Prediction Market Order Book API (2026): Depth Across Every Venue

Why does the order book matter more than the price?

The price you see on a market page is usually a midpoint or the last trade. The order book shows what is actually available: every resting bid and ask with its size. If the best ask is $0.13 for 500 shares and you want 5,000, you will walk up the book and pay more on average. Any bot, arbitrage check or execution tool has to read depth, not just the top price.

How do order books differ between venues?

Kalshi's design follows from binary contracts: buying Yes at $0.14 is the same trade as selling No at $0.86, so one list of No bids is also the list of Yes asks. Kalshi's docs state it directly: a Yes bid at price X is equivalent to a No ask at 100 − X cents.

How do you read and normalize books from Polymarket and Kalshi?

This Node 18+ script reads both venues' books with no keys and returns the same best-first shape for each:

// Normalize Polymarket and Kalshi books to { bids, asks } with probabilities, best first
const pmBook = async (tokenId) => {
  const b = await (await fetch(`https://clob.polymarket.com/book?token_id=${tokenId}`)).json();
  const lv = (xs) => xs.map((x) => ({ price: Number(x.price), size: Number(x.size) })).reverse();
  return { bids: lv(b.bids), asks: lv(b.asks) }; // Polymarket lists the best price last
};

const kalshiBook = async (ticker) => {
  const { orderbook_fp: ob } = await (await fetch(`https://external-api.kalshi.com/trade-api/v2/markets/${ticker}/orderbook?depth=10`)).json();
  const yes = (ob.yes_dollars ?? []).map(([p, q]) => ({ price: Number(p), size: Number(q) }));
  const no = (ob.no_dollars ?? []).map(([p, q]) => ({ price: Number(p), size: Number(q) }));
  return {
    bids: yes.sort((a, b) => b.price - a.price),
    // A No bid at X is a Yes ask at 1 - X
    asks: no.map((l) => ({ price: +(1 - l.price).toFixed(4), size: l.size })).sort((a, b) => a.price - b.price),
  };
};

const [pm] = await (await fetch('https://gamma-api.polymarket.com/markets?limit=1&active=true&closed=false&order=volume24hr&ascending=false&liquidity_num_min=50000&end_date_min=2026-11-01')).json();
const token = pm.version === 'v2' ? pm.positionIds[0] : JSON.parse(pm.clobTokenIds)[0];
const p = await pmBook(token);
console.log('Polymarket', pm.question, '\n  best bid', p.bids[0], 'best ask', p.asks[0]);

const { markets: [km] } = await (await fetch('https://external-api.kalshi.com/trade-api/v2/markets?status=open&limit=1&series_ticker=KXFEDDECISION')).json();
const k = await kalshiBook(km.ticker);
console.log('Kalshi', km.ticker, '\n  best bid', k.bids[0], 'best ask', k.asks[0], '| market record ask', km.yes_ask_dollars);

How do you get order books from every venue in one shape?

The normalizer above is two venues' worth of edge cases. Predictefy does it for 15+ venues: every book comes back as bids and asks of { price, size }, prices as probabilities from 0 to 1, keyed by the same outcomeId the market record gives you.

import Predictefy from '@predictefy/sdk';

const client = new Predictefy({ apiKey: process.env.PREDICTEFY_API_KEY });

// One book
const [market] = await client.kalshi.fetchMarkets({ limit: 1, status: 'active' });
const outcomeId = market.outcomes[0].outcomeId;
const book = await client.kalshi.fetchOrderBook({ outcomeId });
console.log(book.bids[0], book.asks[0], book.asOf);

// Many books in one call (Builder and above)
const many = await client.polymarket.fetchOrderBooks(['OUTCOME_ID_1', 'OUTCOME_ID_2']);
for (const [id, b] of Object.entries(many)) console.log(id, b.bids[0]?.price, b.asks[0]?.price);

// Stored snapshots for a past window
const series = await client.kalshi.fetchOrderBook({ outcomeId, since: '2026-10-15T00:00:00Z', until: '2026-10-16T00:00:00Z' });

How do you use an order book to price a trade?

Walk the asks until your size is filled and average what you paid. That average, plus the venue's fee, is your real entry price, and it is the number to compare across venues or against a fair value. If the book cannot fill your size at a price you accept, the trade is smaller than you planned. Trading involves risk, and the book can move between the moment you read it and the moment you order.

Frequently Asked Questions