A prediction market API is the only way to build anything real in this space — a trading bot, a portfolio tracker, an arbitrage scanner, a Discord alert feed. The two dominant platforms, Polymarket and Kalshi, now handle roughly 97.5% of the industry's volume, and both ship developer-facing APIs that expose order books, historical prices, and trade execution. That's a meaningful shift from two years ago, when most "prediction market data" meant scraping a webpage or paying for a third-party feed of uncertain quality.
This guide ranks the best prediction market APIs available right now, covers what each one actually lets you build, and flags the gaps that trip up new developers. Combined monthly volume across the industry hit roughly $20 billion in February 2026, with 840,000 monthly active wallets according to TRM Labs — enough liquidity and enough users that building tools on top of these markets is now a legitimate business, not a hobby project. We'll cover Polymarket's CLOB and Gamma APIs, Kalshi's Trade API, the on-chain data layer through Dune Analytics, and where the smaller platforms stand.
We evaluated each API on documentation quality, rate limits, authentication complexity, real-time data (WebSocket vs. polling), and whether it supports order execution or read-only data. If you're deciding where to build first, start with the platform that matches your use case rather than the one with the biggest brand name — a sports-betting bot has different needs than a macro dashboard.
Polymarket API: CLOB and Gamma for On-Chain Builders
Polymarket runs two separate public APIs, and understanding the split matters before you write a line of code. The CLOB API (Central Limit Order Book) handles live order book data, order placement, and trade execution — this is what you use to build a trading bot. The Gamma API is the read-only data layer: market metadata, historical prices, event details, and resolution data, which is what most dashboard and analytics builders actually want.
Both are REST-based with WebSocket support for live order book updates, and both are free to query at reasonable volumes — Polymarket doesn't charge for API access itself, though every trade you execute still eats the platform's taker fees of 0.75-1.80% depending on category (fee tiers expanded March 30, 2026, and you should read our breakdown of Polymarket's fee structure before wiring up automated execution). Maker orders remain free with 20-50% rebates, which matters enormously if you're building a market-making bot rather than a directional one.
Concrete example: a builder pulling live odds for a World Cup dashboard hits the Gamma API for market lists and current prices, refreshes every 15-30 seconds, and never touches the CLOB API at all — no wallet, no signing, no gas. A builder running an automated arbitrage strategy between Polymarket and Kalshi needs the CLOB API on the Polymarket side for order placement, plus a signed wallet connection via Polygon, since settlement runs in USDC on Polygon.
The catch: authentication for trade execution requires wallet signing and API key generation tied to your Polymarket account, which is more friction than a typical SaaS API key. Order book depth on lower-volume markets can be thin enough that your bot's own orders move the price. And because Polymarket is pursuing a CFTC license through its $112 million QCEX acquisition, expect API terms and geo-restrictions to keep shifting through 2026 — build with that instability in mind. For general strategy context before you automate anything, see our guide on prediction market strategies.
| Metric | Polymarket API |
|---|---|
| Type | REST + WebSocket |
| Auth | Wallet signing / API key |
| Execution | Yes (CLOB) |
| Data-only tier | Yes (Gamma) |
| Cost | Free access, standard trading fees apply |
| Settlement | USDC on Polygon |
Kalshi API: The Regulated Choice for US-Based Builders
Kalshi's Trade API is the most "traditional fintech" of the group — REST endpoints, WebSocket streaming for live market data, and a structure that will feel familiar if you've worked with a brokerage API before. As a CFTC-regulated designated contract market, Kalshi requires full account verification before you can execute trades through the API, which adds onboarding time but also means the data you're pulling comes from a fully regulated exchange rather than a crypto-native order book.
Fees on Kalshi run around $0.02 per contract on a variable schedule rather than a percentage taker fee, which changes your cost modeling if you're building a bot that trades small positions across many contracts. Idle cash in a Kalshi account earns roughly 4% APY, a detail worth building into any automated system that holds cash between trades rather than staying fully deployed. Kalshi posted around $260 million in 2025 revenue and closed a $1 billion Series E at an $11 billion valuation in December 2025 — a signal the platform is investing heavily in infrastructure, including developer tools.
