Scrape Polymarket Odds: Build a Prediction Market Tracker (and Find Arbitrage)

dev.to Data2026-10-20 21:320 views

Prediction markets move billions and their odds are data gold — elections, sports, crypto, weather. Polymarket has an API, but it's rate-limited and incomplete for history and order books. Apify actors fill the gap: live odds, whale trades, and Kalshi-vs-Polymarket arbitrage detection.

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The stack

Apify — dedicated prediction-market actors, some with 800+ users. Free monthly credits.
Python — odds → CSV, alerts, dashboards.

Step 1 — Token

Create an Apify account → Settings → Integrations → API token.

Step 2 — Pull live market odds

curl -X POST "https://api.apify.com/v2/acts/scrapemint~polymarket-market-monitor/run-sync-get-dataset-items?token=$APIFY_TOKEN" \ -H "Content-Type: application/json" \ -d '{"category": "politics", "maxItems": 100}'

Fields per market: question, outcomes (YES/NO prices), volume, liquidity, endDate, url.

Step 3 — Detect mispriced markets

from apify_client import ApifyClient client = ApifyClient("APIFY_TOKEN") run = client.actor("scrapemint/polymarket-market-monitor").call(run_input={ "category": "crypto", "maxItems": 200 }) for m in client.dataset(run["defaultDatasetId"]).iterate_items(): prices = [o.get("price", 0) for o in m.get("outcomes", [])] # binary market where YES+NO != $1 = inefficiency  if prices and abs(sum(prices) - 1.0) > 0.03: print("GAP:", m["question"][:70], "| prices:", prices, "| vol:", m.get("volume"))

The arb play

congism/polymarket-kalshi-arb-finder (800+ users — the proven demand) compares the same event on both platforms. If Kalshi says YES at $0.52 and Polymarket says NO at $0.45, buying both locks ~3% risk-free. The actor outputs the pairs; you decide.

Whale watching

jy-labs/polymarket-whale-tracker surfaces the largest positions — when a wallet that called the last three elections moves $50k, that's a signal worth having before the odds move.