SimpleFunctions

Compare · APIs

Prediction Market APIs,
11 Compared.

11 APIs across direct venues and aggregators — Kalshi, Polymarket, Manifold, Metaculus, Opinion, SimpleFunctions, Prediction Hunt, PolyRouter, FinFeedAPI, Adjacent News, Probalytics. Compared by venue coverage, auth model, pricing, and agent compatibility.

APIs compared

11

Direct venues

5

Aggregators

6

First-party MCP

SimpleFunctions only

Quick recommendation

If you want X, use Y.

  • AI agents + thesis + CLI/API + cross-venue + calibration → SimpleFunctions
  • Institutional Kalshi with FIX 4.4 → Kalshi REST + FIX 4.4
  • Polymarket-only on-chain trading → Polymarket CLOB v2
  • Maximum venue breadth (PredictIt + ProphetX + Opinion + K + P) → Prediction Hunt v2
  • Three-venue (K + P + Limitless) clean read API → PolyRouter
  • Enterprise SLA-grade feed → FinFeedAPI
  • News-correlated PM odds → Adjacent News
  • Tick-level HFT + SQL + Parquet → Probalytics
  • Play-money community forecasting → Manifold
  • Long-horizon panel forecasts → Metaculus
  • On-chain OPN ecosystem (Opinion) → Opinion Open API

Side by side

11 APIs, comparison table.

APITypeVenuesAuthPricing
SimpleFunctionsAggregatorKalshi, PolymarketAPI key (anonymous public reads available)Public CLI + REST/Data API free for reads · MCP adapter available · authenticated thesis/intents free up to 15M tokens, then pay-per-token
Kalshi REST + FIX 4.4DirectKalshiRSA-PSS signed requests; FIX 4.4 for institutional connectivityFree for retail use; institutional FIX tier negotiated
Polymarket CLOB v2DirectPolymarketEIP-712 signature + Polygon wallet; gamma read API allows anonymous readsFree for read endpoints; on-chain trading fees applied at settlement
Manifold public APIDirectManifoldAPI key for write; anonymous readsFree; play-money venue (Mana, not USD)
Metaculus APIDirectMetaculusAnonymous reads; key for writeFree
Opinion Open APIDirectOpinionAPI key + Bearer authFree for read; OPN token used for fee-tier discounts
Prediction Hunt v2AggregatorKalshi, Polymarket, PredictIt, ProphetX, OpinionFree + paid tiersFree tier + paid plans
PolyRouterAggregatorKalshi, Polymarket, LimitlessAPI keyTiered — see polyrouter.com/pricing
FinFeedAPIAggregatorPolymarket, Kalshi, Manifold, MyriadAPI key, enterprise SLA contractsEnterprise-tier pricing; quote-driven
Adjacent NewsAggregatorKalshi, Polymarket, Limitless, othersAuth0 + JWT bearerFree + paid tiers
ProbalyticsAggregatorPolymarket, KalshiBearer token (api_xxx:sk_xxx)Tiered — see probalytics.io/pricing

Detail · 11 APIs

Each API, in one paragraph.

SimpleFunctions

https://simplefunctions.dev

KalshiPolymarket

Aggregator

Cross-venue normalized API plus the agent layer above raw data — causal-tree thesis system, autonomous trading (Portfolio Autopilot), live calibration scores, computed indicators (IY/CRI/LAS/EE), cross-venue arb pair detection, CLI command catalog, HTTP tool manifest, and MCP adapter. The only PM API that ships an agent-callable surface end-to-end.

Best for: AI agents and autonomous trading systems that need normalized cross-venue data + thesis decomposition + calibration in one install.

Agents: CLI command catalog, HTTP tool manifest, NL query at /api/public/scan, CrossLinks for AI ingest, MCP adapter

Kalshi REST + FIX 4.4

https://trading-api.readme.io/reference/getting-started

Kalshi

Direct venue

Kalshi's own REST + FIX 4.4 trading API. CFTC-regulated venue. Native source of Kalshi orderbook, candles, trades, settlements. Single-venue surface — no Polymarket data, no normalization, no derived signals.

Best for: Production order routing into Kalshi, institutional trading desks needing FIX, single-venue Kalshi-only systems.

Agents: No first-party MCP; community wrappers exist

Polymarket CLOB v2

https://docs.polymarket.com/developers/CLOB/introduction

Polymarket

Direct venue

Polymarket's native order book API on Polygon. Source of truth for Polymarket orderbook, trades, market metadata. Single-venue surface — no Kalshi data, no calibration, no thesis layer.

