FAQ: 15 Common Questions About Prediction Market APIs
Answers to the 15 most common questions about prediction market APIs — covering getting started, technical details, platform differences, building applications, and legal considerations.
Introduction
We get a lot of questions from developers integrating prediction market data for the first time. This FAQ covers the 15 most common questions about prediction market APIs, organized into five categories: getting started, technical details, platform specifics, building applications, and legal considerations.
If your question isn't covered here, check our full API documentation or reach out to support@propheseer.com.
Getting Started
1. What is a prediction market API?
A prediction market API provides programmatic access to data from prediction market platforms like Polymarket, Kalshi, and Gemini. Instead of manually checking each platform's website, you can fetch market questions, probabilities, volumes, and other data through HTTP requests.
APIs let you build applications on top of prediction market data — trading bots, dashboards, research tools, news integrations, and more. Propheseer provides a unified API that aggregates data from multiple platforms into a single, normalized interface. For a comprehensive introduction, see our complete guide to prediction markets.
2. How do I get started with the Propheseer API?
Three steps:
- Sign up at propheseer.com/signup — free, no credit card required
- Copy your API key from the dashboard
- Make a request:
curl -H "Authorization: Bearer YOUR_API_KEY" \
"https://api.propheseer.com/v1/markets?limit=5"
That's it. You'll get back market data from Polymarket, Kalshi, and Gemini in a single, consistent JSON format. For a full walkthrough, see Get Your First API Response in 5 Minutes.
3. Is the API free to use?
Yes, the free tier includes 100 requests per day and 10 requests per minute — enough to prototype and test your application. No credit card required.
For production use, the Pro plan ($19.99/month) provides 10,000 requests per day, plus access to advanced endpoints like arbitrage detection and unusual trade monitoring.
| Plan | Daily Limit | Per-Minute Limit | Price |
|---|---|---|---|
| Free | 100 | 10 | $0 |
| Pro | 10,000 | 100 | $19.99/mo |
| Business | 100,000 | 1,000 | Custom |
4. What programming languages can I use?
Any language that can make HTTP requests. The API is a standard REST API that returns JSON. We provide examples in:
- Python (requests library)
- JavaScript/TypeScript (fetch, axios)
- curl (command line)
Community members have also used Go, Ruby, Rust, Java, and PHP. If your language can send an HTTP GET request with an Authorization header, it works with the API.
Technical Details
5. What data does the API return?
Each market object includes:
{
"id": "pm_12345abc",
"question": "Will Bitcoin reach $150,000 by end of 2026?",
"source": "polymarket",
"category": "crypto",
"status": "open",
"outcomes": [
{ "name": "Yes", "probability": 0.42 },
{ "name": "No", "probability": 0.58 }
],
"volume": 2450000,
"url": "https://polymarket.com/..."
}
Key fields: question (the market question), outcomes (with probabilities as 0-1 decimals), source (which platform), category, and volume. The data is normalized across platforms so every market follows the same schema regardless of its source.
6. How often is the data updated?
Market data is refreshed approximately every 60 seconds via the REST API. For real-time updates, the WebSocket API provides near-instant price change notifications as trades execute on the underlying platforms.
The /v1/arbitrage endpoint has a 30-second server-side cache to ensure efficient computation.
7. What are the rate limits, and what happens if I exceed them?
Rate limits depend on your plan:
| Plan | Daily | Per-Minute |
|---|---|---|
| Free | 100 | 10 |
| Pro | 10,000 | 100 |
| Business | 100,000 | 1,000 |
When you exceed a limit, the API returns a 429 Too Many Requests response with a Retry-After header indicating how long to wait. Every response includes rate limit headers so you can track your usage:
X-RateLimit-Remaining-Day: 95
X-RateLimit-Remaining-Minute: 8
Build your application to check these headers and back off when limits are approached. Our Python trading bot tutorial includes a client class with built-in rate limit handling.
8. What's the difference between REST and WebSocket APIs?
REST API (polling): Send a request, get a response. Simple, stateless, and good for dashboards that refresh every 30-60 seconds.
WebSocket API (streaming): Open a persistent connection and receive updates as they happen. Better for real-time applications where you need instant price movement notifications.
