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.
Top comments (2)
How did you handle API rate limiting for WhaleTrack, and are you planning to open source the project? I'd love to swap ideas on this.
Hey Frank! Thanks for the comment 🙌
Rate limiting: Polymarket's public data API is pretty generous — no official rate limits documented. I poll every 5 minutes per whale wallet which keeps it well within safe territory. The only gotcha was parallelizing too many wallet fetches at once, so I batch them with Promise.allSettled.
Open source: Not planning to fully open source it right now since I'm actively building features, but happy to share specific parts — the win rate calculation logic was the trickiest bit. Drop your GitHub and let's swap notes!