Prediction Market Odds API — Polymarket + Kalshi, De-vigged Fair Probabilities

SEO title: Prediction Market Odds API — Polymarket & Kalshi Scraper, extract de-vigged probabilities as JSON SEO description: Search or look up Polymarket and Kalshi prediction markets and get one unified JSON schema back: prices, volume, liquidity, order books, and de-vigged fair probabilities (multiplicative/power method). No API key needed — built for research, arbitrage-scanning, and AI agents.

One actor, one schema, two prediction market platforms. Query by keyword or by exact market ID, get back current prices AND the de-vigged "fair" probability for every outcome — the overround-adjusted number you actually want if you're comparing markets, building a model, or looking for mispriced longshots.

Why this exists

Polymarket (Gamma + CLOB APIs) and Kalshi (trade-api v2) both expose public, unauthenticated market data — but in two different shapes, with prices that still include the platform's own vig/overround. This actor:

1. queries both (or either) platform for markets matching a keyword, or fetches specific markets by ID,
2. normalizes them into one schema,
3. removes the vig with a standard de-vig method (multiplicative by default) so devigged_prob values for a market's outcomes actually sum to 1.0,
4. optionally attaches a top-of-book order book snapshot.

No scraping, no anti-bot risk, no login — both platforms' public market-data endpoints are called directly (over HTTPS, no browser).

Input

Field	Type	Default	Description
query	string	—	Free-text search keyword, e.g. "bitcoin", "2028 election". Use this OR market_ids.
market_ids	array of strings	[]	Exact market IDs to fetch directly: Polymarket condition IDs (0x...) or slugs, and/or Kalshi tickers (e.g. "KXBTCD-26JUL0517-T52999.99"). Mixing IDs from both platforms in one list is fine — each platform only matches its own IDs.
platforms	array of strings	["polymarket", "kalshi"]	Which platform(s) to query.
include_orderbook	boolean	false	Attach a top-10-level order book per market. Bills an extra event (see pricing) and roughly doubles request count.
max_markets	integer (1–200)	20	Total markets to return across all platforms combined. Split evenly between the requested platforms so one platform's abundance of matches (Polymarket tends to have far more markets matching a broad query than Kalshi) doesn't crowd out the other.

You must provide query and/or market_ids — an actor input with neither fails immediately with a message telling you exactly that.

Worked example — input

{
  "query": "bitcoin",
  "platforms": ["polymarket", "kalshi"],
  "include_orderbook": false,
  "max_markets": 5
}

Worked example — output (2 of 5 items, real data)

[
  {
    "platform": "polymarket",
    "market_id": "0x4863841fee98ae432b657dbad973cd5562e7fa6bab5e1a725ebd833723c9493d",
    "question": "Will the price of Bitcoin be above $50,000 on July 5?",
    "outcomes": [
      { "name": "Yes", "price": 0.9995, "implied_prob": 0.9995, "devigged_prob": 0.9995 },
      { "name": "No",  "price": 0.0005, "implied_prob": 0.0005, "devigged_prob": 0.0005 }
    ],
    "volume": 132828.07,
    "liquidity": 126913.72,
    "close_time": "2026-07-05T16:00:00Z",
    "url": "https://polymarket.com/event/bitcoin-above-50k-on-july-5-2026",
    "scraped_at": "2026-07-05T14:57:29.520Z"
  },
  {
    "platform": "kalshi",
    "market_id": "KXBTCMAX150-25-26OCT31-149999.99",
    "question": "When will Bitcoin cross $100k again? — Before October 2026",
    "outcomes": [
      { "name": "Yes", "price": 0.055, "implied_prob": 0.055, "devigged_prob": 0.0524 },
      { "name": "No",  "price": 0.945, "implied_prob": 0.945, "devigged_prob": 0.9476 }
    ],
    "volume": 3678944.42,
    "liquidity": 1104224.42,
    "close_time": "2026-10-31T03:59:00Z",
    "url": "https://kalshi.com/markets/kxbtcmax150/kxbtcmax150-25-26oct31-149999.99",
    "scraped_at": "2026-07-05T14:57:29.522Z"
  }
]

