What is a sweep in trading?

In US options markets, a sweep is a single order split into pieces and routed to several exchanges at once, taking the available liquidity on each. Options-flow tools flag these because the routing pattern suggests a trader who valued speed over price. For a market whose liquidity is spread across more than a dozen venues, that definition is accurate and useful.

Kalshi is built differently. Each Kalshi market has a single central order book, so there is no second venue to route to and nothing to sweep across. The options definition describes a path through market structure that a prediction market does not have. What survives the translation is the behavior underneath: a trader taking all the liquidity available right now rather than waiting for a better price. On a single book, that behavior leaves a different fingerprint, a burst in time instead of a path across venues.

The usual cause is one large order walking the book. Suppose the YES side of a market is offered at 61, 62, 63, and 64 cents, a few hundred contracts at each level. A single order for 2,000 contracts fills through all four levels, and the exchange reports each level as its own print. The tape now shows a run of YES prints inside a second or two.

Read naively, that looks like a crowd. It is usually one decision. Five prints landing on the same side within seconds is almost always a single order meeting a tiered book, not five traders arriving independently. Counting prints as traders is the most common way to misread a burst.

Occasionally a genuine rush produces the same shape: several takers hitting the same side within seconds of a headline. The anonymous tape does not distinguish the two cases, which is one reason WhaleScanr treats the burst as a single event and shows its parts.

WhaleScanr flags a sweep at 5 or more same-side prints with gaps of 10 seconds or less, treated as one event. Both thresholds are fixed and published: a run of 4 prints is not a sweep, and a gap of 11 seconds starts a new count.

Each sweep event carries the combined notional of the burst and lists every constituent print, so the grouping is inspectable rather than asserted. Whether a sweep also qualifies as a whale event depends on the market it lands in: size thresholds are relative to each market's own liquidity, never a flat dollar bar. Sweep events stream live on /feed, and the wider mechanics of the tape are covered in Reading the Kalshi tape.

A sweep measures urgency, not accuracy. Someone wanted size on one side filled immediately and accepted successively worse prices to get it. That says something about conviction and impatience. It says nothing about being right, and sweeps print on both YES and NO. A NO sweep looks identical to a YES sweep on the tape apart from the side field. WhaleScanr reports the sweep, the side, and the notional. Described, never advised.