Kalshi vs Polymarket: A Developer's Guide to Both APIs

If you're building anything on prediction markets — a bot, a dashboard, an AI agent — your first decision is: Kalshi or Polymarket? I've built on both. Here's what I wish I knew upfront. The Basics Kalshi Polymarket Regulation CFTC (US) Unregulated (crypto) Settlement USD bank account USDC on Polygon Auth RSA-PSS signatures EIP-712 + HMAC Main volume Sports (90%) Crypto + geopolitics Rate limits ~100 req/10s 15,000 req/10s Market Discovery Kalshi uses Series → Events → Markets. Search by series ticker: curl "https://api.elections.kalshi.com/trade-api/v2/series?include_volume=true" Polymarket uses Events → Markets with the Gamma API: curl "https://gamma-api.polymarket.com/events?active=true&limit=50" Orderbook Data Both expose full orderbook depth. The key difference: Kalshi returns YES/NO dollar prices: { "yes_bid_dollars" : "0.48" , "no_bid_dollars" : "0.53" } YES ask = 100 - NO bid. Mental math required. Polymarket returns direct probabilities: { "bids" : [{ "price" : "0.48" , "size"