Data

Four JSON endpoints, open CORS, no key. Everything here is an observed price recorded by our own ingest — no estimates, no modelled values, no predictions. Fields are documented below; terms are defined in the glossary.

Licence: free to use, including commercially, with attribution to MarketMaster (marketmaster.live). If you are publishing a figure from these files, please link the page it came from so a reader can check it.

Kalshi–Polymarket cross-venue spreads

The 25 widest price gaps currently open between the same outcome on Kalshi and on Polymarket, each shown gross and after venue fees, with a settlement-risk flag where the two contracts do not resolve on the same terms. Refreshed every five minutes.

https://marketmaster.live/data/top-spreads.json

• edge_pct — gap between the two venues, as a percentage of the capital the trade ties up, before fees
• edge_after_fees — the same gap after the venue fee on the filled side
• settlement_risk — 'divergent' where the two contracts do not close within the same window

Prediction market price moves

The 25 largest price moves among the 300 most recently recorded, across open Kalshi and Polymarket markets, with the price before and after, the window, and 24-hour volume. Observed prices only — nothing here is a forecast.

https://marketmaster.live/data/movers.json

• move_pp — change in the contract price over the stated window, in percentage points
• price_before — observed venue price at the start of the window, in dollars per contract
• price_now — observed venue price at the end of the window, in dollars per contract

Single-market cross-venue snapshot

One market's current price on each venue and the gap between them, addressed by slug. Powers the embeddable badge; usable directly with open CORS.

https://marketmaster.live/embed/data.json?slug=

• kalshi — Kalshi YES price in dollars per contract
• polymarket — Polymarket YES price in dollars per contract
• gap_pp — absolute difference between the two, in percentage points

Daily price history, both venues

Up to 30 days of daily closing prices for the same outcome on Kalshi and on Polymarket, as two parallel series. The only place the two venues' histories for one question are published side by side.

https://marketmaster.live/m/{slug}/history.json

• t — Unix epoch seconds at the end of the period
• p — closing price in dollars per contract, 0 to 1

Larger or live

The full board, historical series and streaming updates are on the Data API, which also speaks MCP if you want an assistant to query it directly. Price moves are also published as Atom.