What Is a Prediction Market API? A Complete Beginner's Guide
The Short Answer
A prediction market API lets software read live probabilities, order books, and price history from platforms like Polymarket and Kalshi, and often place trades. Reading data is usually free: Polymarket doesn't even require an API key. Below: how these APIs work, venue vs multi-venue, and how to make your first call in minutes.
A prediction market API is an interface that lets software read data from prediction market platforms, and often trade on them, without a human clicking through a website. Instead of opening Polymarket in a browser to check the odds on a Fed decision, your code requests that price directly and gets back structured data it can store, compare, chart, or act on.
If you are a developer, trader, or researcher hearing about prediction markets for the first time, this guide covers everything you need: what these APIs actually expose, how they work under the hood, the difference between venue APIs and multi-venue APIs, what people build with them, and how to make your first call without spending anything.
Key Takeaways
- A prediction market API gives software access to market prices, order books, historical data, and often trading on venues like Polymarket, Kalshi, and SX Bet.
- Reading data is cheap or free: Polymarket and SX Bet serve market data with no API key at all, though Kalshi gates its order book endpoint and its WebSocket behind signed keys even for public data.
- The key architectural choice is venue API vs multi-venue API: direct integration gives the deepest access to one exchange, while an aggregation layer normalizes many venues into one schema.
What Is a Prediction Market API?
A prediction market lets people trade contracts on real-world outcomes. A YES share in "Will the Fed cut rates in September?" pays $1.00 if the cut happens and nothing if it does not, so its price is the market's probability estimate. When that share trades at $0.62, the crowd is saying 62 percent.
An API, or application programming interface, is the machine-facing version of a website. The venue exposes endpoints, which are structured addresses your code can call over the internet, and each endpoint returns data in a predictable format, usually JSON. One endpoint lists all active markets. Another returns a specific market's current price and order book. Another returns its price history.
Put together, a prediction market API turns "check the odds" from a browsing task into a data feed. That single change is what makes everything downstream possible: dashboards that track hundreds of markets, bots that trade on price moves, and models trained on years of probability history.
How Prediction Market APIs Work
Almost every venue exposes two kinds of access. REST endpoints answer one question per request: give me the list of markets, give me this market's price, give me its candles. They are simple and perfect for research, dashboards, and anything that can tolerate data being a few seconds old. WebSocket streams hold a connection open and push updates to you the moment something changes: a new trade, an order book update, a price move. Trading systems and alerting need streams, because polling REST in a loop is always late.
Authentication varies more than beginners expect, and it splits into two separate questions: what you need to read, and what you need to trade. The table below was checked against each venue's official documentation in August 2026; auth models change, so treat the venue's own docs as the live source.
| Venue | To read market data | To trade |
|---|---|---|
| Polymarket (global) | Nothing, including the WebSocket market channel | Crypto wallet signature (EIP-712) |
| SX Bet | Nothing for REST; key only for the realtime token | Crypto wallet signature (EIP-712) |
| Kalshi | Most REST reads public; order book and WebSocket need keys | API key plus per-request signatures |
| Opinion | Public for markets, prices, and order book; key beyond that | API key plus SDK signing |
The pattern to notice: crypto-native venues authenticate trading with wallet signatures, meaning your code signs orders with a private key and funds stay in your own wallet. Regulated exchanges like Kalshi use accounts and API keys, and your funds sit at the exchange. Neither is better in general; they are different custody models, and which one suits you depends on where you are and what you are building.
Two things catch people out. Kalshi's open REST reads cover series, events, and markets, but its order book endpoint is documented as needing the same signed headers as trading, and there is no unauthenticated WebSocket, so "public REST" does not mean "public depth". And Polymarket now means two products: the global onchain platform in the table above, and Polymarket US, a separate CFTC-regulated exchange with its own base URL and Ed25519 keys issued after KYC rather than wallet signatures. Our Polymarket API guide covers that split in detail; code written against one stack will not run against the other.
Venue APIs vs Multi-Venue APIs
A venue API is operated by a single exchange and gives the deepest access to that exchange: every order type, every data field, every venue-specific feature. Kalshi's FIX connectivity for institutions and Polymarket's order book internals are only reachable through their own APIs. If your product lives on one venue, direct integration is the obvious choice.
The catch appears the moment you care about more than one venue. The same real-world event now trades on Polymarket, Kalshi, Opinion, Limitless, SX Bet, and others simultaneously, and the roster keeps growing: DraftKings opened its own exchange, DKeX, in June 2026. A new venue does not always arrive with a public API, but each one that does adds another set of identifiers, price formats, and schemas. Building against five venues means five integrations, five formats to reconcile, and five APIs to monitor for breaking changes.