Use REST for most applications. Switch to WebSocket when you need sub-second updates. Our real-time dashboard tutorial shows how to implement both.
Platform Questions
9. What's the difference between Polymarket, Kalshi, and Gemini?
In brief:
- Polymarket — Crypto-native (USDC), highest liquidity, strong on politics and crypto, limited US access
- Kalshi — CFTC-regulated (USD), legal for US users, unique economics and weather markets
- Gemini — Exchange-backed, crypto-focused prediction markets, newer platform
Each has different APIs, data formats, and authentication methods. The Propheseer API normalizes all three into a single interface. For a comprehensive breakdown, see our Polymarket vs Kalshi comparison.
10. Can I access all platforms through a single API?
Yes — that's exactly what Propheseer does. One API key, one request format, one response schema. You don't need separate accounts or API keys for each platform.
# Returns markets from ALL platforms
curl -H "Authorization: Bearer YOUR_API_KEY" \
"https://api.propheseer.com/v1/markets?limit=20"
# Filter to a specific platform
curl -H "Authorization: Bearer YOUR_API_KEY" \
"https://api.propheseer.com/v1/markets?source=kalshi&limit=20"
The source filter lets you narrow to a single platform when needed. Otherwise, results are aggregated and interleaved from all platforms. For more on why this approach saves development time, see Why Use a Unified Prediction Market API?.
11. How many markets are available?
The combined count across all platforms varies, but typically:
- Polymarket: 600-1,000+ active markets
- Kalshi: 400-600+ active markets
- Gemini: 100-300+ active markets
Total: approximately 1,000-2,000 active markets at any given time, covering politics, crypto, economics, sports, weather, entertainment, and more.
Use the /v1/categories endpoint to see the full category breakdown:
curl -H "Authorization: Bearer YOUR_API_KEY" \
"https://api.propheseer.com/v1/categories"
Building Applications
12. How do I detect arbitrage opportunities?
Arbitrage exists when the same event is priced differently on two platforms. The /v1/arbitrage endpoint finds these automatically:
import requests
response = requests.get(
"https://api.propheseer.com/v1/arbitrage",
headers={"Authorization": "Bearer YOUR_API_KEY"},
params={"min_spread": 0.05}
)
for opp in response.json()["data"]:
print(f"{opp['question']}: {opp['spread']:.1%} spread")
The endpoint handles market matching, price normalization, and spread calculation. For a deep dive into how arbitrage detection works and how to build a custom scanner, see How to Detect Arbitrage Opportunities.
13. Can I build a trading bot?
Yes. The API provides the data layer for any trading strategy — arbitrage, momentum, news-driven, or custom models. Our complete Python trading bot tutorial walks through building a production-ready bot with:
- A reusable API client class
- Arbitrage and unusual trade detection
- Slack/Discord alert integration
- Continuous monitoring with error handling
- Deployment to a cloud server
Note: The Propheseer API is read-only (market data). To execute trades, you'll need direct accounts on the trading platforms.
14. Can I build a real-time dashboard?
Yes. Combine the REST API for initial data loading with the WebSocket API for live updates. Our React dashboard tutorial builds a complete Next.js dashboard with:
- Responsive market card grid
- Search and category filtering
- Real-time price update animations
- Connection status indicators
The tutorial includes all the code you need to deploy to Vercel or any hosting platform.
Legal Considerations
15. Are prediction markets legal?
In the United States:
- Kalshi is fully legal — it's a CFTC-regulated Designated Contract Market (DCM)
- Polymarket has restricted access for US users on certain markets
- Gemini operates under existing cryptocurrency regulations
Internationally: Prediction markets are generally legal in most jurisdictions, though regulations vary by country.
Using API data is legal everywhere — you're accessing publicly available market information, similar to pulling stock prices from a financial data provider. Building applications with this data (dashboards, research tools, alerts) does not constitute trading.
Important: This is general information, not legal advice. If you have specific legal questions about trading on prediction markets or using prediction market data in a regulated industry, consult a qualified attorney.
Still Have Questions?
- API Documentation — propheseer.com/docs for complete endpoint reference
- Getting Started — 5-minute quickstart guide
- Email Support — support@propheseer.com
- Pricing — Compare plans to find the right fit
Ready to start building? Sign up for a free account and make your first prediction market API request in minutes.