Is there one API that covers every prediction market?

No official one, and the best-known startup attempt, Dome, was acquired by Polymarket and shut down in April 2026. Today the strongest open option is PMXT, an open-source CCXT-style library covering 15+ venues. For trading rather than building, Kairos aggregates Kalshi, Polymarket and Predict.fun into one live order book without any code.

Which prediction market APIs have a sandbox?

Kalshi has a full demo environment at external-api.demo.kalshi.co and Predict.fun has a testnet. The global Polymarket CLOB has no sandbox at all, so the first real test of an integration is a real trade with real funds.

Are prediction market APIs free?

Venue APIs are free to access at standard tiers; you pay trading fees, not API fees. Commercial data vendors that sell historical order books and normalized feeds charge subscriptions. Kairos, the cross-venue terminal, is free to use.

Best Prediction Market APIs (2026)

Q: Does Kalshi have an official API?

Yes. Kalshi ships REST, WebSocket and FIX APIs for both its event contracts and its perps product, documented at docs.kalshi.com. Auth is an RSA key pair with signed requests, there is a full demo environment, and rate limits use a token-bucket tier system from Basic (200 reads per second) up to Prestige.

Q: Does Polymarket have an API?

Yes, several: the CLOB API for trading, the Gamma API for market metadata, a Data API for positions and holders, and WebSocket feeds. Note that the global Polymarket CLOB blocks order placement from US and UK IPs, and Polymarket US is a separate CFTC-regulated exchange with a completely different API and Ed25519 key auth.

Eleven venue APIs, the SDKs that still work, the aggregators, and the auth models, all verified against live documentation.

Updated August 30, 2026 · every fact verified against live docs or a linked issue

Short answer: build on one venue with the official APIs (Kalshi for regulated US contracts, Polymarket for the largest book), go cross-venue with open-source PMXT, and if you would rather trade than integrate, Kairos puts every venue in one order book.

What changed in 2026 · breaks most older guides

Dec 2025

Polymarket US launched: a separate CFTC-regulated exchange with a completely different API and Ed25519 keys.

Apr 2026

Kalshi moved to token-bucket rate tiers, Basic (200 reads/sec) through Prestige (10,000).

Apr 28

Dome, the YC-backed unified API, acquired by Polymarket and shut down.

Apr 28

Polymarket exchange upgrade: collateral switched USDC.e to pUSD, breaking older integrations.

May 25

py-clob-client archived and non-functional. Use Polymarket/py-sdk instead.

The directory

19 APIs, SDKs and vendors · filter by type

Kairos Cross-venue trading terminal: Kalshi, Polymarket and Predict.fun in one order book, with sub-second data and advanced order types. K + PM + Predict.funfreea16z-backed
Kalshi Trading API REST, WebSocket and FIX for event contracts and perps. RSA-PSS auth, real demo environment, token-bucket rate tiers from 200 to 10,000 reads/sec. sandbox ✓REST + WS + FIXfree
Polymarket CLOB + Gamma The largest global book: CLOB for trading, Gamma for metadata, Data API, WebSocket feeds. EIP-712 + HMAC auth. US and UK IPs blocked from ordering; no sandbox. no sandboxgeo-blocked ordersfree
Polymarket US API A separate CFTC-regulated exchange with a completely different API: ~23 REST endpoints, two WebSocket feeds, Ed25519 key signing, KYC-verified US users. US only, KYCofficial SDKs
Predict.fun API Orderbook markets on BNB Chain. API-key auth, a real testnet needing no key, 240 req/min default, official TypeScript and Python SDKs. testnet ✓REST + WS
Novig API P2P sports exchange: REST, WebSocket and GraphQL with OAuth 2.0 and a downloadable OpenAPI 3.1 spec. Rate limits unpublished. REST + WS + GraphQL
ProphetX API Regulated P2P sports exchange: four purpose-built APIs, limit orders, real-time odds, 99.5%+ uptime SLO. You must be located in an operating state. REST + WSstate-gated
Manifold API Play-money forecasting with the friendliest API: plain key auth, 500 req/min, WebSocket, and the entire platform is open source. play moneyfree
Metaculus API Forecast probabilities rather than tradable markets. Token auth, full schema published. Great for research pipelines. researchfree
IBKR ForecastEx Event contracts through the full Interactive Brokers stack (TWS API and Web API). Buy-only contracts with netted exits; the heaviest setup of any venue. TWS + Web APIpaper trading ✓
Limitless API Prediction markets on Base: REST and WebSocket, wallet-signature auth with EIP-712 orders, CLOB plus AMM markets, TypeScript SDK. REST + WS
PredictIt (older API) Read-only US politics odds, no auth, roughly one request per minute. Fine as a free ticker, nothing more. read-onlyfree
PMXT The CCXT of prediction markets: open-source unified library, 2,100+ stars, Python and TypeScript, 15+ venues including Kalshi and both Polymarkets. 15+ venues2.1k ★MIT
PolyRouter Data-only unified API across Polymarket, Kalshi, Manifold, Limitless, SX Bet, Novig and ProphetX, with an MCP server. Free tier. 7 venuesfree tier
polymarketdata.co Commercial history: minute-level prices and L2 order book snapshots from Aug 2025, 19B+ rows, REST API, Python SDK and S3 dumps. Used by academic researchers. Polymarketpaid tiers
Polymarket blockchain data Polymarket's own pointer page to free on-chain analysis: the official subgraph, Goldsky pipelines and datasets, and Dune. on-chainfree
Polymarket/py-sdk The official successor to the archived py-clob-client: pip install polymarket-client. 115 stars, 621 commits, active. Pythonofficial
kalshi-python-sdk The most complete community Kalshi SDK: 107 of 108 REST operations, 11 typed WebSocket channels, perps support, async FIX engine. Pythoncommunity
pykalshi pip install pykalshi: WebSocket streaming, retries, pandas integration for Kalshi. Pythoncommunity
polymarket-apis (PyPI) Community unified Polymarket library: CLOB, Gamma, Data, Web3, WebSocket and GraphQL in one, Pydantic-validated. Pythoncommunity

