Best Prediction Market APIs for Automated Trading (2026)

TL;DR

The best prediction market API stack depends on what you are building. Raw platform APIs (Kalshi REST/WebSocket, Polymarket CLOB) are required for placing orders. ArbBets API is the best data layer for cross-platform arbitrage — it normalizes Polymarket, Kalshi, and Opinion, matches equivalent events, and returns fee-aware spreads in one call. Arbitrage Agent suits traders who want an execution API, not just market data. Most production bots use platform APIs for execution plus ArbBets or custom matching logic for discovery.

Last updated: July 4, 2026

What We Evaluated

We compared APIs on what bot builders actually need:

1. Order placement — Can you create, cancel, and monitor positions programmatically?
2. Market data — REST snapshots, WebSocket streams, full orderbook depth?
3. Cross-platform matching — Does it link the same event across Polymarket, Kalshi, and Opinion?
4. Arbitrage math — Pre-computed spreads, fee models, fill simulation?
5. Documentation & SDKs — Official client libraries, examples, sandbox/paper trading?

Best Prediction Market APIs Compared (2026)

ArbBets API surfaces 100+ cross-platform arbitrage opportunities daily. Premium includes 5,000 API credits per month. Primary Polymarket pair endpoints cost 1 credit per call — enough for polling every few minutes around the clock.

— Source: ArbBets real-time market data

API Breakdown

1. ArbBets API — Best for Cross-Platform Arbitrage Data

The ArbBets API is a read-only data API for prediction market arbitrage. It does not place trades. It returns matched pairs, full orderbooks, optimal YES/NO legs, ROI after fees, and deep links to each platform — so your agent can focus on execution logic instead of market matching.

Key endpoints:

Authentication:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://getarbitragebets.com/api/internal/kalshi-polymarket-full?investment=100&min_profit=1.0"

Best for: Developers building prediction market bots, Discord/Telegram alert systems, or autonomous agents that need normalized cross-platform arb data without maintaining three integrations.

Pros:

• Market matching across Polymarket, Kalshi, and Opinion handled server-side
• Fee-adjusted ROI and orderbook-aware fill data included
• 1-credit endpoints enable high-frequency polling
• Links to full Python examples

Cons:

• Does not execute trades — you still need Kalshi and Polymarket credentials
• Premium plan required ($299/month)
• Not a substitute for native exchange APIs when placing orders

Pricing: Premium $299/month — scanner + API, 5,000 credits/month.

2. Kalshi API — Best Regulated US Execution API

Kalshi provides a full REST and WebSocket API for US-regulated prediction market trading. Official Python SDK (kalshi-python) supports market data, order placement, position monitoring, and a paper-trading environment for testing.

Capabilities:

• Read markets, orderbooks, and positions
• Place and cancel limit orders via REST
• Stream real-time prices over WebSocket
• Paper trading for strategy validation

Best for: US-based bots that execute on a CFTC-regulated exchange with USD settlement.

Pros:

• Free API access with a funded account
• Strong documentation and official SDK
• Paper trading reduces live-capital risk during development

Cons:

• Kalshi-only — no Polymarket or Opinion data
• Taker fees (up to ~7%) must be modeled in your arb math
• Position limits cap exposure per market

Pricing: Free API; trading fees apply per contract.

3. Polymarket CLOB API — Best for Crypto-Native Execution

Polymarket uses a Central Limit Order Book (CLOB) on Polygon. The API supports reading orderbooks, placing signed orders, and managing positions via the official py-clob-client Python library.

Capabilities:

• Full orderbook depth (YES/NO asks and bids)
• Programmatic order creation and cancellation
• On-chain settlement via USDC

Best for: Bots executing on the highest-liquidity prediction market, especially politics and crypto event contracts.

Pros:

• Zero trading fees on most markets
• Deepest liquidity in the prediction market ecosystem
• Active algorithmic trading community

Cons:

• Crypto wallet and USDC management required
• US access restrictions — check current terms and eligibility
• No built-in cross-platform matching with Kalshi

Pricing: Free API; gas/network costs minimal on Polygon.

4. Opinion API — Best for Third-Platform Coverage

Opinion is a growing prediction market platform. API access exists but documentation is less mature than Kalshi or Polymarket. Useful when the best arb leg is Opinion vs Polymarket or Opinion vs Kalshi.

Best for: Traders who want exposure to a third venue where spreads vs Polymarket are often wider.

