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What it means for a DSP to support Ads.txt

What it means for a DSP to support ads.txt – and why it’s important

By now your DSP has probably said that they “support ads.txt”. But what does that actually mean, and should you care?

The ads.txt specification aims to address counterfeit inventory by giving content owners and distributors a simple way to declare authorized supply sources for their inventory. This additional transparency provides advertisers and programmatic buyers a means to know and validate the authentic sources of media in real-time, and makes it much harder for bad actors to profit from selling counterfeit inventory across the ecosystem.

There are a few ways in which a DSP can support ads.txt. Let’s look at each of them in turn.

Not at all

At this point, few DSPs active in classic display would publicly admit to not supporting ads.txt in any way. But they are likely to be out there and some of them may even have good reasons.

Manual Review

For some small DSPs it makes sense to use ads.txt data to manually review the most active supply paths. As part of an ongoing supply quality process, purchased inventory is validated against the publicly visible, authorized supply paths in ads.txt for a match on exchange and seller IDs. Unmatched supply paths are blacklisted and subsequently unavailable for purchase.

While this form of manual review may be acceptable for a small number of domains, it suffers from two obvious problems: as a process it will not (profitably) scale to even moderately large domain counts; and it is unlikely to account for changes in ads.txt content on previously audited domains.

The behavioral effect on the bidder for audited domains is likely to be the same as described in the section below. However, unaudited domains will see no change (the DSP will continue to purchase through unauthorized supply paths).

Blocking Unauthorized Inventory

The first step platforms typically take is to block all unauthorized inventory, usually by default and without the possibility for a buyer override. This may not be exposed to the buyer; it’s just one of the things the bots behind your DSP perform automatically billions of times per day without anyone having to think about it.

Bidder behavior should be something like this:

Inventory Without Ads.txt Inventory With Ads.txt
Matching exchange and seller ID vs ads.txt

Mismatch in exchange or seller ID vs ads.txt


There is virtually no reason for this behavior not to be enabled on every impression. Inventory supporting ads.txt will continue to see bids as normal; unauthorized inventory will not.

Bidding Only on Authorized Inventory

As a next step, and often as a campaign-level choice to the buyer, DSPs may bid only on authorized inventory. This means inventory that is explicitly authorized by the proper (matching exchange and seller ID) ads.txt entry on the relevant domain will receive bids; all other inventory will not receive bids.

Careful buyers are already choosing this behavior on both open market campaigns and private market.

Inventory Without Ads.txt Declarations Inventory With Ads.txt Declarations
Matching exchange and seller ID vs ads.txt

Mismatch in exchange or seller ID vs ads.txt


Bidding behavior will change in ways that may limit scale and this may have unintended consequences for reach campaigns without a domain whitelist.

However, with over 60% of the Comscore 1,000 supporting ads.txt, this is probably a reasonable default choice for private marketplaces and should be exposed as an option to the buyer for all campaigns.


Ads.txt provides discerning DSPs and buyers with the capability to target explicitly authorized supply paths. It’s critical for advertisers and agencies to understand how that capability is being used, and when, and to confirm with spend reporting.


IAB Tech Lab’s ads.txt aggregator: a simple tool to help you achieve brand safety


Sam Tingleff, Chief Technology Officer, IAB Tech Lab