Prediction Market WebSocket vs REST API (2026): When to Use Each

What is the difference between a REST and a WebSocket API?

When should you use REST?

• Discovery: listing markets, events and series, or searching by keyword.
• Snapshots: a price or a book at the moment a user opens a page.
• History: candles and price points for charts and backtests.
• Anything scheduled: a dashboard refreshed every minute, a nightly report.

REST is also where rate limits bite first. Kalshi meters REST in tokens per second, with separate read and write budgets, and most requests cost 10 tokens. A 429 response means slow down, and Kalshi recommends exponential backoff. See Kalshi API rate limits and Polymarket API rate limits for the full tables.

When should you use a WebSocket?

When the answer changes faster than you would want to poll for it: order books for a bot, live prices on a trading screen, trade prints, and your own order and fill updates. A rule of thumb: if you would poll the same endpoint more often than every few seconds, open a stream instead.

How do Polymarket's and Kalshi's WebSockets work?

Kalshi signs the handshake the same way as REST: KALSHI-ACCESS-KEY, KALSHI-ACCESS-SIGNATURE and KALSHI-ACCESS-TIMESTAMP, with the signed message being the timestamp plus GET plus /trade-api/ws/v2. Polymarket's crypto and equity reference prices moved from the older RTDS stream to PolyBolt in 2026, covered in our RTDS to PolyBolt migration guide, and keeping connections alive is covered in the Polymarket heartbeat guide.

What goes wrong with WebSockets, and how do you handle it?

• Silent disconnects. Send or answer heartbeats on schedule, and treat a missed one as a dead connection.
• Gaps after a reconnect. Updates sent while you were offline are gone. Take a fresh REST snapshot, or wait for the stream's snapshot message, before trusting the book again.
• Out-of-order deltas. Apply delta streams such as Kalshi's in sequence, and resubscribe for a new snapshot if a sequence breaks.
• Reconnect storms. Back off with jitter so a venue outage does not turn into a burst of thousands of reconnects.

How do you stream every venue from one connection?

Each venue's socket has its own URL, auth, heartbeat and message shapes. Predictefy's stream at wss://stream.predictefy.com/v1/stream puts order books and trades from many venues on one connection. Every book message is a full book in one normalized shape, a snapshot on subscribe and then update frames, so there are no deltas to stitch together:

import Predictefy from '@predictefy/sdk';

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

const stop = client.watchOrderBook({ venue: 'polymarket', marketId: 'YES_TOKEN_ID' }, (u) => {
  console.log(u.type, u.book.bids[0]?.price, u.book.asks[0]?.price);
});

setTimeout(stop, 60_000); // close after a minute

Frequently Asked Questions