DEV Community: Manpreet Brar

How I built a real-time whale tracker for Polymarket using their public API

Manpreet Brar — Mon, 13 Jul 2026 01:45:00 +0000

Polymarket has a public data API that almost nobody talks about. I used it to build WhaleTrack — a live dashboard that tracks what the biggest traders on the platform are betting on.

The stack:

Vanilla JS frontend (no framework — just fast)
Vercel serverless functions to proxy all API calls (avoids CORS, keeps keys server-side)
Polymarket's data-api.polymarket.com for positions, leaderboard, and activity
The interesting parts:

The win rate calculation was tricky. Polymarket doesn't give you a clean "won/lost" field. You have to infer it:

redeemable === true = won (market resolved in their favour)
currentValue === 0 && redeemable === false = lost
Everything else = still open
Also learned the hard way that slug (market-level) and eventSlug (event-level) are different things on Polymarket. Market slugs 404 on polymarket.com/event/. Took me longer than I'd like to admit to figure that out.

Live at whaletrack.app — open to questions about the API or architecture.

How I built a real-time whale tracker for Polymarket using Node.js and a CLI

Manpreet Brar — Sun, 05 Jul 2026 03:05:29 +0000

The 2026 World Cup has $3.89 billion bet on it across Polymarket. That's not retail money — that's whales.

I built WhaleTrack to track exactly what those big wallets are doing. Here's the stack:

Backend: Node.js server fetching live data via Bullpen CLI
Frontend: Vanilla JS, real-time updates
Data: Polymarket CLOB API via Bullpen
Analytics: Google Analytics for traffic tracking
The hardest part wasn't the code — it was getting users. Pure SEO and content distribution (Reddit, Twitter, IH).

The site is live at whaletrack.app — would love feedback from devs on the UX and performance.

Happy to open source parts of it if there's interest.

How I Built a Real-Time Whale Tracker for Polymarket in a Weekend

Manpreet Brar — Sun, 28 Jun 2026 18:05:09 +0000

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.

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

Manpreet Brar — Sat, 27 Jun 2026 03:16:11 +0000

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.