Why Use a Unified Prediction Market API?
Discover why a unified prediction market API saves developers weeks of integration work compared to building direct connections to Polymarket, Kalshi, and Gemini separately.
Introduction
There are three major prediction market platforms today — Polymarket, Kalshi, and Gemini — each with its own API, data format, and authentication method. If you want comprehensive market coverage, you need all three.
Building direct integrations with each platform is straightforward in theory. In practice, it means managing three separate API clients, three different data schemas, three authentication flows, and three sets of quirks. A unified prediction market API eliminates this overhead by providing a single endpoint that aggregates, normalizes, and serves data from all platforms.
This article explains the concrete pain points of direct integration, how a unified API solves them, and when you might still want to go direct.
The Pain Points of Direct Integration
1. Three Different Data Formats
Each platform structures market data differently. Polymarket stores prices as decimal strings in a JSON array. Kalshi uses integer cents with bid/ask spreads. Gemini has its own format entirely.
To display a simple comparison table of market probabilities, you need three separate parsing functions:
# Polymarket: parse string array
import json
prices = json.loads(market["outcomePrices"]) # '["0.42", "0.58"]'
yes_prob = float(prices[0])
# Kalshi: calculate midpoint from integer cents
yes_prob = (market["yes_bid"] + market["yes_ask"]) / 2 / 100
# Gemini: parse decimal string
yes_prob = float(market["yes_price"])
This is just prices. Field names, status indicators, category taxonomies, date formats, and volume representations all differ too. For a complete breakdown, see our data normalization guide.
2. Three Authentication Systems
- Polymarket: Public reads (no auth), but trading requires API key + cryptographic HMAC signatures + EIP-712 typed data
- Kalshi: Email/password login → token → Bearer auth on every request, with periodic token refresh
- Gemini: API key + secret, HMAC signatures on payloads
Each system has different error codes, different rate limit responses, and different token lifecycle management.
3. Three Sets of Rate Limits
You need to track and respect different rate limits per platform:
| Platform | Limits | Behavior |
|---|---|---|
| Polymarket | ~10 req/sec | 429 with retry-after |
| Kalshi | Varies by endpoint | 429 with backoff |
| Gemini | Tiered by auth level | 429 |
Building proper backoff logic, retry queues, and rate limit tracking for three services is significant engineering work — easily a week or more.
4. No Cross-Platform Features
With direct integrations, features like arbitrage detection, cross-platform search, and unified WebSocket streams don't exist — you have to build them yourself. Matching markets across platforms (determining that two differently-worded questions refer to the same event) is a non-trivial NLP problem.
5. Ongoing Maintenance
APIs change. Polymarket has updated their Gamma API endpoints multiple times. Kalshi has released API v2 with breaking changes from v1. When a platform changes its API, you need to update your integration, test it, and deploy — per platform.
How a Unified API Solves This
A unified API acts as a normalization and aggregation layer between your application and the underlying platforms:
Your App → Unified API → Polymarket
→ Kalshi
→ Gemini
Single Authentication
One API key works for all platforms. No crypto wallets, no token refresh, no platform-specific auth flows:
curl -H "Authorization: Bearer YOUR_API_KEY" \
"https://api.propheseer.com/v1/markets?limit=10"
That single request returns markets from Polymarket, Kalshi, and Gemini in the same format.
Normalized Data Schema
Every market follows the same structure regardless of source:
{
"id": "pm_12345abc",
"question": "Will the Fed cut rates?",
"source": "polymarket",
"category": "economics",
"status": "open",
"outcomes": [
{ "name": "Yes", "probability": 0.42 },
{ "name": "No", "probability": 0.58 }
],
"volume": 2450000
}
The source field tells you where it came from. The id prefix (pm_, ks_, gm_) makes the origin immediately obvious. Everything else is consistent.
