What is ads.txt?

ads.txt (Authorized Digital Sellers) is a public text file where a website lists the ad companies allowed to sell its ad space. Ad buyers read it before they bid, so someone who claims to sell ads on your site without being listed gets ignored. It lives at the root of the domain, like https://example.com/ads.txt.

Each line names an ad system, your account ID there, and whether you control that account yourself (DIRECT) or a partner resells your inventory (RESELLER):

google.com, pub-1234567890123456, DIRECT, f08c47fec0942fa0
appnexus.com, 1356, RESELLER, f5ab79cb980f11d1
OWNERDOMAIN=example.com

Apps use the same format in app-ads.txt, placed on the developer website from their app store listing.

Tool description

Enter a website or domain, pick the file and press the check button. Our server fetches the file where ad systems look for it and checks it against the IAB Tech Lab ads.txt 1.1 specification:

  • Whether the file exists and is served as text/plain. An HTML page at that address doesn't count.
  • Every line that breaks the format, with its line number, such as a missing field, an ad system that isn't a domain or a relationship that isn't DIRECT or RESELLER
  • The authorized ad systems, with how many DIRECT and RESELLER records each one has

ads.txt is read from the root domain, so www.example.com checks example.com/ads.txt. For app-ads.txt the tool follows the app-ads.txt rules: it tries the first subdomain without www. or m., then the root domain. Redirects are followed the way the specification allows.

AI assistants connected to the Rapid Tool Set MCP server can run the same check as ads_txt_checker.

Tips

  • Copy lines exactly from your ad network's dashboard. A typo in the account ID makes buyers skip that seller.
  • Add OWNERDOMAIN= with your company's domain. The specification recommends it, and buyers use it to match your entry in sellers.json.
  • A site that sells no ads programmatically can publish the placeholder line placeholder.example.com, placeholder, DIRECT, placeholder to say so.