price-${market.condition_id}`).textContent = yesPrice;
        }
    });
}
For Polymarket, each market has two token IDs (left/right or YES/NO). Subscribe to both to show complete pricing. The best bid on YES roughly equals 1 minus the best ask on NO.