Kalshi vs Polymarket: A Developer's Comparison of APIs, Orderbooks, and Liquidity

Kalshi vs Polymarket: Which Prediction Market Should You Build On?

If you're building a trading bot, analysis tool, or AI agent for prediction markets, your first decision is: Kalshi or Polymarket? Here's a data-driven comparison based on actually building on both APIs.

Architecture

API Comparison

Market Discovery

Kalshi organizes markets as Series → Events → Markets. You search by series ticker (e.g., KXWTIMAX) to find all related markets.

# Kalshi: ~9000 series, search by keyword
sf scan "oil recession"

# Polymarket: search by natural language
sf scan --venue polymarket "oil price above 100"

Polymarket uses Events → Markets with tag-based categorization. The Gamma API (gamma-api.polymarket.com) handles discovery, while the CLOB API (clob.polymarket.com) handles pricing.

Orderbook Data

Both exchanges expose full orderbook depth. Here's how to compare them side by side:

# Compare orderbooks across venues
sf book KXWTIMAX-26DEC31-T135 --history
sf book --poly "oil price above 100"

Key difference: Kalshi's orderbook uses YES/NO dollar prices (e.g., yes_bid_dollars: "0.48"). Polymarket's CLOB returns decimal prices (e.g., price: "0.48") that represent probability directly.

Liquidity

In our testing across 18 market categories:

• Crypto markets: Polymarket has 5-10x more depth
• Macro/geopolitics: Roughly equal, Kalshi slightly ahead on US-centric events
• Sports: Kalshi dominates (90% of their volume)

# Scan liquidity across both venues
sf liquidity crypto    # Compare crypto orderbooks
sf liquidity oil       # Compare oil/energy orderbooks

Price History

Kalshi: Authenticated candlestick API with OHLC data (1min to daily intervals). Requires API keys.

Polymarket: Public OHLC endpoint (/ohlc) with 1m to 1w intervals. Also has a simpler /prices-history endpoint for time series. No auth required.

Rate Limits

Polymarket is significantly more generous with rate limits.

When to Use Which

Use Kalshi when:

• You need CFTC regulation and USD settlement
• Trading sports or US-centric events
• You want FIX protocol for low-latency trading

Use Polymarket when:

• You need higher liquidity on crypto/geopolitical events
• You want simpler API access (no auth for most data)
• You're building for a global audience

Use both when:

• You want cross-venue edge detection
• You're building an arbitrage strategy
• You want the best price across venues

Tools for Building on Both

# Install SimpleFunctions CLI — works with both venues
npm install -g @spfunctions/cli
sf setup

# Scan both venues at once
sf scan "oil recession iran"

# Deep orderbook analysis
sf book KXWTIMAX-26DEC31-T135

# Liquidity comparison
sf liquidity oil

# Connect to Claude Code via MCP
claude mcp add simplefunctions --url https://simplefunctions.dev/api/mcp/mcp

Conclusion

Neither platform is strictly better. Kalshi wins on regulation and US market depth; Polymarket wins on API ergonomics, rate limits, and crypto/geopolitical liquidity. The best tooling works with both.