Quick answer
Both offer free public market data. Polymarket is crypto-settled with Gamma and CLOB APIs; trading needs an EIP-712 wallet signature plus HMAC-signed L2 credentials. Kalshi is CFTC-regulated in the US with REST, WebSocket and FIX; trading requests are RSA-PSS signed with a downloaded private key. Neither signature fits plain HMAC code, so use their SDKs.
Side-by-side comparison
| API | Markets | Auth | Paper / sandbox | Pricing | In AlgoGP |
|---|---|---|---|---|---|
| Polymarket | Event contracts, Crypto | Asymmetric / wallet signing | — | Public data free; trading fees per market | Guide |
| Kalshi | Event contracts | Asymmetric / wallet signing | Demo environment | API free; per-contract trading fees | Guide |
Which one should you pick?
Kalshi if you are in the US and want a regulated exchange.
Build with KalshiPolymarket if you want the deepest crypto prediction markets and on-chain settlement.
Build with Polymarket
Frequently asked questions
Can I backtest prediction market strategies?
Yes: both publish price history; Polymarket via prices-history, Kalshi via market candlesticks.
Test the strategy before you pick the API
Describe your idea to AlgoGP’s copilot, backtest it on years of stock, crypto or forex data with realistic costs, then export Python wired to Kalshi or any built-in broker. Swap the broker later without touching the strategy.
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AlgoGP is not affiliated with any provider listed. Details change; check each provider’s documentation. Nothing here is investment advice.