Auth models, side by side

Venue	Scheme	Timestamp unit	Classic failure
Kalshi	RSA-PSS signed requests	Milliseconds	Signing the query string (strip it), or mixing demo and prod keys
Polymarket	EIP-712 (L1) + HMAC-SHA256 (L2)	Seconds	Wrong signatureType for your wallet kind; keys bound to the EOA, not the proxy or deposit wallet
Polymarket US	Ed25519 signed headers	Per docs	Assuming it shares anything with the global CLOB. It does not.
Predict.fun	API key	n/a	Forgetting the testnet needs no key but mainnet does
Manifold	Plain key header	n/a	Rate limit is per IP, not per key

Gotchas the docs will not tell you

Polymarket's recommended wallet flow rejects every order in the official SDKs. With signatureType 3, L1 auth binds the API key to the EOA instead, and orders bounce with the order signer address has to be the address of the API KEY.
py-clob-client-v2 #70, same class in #64 and clob-client-v2 #65

Polymarket WebSockets go zombie. The stream can stop delivering after roughly 20 minutes while ping-pong keeps succeeding. Track the last real message and force-reconnect.
real-time-data-client #26, filter bug in #34

Kalshi order books can poison themselves. The stream sends a snapshot only on subscribe, so one dropped frame and a local book runs against emptiness until you detect and resubscribe.
kalshi-python-sdk #189

Gamma returns numbers as strings. outcomePrices arrives as a JSON-encoded string you must parse, and market orders enforce different decimal precision for maker and taker sides.

No venue shares identifiers. A Kalshi ticker and a Polymarket condition ID for the same event have nothing in common, so every cross-venue project rebuilds market matching from scratch. The biggest hidden cost of going multi-venue, and the reason the acquired-and-killed Dome existed.

read the docs→handle auth→match markets→execute

Official APIs for building. One book for trading.

Rather trade the spread than build the plumbing? The free Kairos terminal gives you Kalshi, Polymarket and Predict.fun in one order book.

FAQ

Does Kalshi have an official API?

Yes: REST, WebSocket and FIX for both event contracts and perps, with a demo environment and an OpenAPI spec at docs.kalshi.com. Auth is an RSA key pair with signed requests.

Does Polymarket have an API?

Yes, several: CLOB for trading, Gamma for metadata, a Data API, and WebSocket feeds. Order placement is geo-blocked for US and UK IPs on the global exchange; Polymarket US is a separate venue with its own incompatible API.

Is there one API for all prediction markets?

No official one, and Dome, the best-known attempt, was acquired and shut down in April 2026. PMXT is the strongest open-source unified library; Kairos is the no-code terminal answer for traders.

Which venues have a sandbox?

Kalshi (full demo environment) and Predict.fun (testnet). The global Polymarket CLOB has none, so test small and real.

Are these APIs free?

Venue APIs cost nothing beyond trading fees at standard tiers. Commercial data vendors charge for historical depth. Kairos is free to use.