With include_orderbook: true, each item additionally gets:

"orderbook": {
  "bids": [{ "price": 0.98, "size": 357.0 }, "... up to 10 levels"],
  "asks": [{ "price": 0.99, "size": 45095.0 }, "... up to 10 levels"]
}

Output schema

Field	Type	Notes
platform	string	"polymarket" or "kalshi"
market_id	string	Polymarket condition ID (or slug if no condition ID) / Kalshi ticker
question	string	Human-readable market question
outcomes[].name	string	e.g. "Yes" / "No", or a named outcome for multi-way markets
outcomes[].price	float (0–1)	Raw last/mid price, i.e. the platform's own implied probability including vig
outcomes[].implied_prob	float (0–1)	Same as price — kept as its own explicit field per the output contract
outcomes[].devigged_prob	float (0–1) or null	Vig-removed fair probability (multiplicative method); all outcomes of one market sum to 1.0. null only if de-vig math wasn't possible (e.g. a single-outcome market)
volume	float or null	Platform-reported traded volume
liquidity	float or null	Platform-reported liquidity/open interest
close_time	string (ISO 8601) or null	Market close/expiration time
url	string or null	Link to the market on the platform's site
scraped_at	string (ISO 8601)	When this actor fetched the data
orderbook	object, only if include_orderbook: true	{bids: [{price, size}], asks: [{price, size}]}, top 10 levels each

Pricing (pay-per-event)

Event	Price	When it's charged
market-result	$0.003	Once per market returned
orderbook-snapshot	$0.02	Once per market, only when include_orderbook: true

Plus Apify's own apify-actor-start synthetic event (first 5 seconds of compute free, platform-managed — never charged from this actor's code).

Example: 100 markets, no order books: 100 × $0.003 = $0.30. 100 markets + order books on 10 of them: $0.30 + 10 × $0.02 = $0.50.

If a run's cost would exceed the Max total charge USD you set for it, the actor stops producing further results at exactly that point — it never crashes and never produces unbilled/"free" results past the limit.

Error messages

• No query and no market_ids: "Missing search criteria: provide either 'query' ... or 'market_ids' ..." — add one of the two fields.
• Unsupported platforms value: "'platforms' contains unsupported value(s) [...]. Supported values are: ['kalshi', 'polymarket']." — fix the typo/remove the entry.
• max_markets out of range: "'max_markets' must be between 1 and 200, got N." — pick a value in range.
• Wrong type for any field (e.g. query as a number, market_ids as a bare string instead of an array): the message states the expected type and shows a corrected example.
• 0 results with otherwise valid input: not an error — the run succeeds, but the log has an explicit WARNING: 0 results produced despite valid input (...) line and the run's status message says so, so this is never a silent/invisible "nothing happened."

Known limitations (documented, not hidden)

• Kalshi has no public full-text market search endpoint. query search against Kalshi resolves by substring-matching the query against the ~11k-entry /series catalog's titles/tickers (fetched once per run, cached in-memory for that run), then lists open markets for matching series. This is adequate for the actor's per-run cost/latency budget; a future improvement could cache the catalog in the key-value store across runs.
• Order books are attached per-market, not per-outcome. For binary Yes/No markets this is the whole picture (No is fully determined by Yes); for a multi-outcome market, only the first outcome's book is attached.
• market_ids mixing platforms: the input is a single flat list rather than per-platform lists, so an ID that doesn't belong to a requested platform simply yields nothing for that platform — this is intentional (see Input table), not an error.

Data sources (public, no API key, no scraping)

• Polymarket Gamma API: https://gamma-api.polymarket.com/public-search, /markets
• Polymarket CLOB API: https://clob.polymarket.com/book
• Kalshi trade-api v2: https://api.elections.kalshi.com/trade-api/v2/{series,events,markets}

All are the same public, unauthenticated endpoints the platforms' own web UIs call — no login, no key, no ToS-risk scraping.