Prediction market open interest: where capital actually sits

The pm-cohort-stats harness reads each venue from the best source it has, and for open interest that is more often DefiLlama than the venue. Kalshi publishes open interest natively. Polymarket's gamma field is deprecated and reads far below the protocol's tracked value, so Polymarket and Limitless use DefiLlama protocol TVL; Myriad's native figure is AMM liquidity. Every row says on its own tag which source it came from. The rest of the cohort is read from DefiLlama's prediction-market category, where protocol TVL stands in for open interest and /summary/dexs supplies volume. Gauges are pm_venue_open_interest_usd, pm_venue_volume_24h_usd, pm_venue_volume_30d_usd and pm_venue_active_markets, one series per venue, one writer per gauge. The cohort was widened from five venues to seventeen on 2026-09-23; rows below $500,000 of open interest are not ranked, because a venue holding tens of thousands moves by tens of percent on one position. Where a venue's own API can only be read in part, the partial figure is withheld rather than published as a total.

• ·Open interest here is collateral: cash held against positions still open, not their market value. On the on-chain venues that is DefiLlama protocol TVL, one dollar escrowed per complete set outstanding. On Kalshi it is one dollar per open contract, since each settles at a dollar or nothing and is fully collateralized across the two sides. Each row tag names its source.
• ·Valuing Kalshi's open contracts at last price instead gives the market value of the YES leg. That is a different quantity (14% of the collateral figure on 2026-09-23), and it is published as pm_venue_open_interest_mark_usd rather than dropped. Myriad's native figure is AMM liquidity, not positions.
• ·Volume: each venue's own figure where its API gives one, otherwise DefiLlama /summary/dexs for that protocol. Both windows, 24 hours and 30 days, come from the same source for a given venue so the ratio between them is internally consistent.
• ·Turnover is 24h volume over open interest. Below 1x a venue holds more than it trades in a day, which is what long-dated markets look like; well above 1x the book turns over repeatedly, which is what short-dated contracts look like. It is a description, not a ranking.
• ·Kalshi's volume comes from DefiLlama rather than from the harness's own read. Unauthenticated, the fetcher sums the /events catalog and reaches $2.26M against DefiLlama's $424.8M for the same venue and window on 2026-09-23, because the catalog walk is capped well short of the full book. With an API key the native figure is the better source and the harness prefers it again.
• ·Polymarket's 24h volume is DefiLlama's. The harness also walks gamma, and that walk is truncated, not exhausted: gamma caps limit at 100 however much you ask for, and refuses offsets past 2100 with a 422 naming /markets/keyset for deeper pagination. The walk therefore stops at exactly 2,100 markets and $5.37M against DefiLlama's $61.8M, and the same cut is why no active-market count is published for Polymarket. Measured against gamma directly on 2026-09-23.
• ·Size floor: $500,000 of open interest. Below it one position moves a venue by tens of percent, and the board would rank position-opening events. Venues under the floor keep publishing their gauges and are absent from the ranking.
• ·A venue with no readable open interest is absent rather than shown at zero. Polymarket US is in that state: a CFTC-regulated exchange has no on-chain balance for DefiLlama to measure, and while its own API does publish per-market open interest, reading it costs one request per market. Its volume is among the largest in the category and is published.
• ·Cadence: native fetchers every 5 minutes, the DefiLlama aggregate every 15. The headline is a 24h average, so a single print does not move a row.
• ·Two known limits on the Kalshi figure, stated rather than smoothed. Its catalog walk covers 13,402 open events and 125,777 nested markets, and markets that have already resolved inside a still-open event are excluded (2,229 of them, holding 48.5M contracts, on 2026-09-23). And Kalshi offers reduced margin across mutually exclusive outcomes in one event group, so $1 per open contract is an upper bound on cash actually escrowed; the exchange publishes no netted figure.