Pros:

• Expands the opportunity set beyond two-platform pairs
• USD settlement options for some users

Cons:

• Thinner documentation and smaller ecosystem than Kalshi/Polymarket
• You will likely pair it with ArbBets for matching rather than building matchers yourself

Pricing: Free with account (verify current API access on site).

5. Arbitrage Agent API — Best for Managed Execution

Arbitrage Agent exposes an execution-oriented API on top of its matching and two-leg firing engine. It handles WebSocket ingestion, semantic market pairing, Kelly sizing, and parallel order placement with circuit-breaker unwind logic.

Best for: Traders who want execution as a service rather than building a full bot from raw exchange APIs.

Pros:

• Automated two-leg execution with failure handling
• Live-editable risk parameters via API or dashboard
• Built specifically for Polymarket ↔ Kalshi pairs

Cons:

• Less flexible than owning your full stack
• Requires trusting a third party with API keys and capital deployment
• Narrower than ArbBets for pure data exploration or custom strategies

Pricing: Paid tiers (verify on site).

6. DIY Multi-API Stack — Best for Full Control

Wire up Kalshi + Polymarket (+ Opinion) directly. Build your own normalizer, embedding-based matcher, fee engine, and execution layer.

Typical architecture:

Kalshi WebSocket ──┐
Polymarket CLOB ───┼──▶ Your matcher ──▶ Arb detector ──▶ Execution
Opinion REST ──────┘

Best for: Quant teams with dedicated engineering time who need proprietary logic or latency advantages.

Pros:

• No monthly data API fee
• Complete control over matching, sizing, and execution
• Potential latency edge if optimized

Cons:

• Market matching alone is weeks to months of work
• Ongoing maintenance as APIs and market titles change
• Reinventing what ArbBets already normalizes

Pricing: Free software; high engineering cost (40+ hours/month maintenance is common).

How to Choose: Decision Matrix

Recommended Bot Architecture

Most production prediction market arbitrage agents use a two-layer stack:

Layer 1 — Discovery (where is the edge?)

Poll ArbBets or stream native orderbooks and run your own matcher. ArbBets collapses this to one HTTP call:

import requests
 
API_KEY = "ak_your_api_key_here"
headers = {"Authorization": f"Bearer {API_KEY}"}
 
response = requests.get(
    "https://getarbitragebets.com/api/internal/kalshi-polymarket-full",
    headers=headers,
    params={"investment": 500, "min_profit": 1.5},
)
opportunities = response.json()
 
for opp in opportunities:
    print(opp.get("market_name"), opp.get("roi_percent"), "% ROI")

For a full walkthrough, see how to programmatically identify Polymarket arbitrage.

Layer 2 — Execution (capture the edge)

When an opportunity clears your threshold:

1. Verify prices still match on both platforms (stale data kills arbs)
2. Place both legs via Kalshi REST and Polymarket CLOB
3. If one leg fails, unwind or hedge the other immediately
4. Log net PnL after fees

ArbBets does not place orders — you connect Layer 2 yourself or use an execution platform like Arbitrage Agent.

Layer 3 — Risk controls

• Maximum capital per opportunity
• Minimum net ROI after fees (Kalshi taker fees can erase thin spreads)
• Kill switch for API errors or partial fills
• Separate capital pools: USDC on Polymarket, USD on Kalshi

API vs Scanner: When Do You Need Which?

If you are only trading by hand, the ArbBets scanner is enough. The API is for automation.

Common Integration Pitfalls

Stale orderbook data

An arb that existed 30 seconds ago may be gone. Poll frequently (ArbBets: every 1–3 minutes minimum) or use WebSockets on the execution side for confirmation.

Market matching errors

Never trade on string equality between platform titles. Use a verified matcher (ArbBets, Arbitrage Agent) or embedding-based pairing with human review on new markets.

Fee blindness

A 2% gross spread on Kalshi can become negative after taker fees. Always use fee-adjusted calculations. See how to calculate arbitrage.

Capital stranded across rails

Polymarket settles in USDC; Kalshi in USD. Rebalancing between platforms has friction — keep operational balances on both sides.

Frequently Asked Questions

Related Reading

• Best prediction market arbitrage tools — scanner comparison
• How to programmatically identify Polymarket arbitrage — full API tutorial
• Prediction market bots — bot strategies and FAQ
• API documentation — authentication, endpoints, examples
• Polymarket vs Kalshi vs Opinion — platform comparison

Start Building

If you already trade prediction markets manually and want to automate, the fastest path is ArbBets API for discovery plus native Kalshi and Polymarket APIs for execution. That avoids months of matching logic while keeping full control over order placement.