This guide is designed to stand on its own as an educational reference. It separates technical concepts, commercial outcomes and policy considerations so readers can reason about the system rather than memorize terminology.
What ads.txt is
ads.txt stands for Authorized Digital Sellers. It is a public text file hosted by a publisher so buyers can see which advertising systems are authorized to sell that publisher's web inventory. The core idea is simple: a buyer should not have to trust a seller's claim; the publisher can publish the relationship on the domain itself.
Anatomy of a record
A typical record contains the advertising system domain, publisher or seller account identifier, a relationship type such as DIRECT or RESELLER, and sometimes a certification authority identifier. DIRECT means the publisher controls that account. RESELLER means another entity is authorized to resell the inventory.
Why crawlers sometimes disagree
Platforms crawl ads.txt on their own schedules. A file can be publicly reachable while a dashboard still shows an older status. Caches, redirects, hostname differences and temporary response failures can also create inconsistent observations. Check both apex and common hostname variants, status code, content type and exact record.
Common mistakes
Typical mistakes include publishing the file at the wrong path, returning an HTML error page with a 200 status, using the wrong publisher identifier, removing the file during a deployment or redirecting in an unreliable way. Seller relationships also change, so ads.txt needs ongoing maintenance.
What ads.txt cannot prove
ads.txt does not prove that traffic is human, an ad was viewable, content is high quality or every policy was followed. It answers one narrower question: is this selling relationship authorized by the publisher domain? That narrow scope is a feature, not a weakness.
Operational checklist
Keep the file at the root path, serve plain text, test externally, monitor for accidental deletion during deploys and reconcile records with active partners. When a platform shows a stale status, verify the live file before making changes. Rewriting a correct file repeatedly can create more uncertainty.