ads.txt (Authorized Digital Sellers) is a public text file that lets publishers declare which companies may sell their digital ad inventory. Buyers can compare ad offers against it to spot unauthorized sellers. IAB Tech Lab describes it as a way to make counterfeit inventory harder to sell.
How is an ads.txt file built?
The file sits in the root directory of the site, for example https://example.com/ads.txt. Each line holds comma-separated fields:
- the domain of the advertising system
- the publisher’s account ID there
- the relationship,
DIRECTorRESELLER - optionally, a certification authority ID
Version 1.1 of the specification (August 2022) added the OWNERDOMAIN and MANAGERDOMAIN variables. Specifications, including app-ads.txt for mobile and connected TV apps, are maintained by IAB Tech Lab on GitHub.
Google’s AdSense help calls ads.txt not mandatory but highly recommended. It shows the status “Not found” when the file is missing and “Unauthorized” when the publisher ID is not listed.
| File | Standard or proposal? | Location | Purpose | Who reads it (documented) |
|---|---|---|---|---|
| robots.txt | IETF standard: RFC 9309 (September 2022) | / | tells crawlers which URLs they may fetch; per the RFC not a form of access authorization | Crawlers; Google's common crawlers always respect it for automatic crawls |
| llms.txt | Proposal (Jeremy Howard, 2024; version 2 from August 2026) | / or in a subpath | Markdown overview linking to Markdown versions of pages, for AI agents | per the proposal mainly coding agents; Google Search ignores it, says Google; Lighthouse only checks the fetch |
| llms-full.txt | no specification; not part of the llms.txt proposal, a convention of individual vendors (e.g. Mintlify) | / (Mintlify also serves /.) | entire documentation as Markdown in one file | no reader documented; Google Search doesn't use such files, says Google |
| ai.txt | Individual IETF Internet-Draft (June 2026, expires 2026-12-14), not a standard; predecessor by Spawning (2023) | /. (draft), / (Spawning) | AI usage preferences (training, scraping, indexing, caching), licensing, attribution | no reader documented in primary sources; Google Search doesn't use 'AI text files', says Google |
TDMRep (tdmrep.) | W3C Community Group Final Report (2024-05-10), not a W3C standard | /., HTTP header tdm-reservation or HTML meta tag | machine-readable reservation of text and data mining rights (Art. 4 DSM Directive) | parties performing text and data mining (per the specification) |
| security.txt | RFC 9116 (April 2022, informational, not standards track) | /. | contact for reporting security vulnerabilities; required fields Contact and Expires | Security researchers |
| ads.txt (this term) | IAB Tech Lab specification, version 1.1 (August 2022) | / on the root domain, subdomains as needed | declares the companies authorized to sell the site's ad inventory | advertising systems and buyers (ad system crawlers) |
| humans.txt | Initiative (humanstxt.org), no formal specification | / next to robots.txt, optionally linked via rel="author" | names the people who contributed to the website | people who want to know who is behind the website |
Only robots.txt, security.txt and ads.txt are based on a finalised specification (IETF or IAB Tech Lab). Google states that Google Search doesn't use new machine-readable files or 'AI text files' and ignores llms.txt files, which neither help nor harm Search visibility. TDMRep is a technical way to express the reservation under Art. 4 of Directive (EU) 2019/790. Not legal advice. Sources: RFC 9309: Robots Exclusion Protocol; Google: Overview of Google crawlers and fetchers (last updated 2026-06-12); The /llms.txt file, v2 (llmstxt.org); llmstxt.org: Changes (v2, August 2026); Google Search Central: Optimizing for generative AI features (last updated 2026-07-10); Chrome for Developers: Lighthouse llms.txt audit (2026-05-05); Mintlify: llms.txt and llms-full.txt; IETF Internet-Draft draft-car-ai-txt-wellknown-00 (2026-06-12); W3C TDMRep Community Group: TDM Reservation Protocol, Final Report 2024-05-10; RFC 9116: A File Format to Aid in Security Vulnerability Disclosure; IAB Tech Lab: ads.txt; IAB Tech Lab: ads.txt Specification Version 1.1 (GitHub); humanstxt.org. Last checked: .
Example
google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0
Common mistakes
- The file is missing or lists sellers the publisher no longer works with.
- The publisher ID is not included, so AdSense reports “Unauthorized”.
Sources
- ads.txt – IAB Tech Labiabtechlab.com
- ads.txt – Google AdSense Helpsupport.google.com
- Ads.txt specification 1.1 – IAB Tech Lab (GitHub)github.com
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