Programmatic auctions moved from second price to first price, and the change altered how bidding has to be approached. Under first price the bid is the payment, which removes a substantial cushion.
Second price rewarded honest valuation
In a second price auction the winner pays slightly more than the next highest bid, not their own figure.
That structure made bidding true value the sensible strategy, because overbidding cost nothing when the competition was low.
Buyers could therefore set a maximum they were willing to pay and let the mechanism find the clearing price.
First price makes every bid a payment
Under first price, the winner pays exactly what they bid, so any gap between bid and market value is money given away.
Bidding true value now means paying true value on every impression, including those that would have cleared far lower.
The optimal strategy shifts from stating a value to estimating the minimum required to win, which is a prediction problem rather than a valuation one.
Bid shading emerged to fill the gap
Buying platforms responded with shading, which lowers the submitted bid to an estimate of what will clear while still winning.
The estimate is built from historical clearing prices for similar inventory, adjusted for how much the buyer wants to win.
This makes the platform's shading model a significant determinant of media cost, and it is largely opaque to the advertiser using it.
Shading too aggressively loses impressions the campaign wanted, and shading too little overpays, so the model is balancing two costs the advertiser never sees separately.
Why publishers pushed for the change
Second price auctions depended on buyers behaving as the mechanism assumed, and floors were widely used to raise clearing prices artificially.
Once floors were being adjusted dynamically, the auction was no longer genuinely second price, and the pretence added complexity without benefit.
First price made pricing transparent in one respect, since the amount paid is now visible and unambiguous to both sides.
What it means for how accounts are run
Comparing platforms on average cost per thousand now partly compares their shading models rather than their inventory access.
Sudden cost increases are more often a change in competitive pressure or in shading behaviour than a change in inventory quality.
And because every bid is paid in full, reducing the number of exchanges bidding on the same impression has a direct effect on price rather than a theoretical one.