A multi-venue API exists to absorb that problem: one integration that returns normalized data across platforms, ideally with matched markets, meaning it knows that two differently titled contracts on different venues settle on the same outcome. Open-source options like PMXT wrap many venues behind a CCXT-style SDK. Our own product, Predictefy, takes the same idea further with matched markets and normalized data across 12 venues. For a full comparison of both approaches, see our ranking of the best prediction market APIs and SDKs.
What Can You Build With a Prediction Market API?
The common thread in everything below is that prediction market prices are structured probability data, which very few other data sources provide about the future.
Trading bots and market makers read streams and place orders programmatically, from simple "buy when the price crosses X" logic to full market-making systems quoting both sides of a book. Arbitrage scanners compare the same event across venues and flag price gaps large enough to profit from. Research dashboards and backtests use historical prices to study how probabilities moved before elections, rate decisions, and games, and to validate strategies before risking money. AI and forecasting agents consume market probabilities as input signals, an increasingly common pattern now that agents can call APIs directly. Probability trackers power the "chance of X" widgets appearing in news products and newsletters.
How to Get Started in Three Steps
Step 1: Pick a venue with free reads. Polymarket is the standard starting point because its market data, including the streaming WebSocket channel, requires no account and no API key. You can be looking at live order books within minutes of opening the docs.
Step 2: Make your first two calls. Fetch the market list, pick one market, and fetch its price. Every venue's documentation has a variant of this quickstart. Once you can print one live probability in your terminal, you understand the shape of the whole API.
Step 3: Upgrade to streams when polling hurts. The moment your use case cares about being seconds late, switch the price fetch to the WebSocket feed. That is also the point to think about whether you will stay single-venue or need normalized data across several, which is the decision covered in our API comparison guide.
One caveat before you build the trading half: reading data is open to anyone, but trading is not. At the time of writing, Polymarket's global platform restricts more than 30 jurisdictions, including the UK and Singapore, and its published geoblock reference marks most of them close-only rather than fully blocked: users there can close existing positions but not open new ones, on the API as well as the front end. Ontario, Alberta, British Columbia, and Quebec sit in that close-only group. A 2022 CFTC settlement keeps US users off the global platform, which is why the separate Polymarket US exchange exists. Kalshi is CFTC-regulated and US-first, though it began expanding abroad in 2026, starting with Brazil. These lists move, so check the venue's own eligibility docs before you write order placement. None of this is financial, tax, or legal advice.
Frequently Asked Questions
Are prediction market APIs free to use?
Reading data is free on most venues, and Polymarket and SX Bet do not even require an API key for market data. Kalshi's REST reads are free too, though its order book endpoint and its WebSocket both need signed credentials. Costs appear when you trade, through venue fees and spreads, or when you need premium data such as deep history or normalized multi-venue feeds from a dedicated provider.
Do I need a crypto wallet to use a prediction market API?
Only for trading on onchain venues like Polymarket's global platform, Limitless, and SX Bet, where orders are signed with a wallet key. Reading data never requires a wallet, and regulated exchanges use ordinary accounts with API keys instead: that covers Kalshi and also the separate Polymarket US exchange, whose keys are issued after KYC.
What is the difference between a prediction market API and an SDK?
The API is the interface itself, the endpoints your code calls over the internet. An SDK is a client library in a specific language, such as Python or TypeScript, that wraps those endpoints in convenient functions and handles details like authentication and signing for you. Good SDKs shorten integration from days to hours.
Can AI agents use prediction market APIs?
Yes, and it is one of the fastest-growing uses. Market prices are structured probability estimates about future events, which makes them natural input signals for forecasting agents, research assistants, and trading agents that can call APIs directly.
Can I use a prediction market API from any country?
Reading market data usually has no geographic gate, since public endpoints answer wherever you call them from. Trading does. Polymarket's global platform restricts more than 30 jurisdictions, including the UK and Singapore, US users trade on the separate Polymarket US exchange instead, and Kalshi is a CFTC-regulated US venue that only began expanding abroad in 2026. Check the venue's own eligibility page before you write order placement. None of this is legal advice.
Conclusion
A prediction market API is the difference between looking at the odds and building with them. The data is unusually accessible for a financial market: several venues serve prices and order books with no key at all, so the cost of experimenting is an afternoon, not a budget. Start with one venue's quickstart, print a live probability, and go from there.
When you outgrow one venue, the landscape question becomes which API fits your stack, and that is exactly what our guide to the 7 best prediction market APIs and SDKs in 2026 compares in depth: coverage, real-time feeds, historical data, and trading support across every major option.