How I Built a Real-Time Whale Tracker for Polymarket in a Weekend
Prediction markets just hit $3.6B in volume. I wanted to know what the biggest traders were betting on — in real time. So I built WhaleTrack. Here's how it works under the hood. The Problem Polymarket has a public leaderboard. But it only shows P&L totals — not what whales are currently betting on, not their recent activity, not their win rate. If you want to follow smart money, you're flying blind. I wanted something that answered: what are the top traders doing right now? The Stack Vanilla JS frontend (no framework, keeps it fast) Vercel serverless function as a backend proxy (avoids CORS issues) Polymarket's public data API — no auth required Step 1: Finding the Whales Polymarket exposes a leaderboard endpoint: https://data-api.polymarket.com/v1/leaderboard?limit=20 This returns traders ranked by P&L. I pull the top 10, grab their wallet addresses, and that's my whale list. Step 2: Fetching Live Activity For each whale wallet, I hit: https://data-api.polymarket.com/activity?user={address}&limit=20 This returns their recent trades — market name, size in USDC, timestamp. Refreshes every 60 seconds. Step 3: Calculating Win Rate (the tricky part) The key is the redeemable flag — redeemable: true means they won, currentValue: 0 + redeemable: false means they lost. Took a few wrong attempts with cashPnl (always negative, not useful). Step 4: The Whale Alert Banner Every 60 seconds I check for trades over $5,000 placed in the last 10 minutes. When it fires, a green banner slides down with the whale name, market, and amount. Auto-dismisses after 12 seconds. First time I saw it fire live with a $28K bet — genuinely exciting. Results 129+ users in the first few days Zero ad spend Traffic from Twitter, Reddit, Quora What's Next More whale wallets (suggestions welcome) Click-through to open the same market on Polymarket directly Email/push alerts for big trades Check it out: whaletrack.app All feedback welcome — especially if you spot a whale I'm missing.