I built a free whale tracker for Polymarket — here's what I learned

The problem: I kept missing big moves on Polymarket because I had no way to see what the biggest traders were betting on in real time.

So I built WhaleTrack — a free, no-signup tool that shows you exactly what top Polymarket whales are buying and selling.

What it does

• Live whale activity feed — see the last 40 trades from top wallets, updated on refresh
• Whale leaderboard — P&L, win rate, trade count for the biggest accounts
• No login, no ads, no fluff — just the data

How it works

The whole thing is vanilla HTML/CSS/JS deployed on Vercel with two serverless functions:

/api/whales.js — hits the Polymarket leaderboard API, fetches position stats for each whale, calculates win rates from closed positions

/api/activity.js — pulls recent trades for each whale wallet in parallel, filters out internal combo transactions (no title / zero price), and returns the 40 most recent trades

The serverless layer solves CORS — Polymarket's data API doesn't allow browser requests, so everything goes server-side.

Tech stack

• Frontend: Vanilla HTML/CSS/JS (zero dependencies)
• Backend: Vercel serverless functions
• Data: Polymarket public data API
• Deploy: Vercel (free tier)

Biggest lesson

Filtering bad data is half the work. The raw API returns combo trades and internal transactions that show up as "Unknown Market @ 0¢" — useless noise. Had to figure out which fields to check (title, price > 0) to strip them.

Also: win rate calculation is tricky when most whales have unrealized profits. Showing "—" instead of 0% is more honest.

Try it

WhaleTrack →

Also launched on Product Hunt today if you want to show some love: Product Hunt

Built this in a weekend. Happy to answer questions about the Polymarket API or Vercel serverless setup.