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Trading API

A trading API is a programmatic interface that allows developers and algorithmic traders to interact with a prediction market exchange — submitting orders, retrieving market data, and managing positions — without using the platform's web or mobile interface. Both Kalshi and Polymarket publish public trading APIs with REST, WebSocket, and in Kalshi's case FIX protocol support.

Updated June 24, 2026Platforms & Tools
TL;DR
A prediction market trading API lets bots and algorithms place orders and stream real-time data, enabling systematic trading strategies on platforms like Kalshi and Polymarket.

Key Points

✓Kalshi's API uses base URL https://api.kalshi.com/trade-api/v2 for REST calls and wss://api.kalshi.com/trade-api/ws/v2 for WebSocket streaming.
✓Kalshi authenticates API requests with RSA-PSS key pairs; session tokens expire every 30 minutes and must be refreshed.
✓Polymarket's CLOB API accepts EIP-712-signed orders submitted on-chain to Polygon, with public WebSocket streams for live order book and trade data.
✓Kalshi publishes openapi.yaml and asyncapi.yaml specification files for automated client code generation in any language.
✓Unified third-party SDKs such as PMXT normalize market data across Polymarket, Kalshi, and other venues under a single interface, reducing integration overhead.

Kalshi API Architecture

Kalshi provides a REST API, a WebSocket streaming API, and a FIX protocol interface for institutional participants. The REST API covers market listing, Order Book retrieval, Limit Order and Market Order submission, and position management. The WebSocket API streams real-time Order Book deltas, Last Price ticks, and account position updates. Authentication uses RSA-PSS asymmetric key pairs: a trader generates a key pair, uploads the public key to their account, and signs each request with the private key to prove identity without transmitting a password. Kalshi also publishes OpenAPI and AsyncAPI specification files, allowing developers to auto-generate typed client libraries in Python, TypeScript, or any other language with OpenAPI tooling.

Polymarket API Architecture

Polymarket's public API exposes a Central Limit Order Book (CLOB) built on Polygon. Because all settlement is on-chain, order submission requires an EIP-712 signature from a connected Crypto Wallet, making the API inherently non-custodial. Developers can query live Order Book data, historical trades, and individual position history through REST endpoints, and subscribe to live market updates via WebSocket. Gas fees on Polygon are low (typically fractions of a cent), making high-frequency API usage economically viable. The open nature of on-chain data means Trading Volume, Open Interest, and full trade histories are also queryable directly from the blockchain without using the official API, enabling independent analytics.

Use Cases and Ecosystem Tools

Trading APIs enable Arbitrage strategies that monitor the same event market on both Kalshi and Polymarket simultaneously, executing offsetting trades when Mispricing exists between venues — a core application for Prediction Market Aggregator tools. They also power systematic Value Betting bots, Kelly Criterion position-sizing algorithms, and Hedging workflows that offset prediction market exposure against related financial instruments. The broader developer ecosystem includes unified SDKs that abstract multi-platform differences, alerting tools that notify traders of sudden Implied Probability moves, and Leaderboard scraper tools that track top-wallet performance. Prediction Hunt offers a unified API key covering Polymarket, Kalshi, PredictIt, and ProphetX under a single interface.

Related Terms

Kalshi
Kalshi is a US-based prediction market exchange regulated by the CFTC as a Designated Contract Market, allowing traders to buy and sell event contracts on real-world outcomes. It was the first exchange to receive CFTC approval specifically for event contracts, launching its first markets in 2021.
Polymarket
Polymarket is the world's largest decentralized prediction market platform, built on the Polygon blockchain, where users trade binary outcome shares on real-world events using USDC as collateral. Founded in 2020, it relaunched for US users in May 2026 after securing a CFTC operating license.
Order Book
An order book is the real-time electronic ledger that lists all outstanding buy (bid) and sell (ask) orders for an event contract, organized by price level. It is the central mechanism through which prediction market exchanges match buyers and sellers.
Arbitrage
Arbitrage in prediction markets is the practice of exploiting price discrepancies for the same event across different platforms or outcomes to lock in a guaranteed profit regardless of how the event resolves.
Prediction Market Aggregator
A prediction market aggregator is a tool or platform that collects and displays probability estimates, prices, or odds from multiple prediction market venues in a single interface, enabling traders to compare implied probabilities across platforms and identify discrepancies. Aggregators may also support direct trading across multiple markets through a unified API.

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