ads.txt and the Authorized Seller Chain: A Practical Guide

ads.txt (Authorized Digital Sellers) is a simple text file at yourdomain.com/ads.txt listing every company authorized to sell or resell your ad inventory. Buyers' systems crawl it; bids routed through unauthorized paths get discarded. It exists to kill domain spoofing — fraudsters claiming to sell premium inventory they don't represent.

The format in 30 seconds

Each line has four fields: the ad system domain, your seller account ID, the relationship (DIRECT or RESELLER), and an optional certification authority ID. DIRECT means you hold the account; RESELLER means the partner is authorized to resell through it.

The mistakes that silently cost money

• Missing reseller lines. If your monetization partner routes demand through exchanges that aren't listed, those bids are filtered before you ever see them. This is the most common and most expensive error.
• Stale entries. Old partners left in the file expand your attack surface and confuse buyers' supply-path algorithms.
• Conflicting DIRECT claims. Two companies claiming DIRECT for the same exchange account triggers buyer distrust.
• No app-ads.txt for apps, or missing the file on AMP cache domains.

sellers.json and supply-path transparency

ads.txt's mirror image is sellers.json: every ad system publishes who it pays. Together they let a buyer trace the full path from advertiser dollar to publisher. Buyers increasingly prefer short, clean paths (SPO — supply path optimization), so a tidy authorized-seller chain isn't just fraud hygiene; it actively improves your bid density.

Keep it automated

Authorized seller files should be generated and monitored, not hand-edited. PubMonetX scans every managed domain daily, flags missing or risky lines in the dashboard, and emails the publisher the exact diff to apply — because a single missing line can quietly filter five figures of monthly demand.