Kalshi API vs Polymarket API: Which Should You Build On?

Kalshi API vs Polymarket API: which should you build on?

The correct choice follows the product boundary.

For a Kalshi trading assistant, tax tool or market browser, use Kalshi directly. You keep the venue's native ticker model and its full account and order lifecycle.

For a Polymarket wallet dashboard, CLOB trader or onchain analytics product, use Polymarket directly. Its public APIs expose discovery, prices, books, positions and wallet activity in the venue's own model.

For a prediction market terminal, scanner or agent that needs both, start with a normalized layer. The cost of two direct integrations is not only two base URLs. It is two identifier systems, two order-book shapes, two authentication models and two sets of failure behavior that the rest of your application has to understand.

How do the two APIs compare?

The table explains why changing the venue name in a direct API URL is not enough. Even when both APIs expose the same concept, the object carrying it can be different.

How does market discovery differ?

Kalshi organizes contracts into series, events and markets. A ticker carries structure and is used throughout the API. Market listings come from one Trade API at https://external-api.kalshi.com/trade-api/v2, with cursor pagination for larger result sets.

Polymarket uses its Gamma API for discovery. Events group one or more tradable markets, and each binary market maps to two CLOB token IDs. The market object carries the metadata needed to move from a human-readable question to the tokens used for prices and books.

This distinction matters when an application stores references. A Kalshi market ticker is meaningful to Kalshi. A Polymarket condition ID identifies the market, while the token ID identifies the outcome book. Treating all three as a generic market ID creates lookup failures later.

How are the order books different?

Kalshi returns yes_dollars and no_dollars bid arrays. It does not need separate ask arrays because a YES bid at one price implies a NO ask at one minus that price, and the same relationship works in reverse.

Polymarket returns explicit bids and asks for a single outcome token. Its GET /book endpoint reads one token, while POST /books batches several token IDs. The response also includes fields such as the minimum order size, tick size and last trade price.

Both structures describe a binary order book, but they are not drop-in replacements. A shared application has to normalize sides, sort order, decimal types, identifiers and the relationship between a market and its outcomes before it can compare them.

How does authentication differ?

Public market data is the simple part. Kalshi documents unauthenticated access to markets and books, and Polymarket states that its public market data needs no API key, wallet or authentication.

Private operations diverge sharply.

Kalshi issues an API key ID and an RSA private key. An authenticated request sends three headers: the key ID, a millisecond timestamp and an RSA-PSS signature over the timestamp, HTTP method and request path.

Polymarket uses two authentication levels for CLOB trading. An EIP-712 wallet signature creates or derives L2 API credentials, then HMAC-SHA256 headers authenticate subsequent trading requests. Order payloads still use the user's signer.

Neither model is inherently better. Kalshi fits a conventional signed API workflow. Polymarket ties the trading identity to an onchain wallet. The right choice is the one native to the venue where the order will live.

How do the WebSocket APIs compare?

Kalshi's WebSocket requires API authentication and streams channels such as order-book changes, trades, market status and fills. A client receives an order-book snapshot, then applies incremental deltas in sequence.

Polymarket separates a public market channel from an authenticated user channel. The market channel provides book snapshots, price-level changes, trades and lifecycle events. The user channel carries account-specific orders and fills.

The state-management consequence is easy to miss. A Kalshi consumer maintains a local book by applying deltas after the snapshot. A Polymarket consumer also has to understand its market-channel event types and token IDs. A reconnecting client on either venue needs a fresh snapshot before treating updates as complete.

How can one integration read both APIs?

Predictefy maps both venues into the same market and order-book contract. The venue client changes, while the method and returned shape stay consistent.

npm install @predictefy/sdk@1.0.0-beta.8

import Predictefy from '@predictefy/sdk';

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

const [kalshiMarket] = await client.kalshi.fetchMarkets({
  status: 'active',
  limit: 1,
});

const [polymarketMarket] = await client.polymarket.fetchMarkets({
  status: 'active',
  limit: 1,
});

const kalshiBook = await client.kalshi.fetchOrderBook({
  outcomeId: kalshiMarket.outcomes[0].outcomeId,
});

const polymarketBook = await client.polymarket.fetchOrderBook({
  outcomeId: polymarketMarket.outcomes[0].outcomeId,
});

console.log(kalshiBook.bids, kalshiBook.asks);
console.log(polymarketBook.bids, polymarketBook.asks);

This pattern is most valuable when the next feature crosses the venue boundary. A matched-market view, price alert or arbitrage check can use one shared type instead of converting Kalshi and Polymarket objects every time data enters the application.

Which API is better for a trading bot?

For a single-venue bot, the venue's own API is the shortest path. It exposes the native order lifecycle and every venue-specific field.

For a bot that researches both venues, Predictefy removes the duplicate reads integration while preserving venue-specific execution. Orders are built and submitted through explicit per-venue routes, and signing stays client-side with your credentials. That keeps the data model shared without pretending the two exchanges settle or authenticate in the same way.

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