Built-In Cross-Platform Features
Features that would take weeks to build yourself come out of the box:
| Feature | Direct Integration | Unified API |
|---|---|---|
| Cross-platform search | Build NLP matching | ?q=bitcoin |
| Arbitrage detection | Build matching + comparison | /v1/arbitrage |
| Unusual trade detection | Build per-platform monitors | /v1/unusual-trades |
| Category normalization | Map 3 taxonomies | Already normalized |
| Real-time updates | 3 WebSocket connections | 1 WebSocket connection |
Managed Rate Limits
Instead of tracking three sets of rate limits, you manage one. The unified API handles upstream rate limiting internally, so you never need to worry about hitting Polymarket's or Kalshi's limits directly.
Rate limit headers on every response make tracking simple:
X-RateLimit-Limit-Day: 10000
X-RateLimit-Remaining-Day: 9500
X-RateLimit-Limit-Minute: 100
X-RateLimit-Remaining-Minute: 95
X-RateLimit-Plan: pro
Cost and Time Savings
Here's a realistic comparison of the engineering effort:
| Task | Direct (3 platforms) | Unified API |
|---|---|---|
| Initial integration | 2-3 weeks | 1-2 hours |
| Authentication setup | 3-5 days | 5 minutes |
| Data normalization | 1-2 weeks | Already done |
| Arbitrage detection | 1-2 weeks | 1 API call |
| Rate limit handling | 2-3 days | Built-in |
| Ongoing maintenance | 1-2 days/month | None |
| Total first 3 months | 6-10 weeks | < 1 day |
For an early-stage project, this difference is often the deciding factor. You can prototype and ship with a unified API in a day, then evaluate whether direct integration is needed later.
Features You Get for Free
Arbitrage Detection
The /v1/arbitrage endpoint automatically matches markets across platforms, calculates spreads, and returns ranked opportunities:
response = requests.get(
"https://api.propheseer.com/v1/arbitrage",
headers={"Authorization": f"Bearer {API_KEY}"},
params={"min_spread": 0.05}
)
for opp in response.json()["data"]:
print(f"{opp['question']}: {opp['spread']:.1%} spread ({opp['potentialReturn']} return)")
Building this from scratch requires market matching (an NLP problem), price normalization, and spread calculation. See our full arbitrage guide for more details.
Unusual Trade Detection
The /v1/unusual-trades endpoint surfaces large or anomalous trades across all platforms — whale activity, near-resolution surges, and new large wallets entering markets:
response = requests.get(
"https://api.propheseer.com/v1/unusual-trades",
headers={"Authorization": f"Bearer {API_KEY}"},
params={"min_score": 70, "limit": 10}
)
Historical Data
The Business tier includes 30-day market snapshots, enabling trend analysis and backtesting without scraping or maintaining your own database.
WebSocket Streaming
Real-time updates from all three platforms through a single WebSocket connection, with normalized event payloads.
When Direct Integration Makes Sense
A unified API isn't always the right choice. Consider direct integration when:
You Only Need One Platform
If your application exclusively uses Kalshi economic data or exclusively uses Polymarket political data, the overhead of a unified API provides less value. A single direct integration is manageable.
You Need Trading Capabilities
If you're executing trades (not just reading data), you'll need direct connections to the platform order books. A read-only unified API doesn't replace trading infrastructure.
You Need Sub-Second Latency
For high-frequency trading strategies where milliseconds matter, the extra network hop through a unified API adds latency. Direct WebSocket connections to each platform will be faster.
You Have a Dedicated Data Team
If you have engineers dedicated to maintaining integrations and a data pipeline that already handles multi-source normalization, the unified API may be redundant.
For most applications — dashboards, research tools, alert systems, bots, news integrations — the unified API is the faster and more maintainable choice.
Getting Started
Ready to skip the multi-platform integration headache?
- Create a free account — 100 requests/day, no credit card
- Make your first request in 5 minutes
- Explore the full API documentation
- Compare plans — Free, Pro ($19.99/mo), and Business tiers
For a detailed look at how the underlying platforms differ, see our Polymarket vs Kalshi comparison.
Stop maintaining three integrations. Get your free API key and access Polymarket, Kalshi, and Gemini through a single, normalized interface.