ads.txt and sellers.json for MCM publishers

Programmatic buyers want to know that the company selling an impression is actually allowed to sell it. Two public standards from the IAB Tech Lab make that check possible: ads.txt, which lives on your site, and sellers.json, which lives on the seller's side. Together they let anyone trace an impression back to its owner.

If you already use AdSense, you may have an ads.txt entry for it. Keep entries for demand you still use; joining an MCM partnership may require additional lines supplied by that partner.

ads.txt: your public list of authorized sellers

ads.txt is a plain text file at the root of your domain — for example, example.com/ads.txt. Each line names a company that is allowed to sell your inventory, along with your account ID at that company and the type of relationship.

A line has this shape:

<seller domain>, <your account ID>, <DIRECT | RESELLER>, <optional certification ID>

• DIRECT means you control the account at that seller.
• RESELLER means someone else controls the account and sells on your behalf.

sellers.json: the other side of the check

sellers.json is published by the selling platform. It lists the publishers and intermediaries it pays, each identified by the same account ID used in ads.txt. Buyers compare the two: if your ads.txt says a company may sell your inventory under a given ID, that company's sellers.json should list the same ID and identify who it belongs to.

What changes under MCM

When you join an MCM partnership, your inventory is sold through the parent. Your ads.txt therefore needs to authorize the parent. In practice, the parent will give you the exact lines to add — typically an entry for Google's selling system with the relevant publisher ID and relationship type. Follow your partner's instructions precisely; small typos break the match.

Keep your existing lines for any other demand you still use. ads.txt is additive, not exclusive.

Common mistakes

• Publishing the file somewhere other than the root of the domain that serves the ads.
• Forgetting subdomains or additional domains that also carry ads.
• Leaving lines from partners you stopped working with long ago.
• Using DIRECT for an account you don't actually control.
• Serving the file with redirects, HTML wrappers or the wrong content type, so crawlers can't read it.

A quick self-check

1. Open yourdomain.com/ads.txt in a private browser window. It should load as plain text.
2. Confirm every line belongs to a partner you currently work with.
3. Confirm the account IDs match what each partner gave you.
4. Check Ad Manager's ads.txt status for any domains it flags.