Short answer
ads.txt is a public file on a publisher's domain listing who is allowed to sell its ads, marked DIRECT or RESELLER. sellers.json is the same list from the other side: each exchange publishes who its sellers are. The SupplyChain object travels in every bid request and lists each company that handled the impression. Cross-check the three to buy direct paths, avoid spoofed domains and cut middlemen who take a share of your budget.
Three standards, one question
| Standard | Published by | Lists | Answers |
|---|---|---|---|
| ads.txt (app-ads.txt for apps) | The publisher, on its own domain | Ad system, seller ID, DIRECT or RESELLER | Is this seller allowed to sell this site? |
| sellers.json | Each SSP or exchange | Seller ID, name, domain, type: publisher, intermediary or both | Who is behind that seller ID? |
| SupplyChain object (schain) | Added to each bid request by every seller in the chain | Each node's domain and seller ID | How many hands touched this impression? |
IAB Tech Lab introduced ads.txt in 2017 and sellers.json and the SupplyChain object in 2019. Cross-referencing all three lets a buyer check each step from publisher to exchange.
Why it matters for B2B budgets
Every reseller in the chain takes a share, and spoofed inventory, ads sold as a known business site but served elsewhere, only works where these checks are skipped. B2B campaigns buy small, valuable audiences, so a few points of leakage cost more per target account than in mass-market media.
A five-minute check on any publisher
- Open
publisher.com/ads.txtin a browser. - Find your DSP's main exchanges. Are they listed, and as DIRECT?
- Look up a seller ID in that exchange's sellers.json. Does the name match the publisher?
- Count RESELLER lines. A long list isn't wrong, but it means more paths to compare.
- Ask your DSP for the supply path report on that domain: which seller IDs actually won your impressions.
What to ask your DSP or agency
- Do you bid only on ads.txt-authorised paths?
- Can you report spend by supply path and seller ID?
- Do you prefer DIRECT paths when the same impression is offered several ways?
- How do you treat made-for-advertising sites? Why it matters.
- Can the core of our budget run through deals with named publishers? Deal types.
Questions
What is ads.txt?
A text file at publisher.com/ads.txt listing the ad systems and seller accounts authorised to sell the publisher's inventory, each marked DIRECT or RESELLER. IAB Tech Lab introduced it in 2017.
What is sellers.json?
A public file that each SSP or exchange publishes, listing its sellers and whether each is a publisher, an intermediary or both.
What is the SupplyChain object?
A field in the bid request, often called schain, that lists every company that took part in selling the impression, from the publisher to the exchange.
Do I need to check this myself?
Ask your DSP or agency to buy only authorised paths and to report supply paths. Spot-check your top publishers yourself; it takes minutes.
Facts checked against the sources below on 19 October 2026. Ad platforms change quickly: if something here no longer matches what you see, tell us and we will update it.