Best for: Polymarket-only trading bots, on-chain settlement workflows, builders comfortable with EIP-712 + wallet auth.

Agents: Open-source py-clob-client + community MCPs (guangxiangdebizi, berlinbra)

Manifold public API

https://docs.manifold.markets/api

Manifold

Direct venue

Manifold's play-money community-forecasting API. Excellent for testing forecasting models against a high-volume universe of markets without real-money risk, but Mana-denominated probabilities do not translate directly to USD-pricing decisions.

Best for: Backtesting forecasting models, building public dashboards, training data — anywhere you want PM data without USD-grade calibration.

Agents: No first-party MCP

Metaculus API

https://www.metaculus.com/api/

Metaculus

Direct venue

Metaculus' panel-forecasting REST API. Question text, predictions, community medians, resolution criteria. Different epistemic class from Kalshi/Polymarket — these are forecaster predictions, not market prices on real money.

Best for: Long-horizon forecasts, geopolitical research, training models against well-resolved prediction series, comparing markets vs panels.

Agents: No first-party MCP

Opinion Open API

https://docs.opinion.trade

Opinion

Direct venue

Opinion is an AI-driven on-chain prediction-market protocol that launched its OPN token in March 2026. Reported ~31% share of global PM volume by 2026-Q1. Single-venue surface — markets, prices, orderbook, history.

Best for: Crypto-native applications targeting Opinion liquidity, OPN token holders accessing premium AI insight tiers, on-chain composability use cases.

Agents: No first-party MCP

Full SimpleFunctions vs Opinion Open API comparison →

Prediction Hunt v2

https://predictionhunt.com

KalshiPolymarketPredictItProphetXOpinion

Aggregator

Cross-exchange comparison + arbitrage detection across the broadest venue list on this page (5 venues). 5-minute refresh cadence on the free tier; faster on paid plans. Strong on cross-venue surface area; lighter on derived signals (no calibration, no thesis system, no MCP).

Best for: Quants who want maximum venue coverage and explicit arbitrage detection across PredictIt + ProphetX + Opinion (in addition to Kalshi + Polymarket).

Agents: No first-party MCP

Full SimpleFunctions vs Prediction Hunt v2 comparison →

PolyRouter

https://polyrouter.com

KalshiPolymarketLimitless

Aggregator

Single-API normalized read across Kalshi + Polymarket + Limitless. Developer-focused with one integration point and a uniform schema. Lighter than SimpleFunctions on derived signals — no causal thesis system, no agent-callable thesis surface, no calibration.

Best for: Builders who want a clean three-venue read API and are willing to compose derived signals + calibration themselves on top.

Agents: No first-party MCP

Full SimpleFunctions vs PolyRouter comparison →

FinFeedAPI

https://finfeedapi.com/products/prediction-markets-api

PolymarketKalshiManifoldMyriad

Aggregator

Mature normalized REST API across four venues, built on CoinAPI's enterprise infrastructure. Strong on uptime SLA + production-feed semantics; quiet on derived signals (no MCP, no NL query, no calibration scoring, no thesis system).

Best for: Enterprise customers requiring SLA-grade uptime on a normalized PM data feed and willing to pay for guaranteed availability.

Agents: No first-party MCP

Full SimpleFunctions vs FinFeedAPI comparison →

Adjacent News

https://adj.news

KalshiPolymarketLimitlessothers

Aggregator

News-lens-first PM aggregator across 40,000+ markets. Markets API + Semantic Search + News API + Trade API. Strong on the news-correlation surface; lighter on production-grade derived signals + calibration.

Best for: Newsroom and journalist workflows that want PM odds inline with reporting, semantic search across PM questions, and a TradingView-style chart layer.

Agents: Browser extension; no first-party MCP

Full SimpleFunctions vs Adjacent News comparison →

Probalytics

https://probalytics.io

PolymarketKalshi

Aggregator

Microstructure-first PM data API: REST plus direct ClickHouse SQL access plus Parquet bulk export. 1ms orderbook resolution at 200-500M event updates/day across cited 813K Polymarket + 40M Kalshi markets. Targets quantitative HFT-style buyers.

Best for: HFT or quant teams that need tick-level orderbook history + SQL access + Parquet bulk export and are willing to compose higher-level signals themselves.

Agents: Direct ClickHouse SQL access; planned SSE

Full SimpleFunctions vs Probalytics comparison →

FAQ

Common questions.

What is the best prediction market API in 2026?

