ads.txt — Authorized Digital Sellers — is a plain text file a publisher hosts at a known path listing the companies authorised to sell its inventory. Buyers fetch it to verify that a seller offering an impression is entitled to. It is a rare example of a market solving a trust problem with a flat file at a predictable URL.
ads.txt
ads.txt is one line per authorised seller, in a file at a fixed location, crawled by the buy side. It was introduced to fight domain spoofing, and it worked well enough to be extended to apps as app-ads.txt and complemented by sellers.json on the other side of the transaction.
- A predictable location - The file lives at a known path, which is what makes it crawlable without negotiation.
- Plain text - Deliberately trivial to publish and to parse — no auth, no API, no onboarding.
- Verification, not enforcement - It tells a buyer who is authorised; acting on that remains the buyer’s job.
- A family - sellers.json and the supply chain object complete the picture from the seller’s side.
ads.txt is worth studying by anyone building a discovery artifact. It is the same shape as security.txt and the .well-known catalogs the Kin Score reads: put a machine-readable fact at a predictable URL and let consumers find it. The advertising market adopted that pattern for supply-chain trust and, as The State of Marketing & Advertising APIs shows, has not yet adopted it for anything a customer’s integration needs.
Referenced in API Evangelist papers
This standard shows up in my published research. These reports read the machine-readable evidence provider by provider — and put this standard in the context of a real sector.
The State of Marketing & Advertising APIs
The market proved it will publish a machine-readable fact at a predictable URL when the problem is its own supply-chain trust — 27.5% of the leaders publish a .well-known catalog for everything else.