Concrete example: a builder creating a sports-focused alert bot pulls Kalshi's WebSocket feed for live price moves on NFL or NBA contracts, sets a threshold trigger (say, a 5-point swing in implied probability within 10 minutes), and pushes alerts to a Telegram channel — no execution required, so verification friction doesn't matter. A builder running an automated market-neutral strategy across both exchanges needs full KYC on Kalshi plus the wallet setup on Polymarket, effectively doubling onboarding time before the bot ever places a trade.
The catch: Kalshi's regulatory status is a double-edged sword for API builders. Availability in 40+ states is a strength, but state-level legal fights — Kalshi faces active challenges in Nevada, Massachusetts, Maryland, and Ohio, plus criminal charges filed in Arizona on March 18, 2026 — mean your API integration could face sudden geo-restrictions depending on where your users sit. Check our prediction market legality guide before shipping a product to users in disputed states. For a full side-by-side on the underlying platforms rather than just the APIs, our Polymarket vs. Kalshi comparison covers fees, markets, and UX in depth.
| Metric | Kalshi API |
|---|---|
| Type | REST + WebSocket |
| Auth | Full account KYC required |
| Execution | Yes |
| Data-only tier | Yes |
| Cost | ~$0.02/contract, variable |
| Settlement | USD |
Dune Analytics: The On-Chain Data Layer Behind Polymarket
Dune Analytics isn't a trading API — it's the data infrastructure layer that most serious Polymarket builders end up using alongside the official API. Because Polymarket settles on Polygon, every trade, resolution, and wallet interaction is public on-chain data, and Dune's SQL-based query engine lets you build custom dashboards tracking whale wallets, volume by category, or resolution accuracy without waiting on Polymarket's own endpoints to expose that data.
Concrete example: a builder wants to track which wallets consistently trade ahead of major price moves on political markets — a whale-tracking dashboard. Polymarket's own API won't easily surface "top wallets by realized PnL," but a Dune query against the on-chain transaction history can. This is also the layer researchers use to verify Polymarket's reported Brier scores (around 0.09, implying 94%+ resolution accuracy) independently rather than trusting the platform's own marketing.
The catch: Dune only covers on-chain platforms, so it's useless for Kalshi, which settles off-chain in USD through a regulated clearinghouse. Query performance and cost scale with complexity, and building genuinely useful dashboards requires real SQL skill, not just API-key copy-paste. See our full breakdown of Dune Analytics for prediction markets for query examples and dataset structure.
Emerging Platform APIs: Predict.fun, PredictStreet, and the Long Tail
Beyond the two giants, a handful of smaller platforms are building out developer access, though documentation and stability lag well behind Polymarket and Kalshi. Predict.fun, running on BNB Chain with zero fees and yield on collateral, has on-chain data that's technically accessible the same way Polymarket's is — through blockchain explorers and emerging Dune-style query layers — but it lacks a mature first-party trading API as of early 2026.
PredictStreet, the Gibraltar-licensed platform that landed a FIFA World Cup partnership, is still pre-launch on the API front, and OG (Crypto.com's prediction market) and Opinion currently offer limited or no public developer access. If you're building for the World Cup cycle specifically, our guide to the best prediction markets for World Cup 2026 covers which platforms are live now versus which are still building out infrastructure.
The catch: building on a pre-launch or undocumented API means your integration could break with zero notice and no changelog. Treat these as speculative bets on the platform's future, not production infrastructure, until you see stable, versioned endpoints and real developer support channels.
Master Comparison: Prediction Market APIs Side by Side
| Platform | API Type | Execution | Real-Time Feed | Auth Complexity | Best For |
|---|---|---|---|---|---|
| Polymarket (CLOB + Gamma) | REST + WebSocket | Yes | Yes | Medium (wallet signing) | On-chain bots, dashboards |
| Kalshi (Trade API) | REST + WebSocket | Yes | Yes | High (full KYC) | Regulated US trading products |
| Dune Analytics | SQL query engine | No | Near real-time | Low (API key) | On-chain analytics, whale tracking |
| Predict.fun | Limited/on-chain | No official API | Partial | Medium | Experimental, low-fee builds |
| PredictStreet | Not yet public | No | No | N/A | Future World Cup integrations |
How to Choose the Right API for What You're Building
If you're building a trading bot that needs to execute orders automatically, you need either the Polymarket CLOB API or Kalshi's Trade API — and probably both if your strategy is cross-platform arbitrage. Automated strategies deserve real scrutiny before you deploy capital through them; our piece on AI trading bots eating prediction markets covers how much of current volume is already bot-driven and what that means for your edge.