Depends on your buyer profile. For AI agents and autonomous trading systems that need normalized cross-venue data plus thesis system plus calibration plus CLI/API-first agent surfaces, SimpleFunctions is the deepest agent-layer surface. For institutional Kalshi order routing with FIX 4.4, the native Kalshi REST + FIX is the right answer. For pure on-chain Polymarket trading, the Polymarket CLOB v2 is canonical. For maximum venue breadth across PredictIt + ProphetX + Opinion in addition to Kalshi + Polymarket, Prediction Hunt v2 covers the most ground. For tick-level HFT data, Probalytics. For SLA-grade enterprise feed, FinFeedAPI. Match the API to the buyer.

Which prediction market APIs cover both Kalshi and Polymarket?

Several aggregators normalize across both. SimpleFunctions, Prediction Hunt v2, PolyRouter, FinFeedAPI, Adjacent News, and Probalytics all cover Kalshi + Polymarket from a single integration. They differ in additional venues (Prediction Hunt adds PredictIt + ProphetX + Opinion; PolyRouter adds Limitless; FinFeedAPI adds Manifold + Myriad; Adjacent News covers many venues), in derived signals (only SimpleFunctions ships calibration, thesis system, and CLI/API-first agent surfaces), and in pricing model.

Is the Kalshi API free?

Yes for retail developers. Kalshi exposes a REST API with public read access for markets, events, candles, and orderbook. Authenticated trading endpoints require RSA-PSS signed requests with a generated key pair. FIX 4.4 connectivity is offered to institutional clients on negotiated terms. Aggregators (SimpleFunctions, PolyRouter, etc.) reduce the auth boilerplate and add cross-venue normalization at the cost of a small layer between you and Kalshi.

Is the Polymarket API free?

Read access is free through the gamma API and the public CLOB endpoints. Trading on Polymarket requires an EIP-712 signature from a Polygon wallet and is subject to on-chain fees at settlement. The official Polymarket py-clob-client is the canonical SDK. For a normalized read API across Polymarket plus other venues, aggregators in this list reduce wallet + EIP-712 boilerplate to a simple HTTP call.

What is the difference between a direct venue API and an aggregator API?

A direct venue API (Kalshi, Polymarket, Manifold, Metaculus, Opinion) is the venue's own native interface — source of truth for that one venue. An aggregator (SimpleFunctions, PolyRouter, FinFeedAPI, etc.) sits above multiple direct venue APIs and exposes a normalized schema. Aggregators reduce per-venue auth/schema friction; direct APIs minimize the layers between your code and the underlying venue. For trading execution, direct APIs are usually right. For research, agents, or cross-venue analysis, aggregators almost always win.

Which prediction market APIs ship MCP servers for AI agents?

SimpleFunctions is CLI/API-first: the installed `sf` catalog is the canonical local surface and `/api/tools` is the HTTP manifest for remote agents. It also ships an MCP adapter for hosts that require MCP, spanning Kalshi + Polymarket reads, screener over computed indicators, calibration scores, causal-tree thesis system, and authenticated intent execution. Several open-source MCP servers exist for individual venues — see the dedicated MCP comparison at /compare/prediction-market-mcp-servers.

How do I check the calibration of a prediction market API?

For SimpleFunctions, GET /api/calibration returns live Brier scores by venue, category, and price bucket — currently Kalshi 0.20, Polymarket 0.12 on T-24h price over the past 90 days. The score is computed against resolved contracts; you can re-verify with curl. Most other APIs on this list do not publish calibration data; if calibration is load-bearing for your decisions, SimpleFunctions is the only API in this comparison that exposes it as a programmatic endpoint.

What about FIX or low-latency feeds?

Kalshi offers FIX 4.4 for institutional clients on negotiated terms. Probalytics offers 1ms orderbook resolution via REST + ClickHouse SQL with planned SSE streaming. For direct WebSocket feeds, Polymarket's public WebSocket is canonical for that venue. SimpleFunctions sits at REST + 5-minute indicator snapshot cadence — appropriate for agents and research, not for HFT order routing.

Try the SimpleFunctions API.

Public reads require no auth. Authenticated thesis system, intents, and Portfolio Autopilot are free up to 15M tokens. Cross-venue normalized data + CLI/API surface + calibration in one install.

curl https://simplefunctions.dev/api/public/markets
Get API KeySee the API →
All comparisonsMCP serversSimpleFunctions vs each toolLive calibrationSimpleFunctions API docsReal-time data API