First-price auction

A first-price auction is an auction in which the winning bidder pays the amount it bid, signalled in OpenRTB by at: 1, as opposed to a second-price auction (at: 2, "Second Price Plus") in which the winner pays just above the second-highest bid.

OpenRTB defines at as "auction type, where 1 = First Price, 2 = Second Price Plus," and a deal can override it, including at: 3, where "the value passed in bidfloor is the agreed upon deal price" (OpenRTB 2.6, sections 3.2.1 and 3.2.12). The open web moved from second to first price between 2017 and 2019 as header bidding made exchanges compete on the same impression; a second-price exchange feeding a first-price decision in the ad server was systematically under-pricing.

First price shifts the pricing problem to the buyer. A DSP that bids its true value pays its true value, so DSPs bid below value using bid shading. To make that less of a guess, OpenRTB 2.6 added the ${AUCTION_MIN_TO_WIN} macro, "minimum bid to win the exchange's auction," which an exchange can send to losing and winning bidders (OpenRTB 2.6, section 4.4.1). IAB Tech Lab's Programmatic Standard Practices v1, in public comment until 16 October 2026, is the first attempt to write shared rules for how these mechanisms are used (IAB Tech Lab, 16 September 2026).

Example: bids of $2.10 and $1.80 with at: 1 clear at $2.10; with at: 2 they clear at $1.81 (the second price plus the increment).

Related: bid shading, floor price, ad exchange, deal ID.

Source: IAB Tech Lab, OpenRTB 2.6, BidRequest.at, Deal.at, section 4.4.1.

1. OpenRTB 2.6, section 4.4.1
2. IAB Tech Lab, 16 September 2026