Live order book for one outcome of an event

Keyless live order book for the SAME single outcome price-history describes (the Yes leg of a binary market, else the first live outcome by sort order, else the first outcome). Polymarket and Kalshi only; other sources answer 200 with orderBook: null, which means "not served here", not "no market". A thin or near-expiry market with no resting orders on either side also answers 200 with orderBook: null — a valid empty shape, not an error.

bid/ask/midpoint/level price are probabilities 0-1 (a fraction), NOT the 0-100 scale used by PublicPmOutcome.probability elsewhere in this API. This is fetch-time venue depth, not an executable quote or a tradability guarantee — use the quote/mock-open flow for that.

askEvidence is present only for Polymarket books: it records whether the supplied ask snapshot parsed as a valid non-empty book (validated) and when this server fetched it (receivedAt, its own acquisition time, not an upstream trade time). Kalshi books omit it. Both venues' books carry receivedAt and sideEvidence.

outcomeExternalMarketId selects a different outcome of the event instead (a Kalshi No side is complemented). That book must have been received less than 10 seconds earlier, otherwise orderBook is null; an outcome that is not on the event returns 404.