If you're building a dashboard, alert system, or portfolio tracker with no execution requirement, start with read-only endpoints — Polymarket's Gamma API or Kalshi's data feed — since you avoid KYC friction and wallet-signing entirely. If you want to build something like a copytrading tool that mirrors a specific wallet's positions, you'll need both the on-chain data layer (Dune or direct Polygon indexing) and the CLOB API for execution; see our guide on copytrading on Polymarket for how that mechanic works from the trader side before you automate it.
If you're building anything that touches user funds — deposits, withdrawals, or automated position sizing — read our guides on depositing on Polymarket and withdrawing from Polymarket first, since API-driven wallet flows inherit all the same fee and settlement quirks as manual trading.
The Honest Reality Check for API Builders
Neither Polymarket nor Kalshi has a stable, long-term public API contract the way Stripe or Plaid does. Polymarket is mid-acquisition of a CFTC license through QCEX and actively courting a $20 billion valuation, which means its API terms, fee schedule, and geo-restrictions are all moving targets through 2026. Kalshi faces active state-level legal challenges in Nevada, Massachusetts, Maryland, and Ohio, plus criminal charges in Arizona filed March 18, 2026 — any of these could force sudden API access changes for users in those states, regardless of what your code assumes today.
If your bot or product generates trading profits, the tax picture is unresolved and genuinely messy. The IRS has issued zero formal guidance on prediction market taxation, Kalshi issues a 1099-MISC while Polymarket issues nothing globally, and a new 90% gambling-loss cap under OBBBA starts affecting tax year 2026 filings. Build your reporting features around our prediction market tax guide rather than guessing, because "the API doesn't tell me my tax treatment" is not a defense in an audit.
For a broader view of how these markets function before you commit engineering time, read our complete guide to how prediction markets work, and compare platform quality overall in our ranking of the best prediction market apps. If insider trading and market integrity affect how you design resolution-tracking features, our insider trading regulation breakdown is worth a read before you ship anything that trusts market prices as clean signal.
Frequently Asked Questions
Is there a free prediction market API?
Yes. Both Polymarket's Gamma API and Kalshi's data endpoints offer free read-only access to market data, prices, and history. You only pay standard trading fees once you execute orders — see our Polymarket fees breakdown for the exact schedule.
Does Polymarket have a public API?
Polymarket runs two public APIs: the CLOB API for order book data and trade execution, and the Gamma API for read-only market metadata and historical prices. Trade execution requires wallet signing tied to your Polymarket account, and settlement runs through USDC on Polygon.
Does Kalshi have a trading API?
Yes, Kalshi's Trade API supports both REST and WebSocket connections for live data and order execution. Because Kalshi is a CFTC-regulated exchange, full account verification (KYC) is required before you can place trades through the API, unlike Polymarket's more crypto-native onboarding.
Can I build a trading bot on prediction markets?
Yes, using either Polymarket's CLOB API or Kalshi's Trade API for execution. A meaningful share of current volume is already bot-driven — see our analysis of AI trading bots in prediction markets before assuming a simple bot will find easy edge.
What's the best data source for on-chain prediction market analytics?
Dune Analytics is the standard tool for querying Polymarket's on-chain activity, since the platform settles on Polygon and every transaction is publicly queryable. It's not useful for Kalshi, which settles off-chain in USD through a regulated clearinghouse.
Do I need a license to build tools on prediction market APIs?
No license is required to build read-only tools like dashboards or alert systems. If you're building something that handles user funds or executes trades on their behalf, review our guide on prediction market legality in the US, since state-level restrictions vary and are actively shifting in 2026.
Which API is better for arbitrage bots, Polymarket or Kalshi?
You need both if you're arbitraging between the two exchanges, since price discrepancies emerge specifically from differences in liquidity and fee structure across platforms. Our step-by-step arbitrage guide covers the mechanics and realistic margins involved.
Are prediction market API terms stable enough to build a real business on?
Not fully. Polymarket is mid-acquisition of a CFTC license and adjusting fee tiers as recently as March 30, 2026, while Kalshi faces active state-level legal challenges that could force sudden access changes. Build with version pinning, error handling for sudden API changes, and a plan for regulatory shifts rather than assuming today's terms hold through the year.
