For developers and AI assistants

ComboCheck API and MCP server

Everything ComboCheck knows about Polymarket Combo (parlay) prices is free to use: a JSON API and an MCP server that Claude, Cursor, ChatGPT and other assistants can call. Read-only, no key, no sign-in.

MCP server

Server URL: https://www.combocheck.xyz/mcp (Streamable HTTP). Ask your assistant things like "Is 32 cents a fair price for a Polymarket combo of the Chiefs to win and over 47.5 in Chiefs vs. Colts?" and it searches the markets, checks the quote and answers with a link that opens the combo in ComboCheck.

Tool	What it does
`search_markets`	Find the Polymarket markets that can go into a Combo (parlay), by team, player or event name. Returns each market's two sides with their position_id, the pick they stand for (a team, Over or Under, or Yes/No) and the current probability. Use the position_ids with check_combo_quote.
`check_combo_quote`	Judge a Polymarket Combo (parlay) quote: the legs' fair price, Polymarket's taker fee, the all-in price, the margin over fair, and a 0 to 100 deal score against similar real combo trades. Needs 2 to 10 position_ids from search_markets and the quoted price per share in cents.
`get_market_stats`	Dated figures on how much Polymarket combo buyers pay over the fair price: today's medians for pre-game, same-game and live combos, and the last 7 days by number of legs.
`get_popular_combos`	The exact leg sets bought most since yesterday that can still be placed, with how often they were bought and what buyers paid against fair.

Connect it

Claude (claude.ai and the desktop app)

Settings, then Connectors, then Add custom connector. Name it ComboCheck and paste https://www.combocheck.xyz/mcp.

Claude Code

claude mcp add --transport http combocheck https://www.combocheck.xyz/mcp

Cursor

// ~/.cursor/mcp.json
{ "mcpServers": { "combocheck": { "url": "https://www.combocheck.xyz/mcp" } } }

VS Code

// .vscode/mcp.json
{ "servers": { "combocheck": { "type": "http", "url": "https://www.combocheck.xyz/mcp" } } }

ChatGPT

In connector settings with developer mode on, add a custom connector with the URL https://www.combocheck.xyz/mcp. Availability depends on your plan.

Anything else

Any MCP client that speaks Streamable HTTP: https://www.combocheck.xyz/mcp. No key, no sign-in.

REST API

Base URL https://www.combocheck.xyz. JSON in and out; prices are fractions of a dollar (0.30 is 30 cents) and margins are fractions where positive means paying more than fair.

Method	Path	What it returns
`POST`	`/v1/quotes/evaluate`	Check a quote: fair price, fee, all-in price, margin over fair and the comparison with similar trades. Body: leg_position_ids (2 or more), price (0 to 1).
`GET`	`/v1/markets`	Open combo-enabled markets with both legs' position ids and live probabilities.
`POST`	`/v1/legs/resolve`	Look up legs by position id, with live probabilities.
`GET`	`/v1/legs/{position_id}`	One leg: live probability, 24 hours of price history, recent combos with it.
`GET`	`/v1/combos/popular`	The leg sets bought most since yesterday that can still be placed.
`GET`	`/v1/legs/popular`	The legs that appear in the most combos.
`GET`	`/v1/stats/summary`	Today's medians over fair: all, pre-game, live and same-game combos.
`GET`	`/v1/stats/overview`	Median margin over fair by sport, number of legs and phase, last 7 days.
`GET`	`/v1/trades/recent`	Recent combo trades with their legs and margins (no wallet addresses). Page back with before=<unix time>.
`GET`	`/v1/trades/best-value`	The lowest-margin pre-game combos of the last 24 hours, one row per combo.
`GET`	`/v1/combos/last-paid`	The last trade of an exact leg set. legs=ID,ID,...
`GET`	`/health`	When the last combo trade was seen.

Check a quote

curl -s https://www.combocheck.xyz/v1/quotes/evaluate \
  -H 'content-type: application/json' \
  -d '{"leg_position_ids": ["<yes or no position id>", "<another>"], "price": 0.30}'

{
  "fair": 0.27,               // the legs' probabilities multiplied
  "price": 0.30,              // your quote
  "fee_per_share": 0.0105,    // 0.05 x p x (1 - p)
  "all_in_price": 0.3105,
  "all_in_margin": 0.15,      // all-in / fair - 1: +15% over fair
  "better_than": 0.41,        // share of similar trades this price beats
  "verdict": "typical",
  "comparable": {"n": 212, "median": 0.12, "sport": "nfl", "legs": "2", ...},
  "legs": [...], "warnings": []
}

Leg position ids come from /v1/markets (each market has a Yes and a No leg) or from the MCP search_markets tool.

Link straight to a combo

https://www.combocheck.xyz/#build?legs=ID,ID&quote=30 opens ComboCheck with those legs in the slip and the quote typed in (cents), ready to check.

Limits and credit

Quote checks and leg lookups: 40 a minute per IP; MCP tool calls: 120 a minute. Other responses are cached for 30 seconds to 10 minutes.
No uptime guarantee, and endpoints may change. Numbers are for information, not financial advice.
If you show ComboCheck's numbers, please credit it with a link to combocheck.xyz.

How every number is calculated: methodology. For language models: llms.txt.

Check your own combo quoteFree, no sign-in.