A plain text file placed at the root of a domain listing authorized ad sellers for selling the publisher's inventory.
Readers involved in programmatic advertising and AI-driven ad systems who need to verify seller legitimacy.
01What it is and how it works
ads.txt stands for Authorized Digital Sellers. It is a plain text file placed at the root of a domain, for example https://example.com/ads.txt. The file contains lines that list each seller authorized to sell the publisher's inventory. Each line has four fields: the domain of the seller, the seller's account ID, the type of relationship (direct or reseller), and an optional tag ID. When an ad exchange or a demand-side platform (DSP) wants to buy inventory, it can fetch the ads.txt file and compare the seller information in the bid request against the list. If the seller is not listed, the buyer can reject the bid. This reduces the risk of domain spoofing and unauthorized reselling. In the context of AI-search, ads.txt becomes relevant because AI-driven ad systems can automatically check this file to filter out unauthorized sellers, making the programmatic buying process more transparent and trustworthy.
It's a simple list of who is allowed to sell ads on your site. You put it on your website, and ad buyers check it to confirm they are buying from a real seller.
02What to do about it
If you are a publisher, you should create an ads.txt file and host it on your domain. Start by listing all the ad sellers you work with directly, such as your ad network or ad exchange. Include their seller IDs and the relationship type. If you use resellers, list them as well. Once the file is live, verify that it is accessible at the correct URL and that it is formatted correctly. You can use Google's ads.txt validator or a similar tool to check for errors. If you are an advertiser or a buyer, you should check the ads.txt file of any site you plan to buy inventory from. Many DSPs already do this automatically, but you can manually inspect the file to understand the seller landscape. For AI-search products, ensure that your system fetches and parses ads.txt when evaluating ad placements or generating ad recommendations.
03How it is measured or noticed
The presence and correctness of ads.txt are measured by how often the file is fetched and how well it matches the bid requests. Ad exchanges and DSPs log the number of times they check ads.txt and flag mismatches. You can monitor the file's accessibility and its content through server logs or third-party monitoring tools. In an AI-search context, you might notice ads.txt when an AI system uses it to filter out unauthorized sellers. For example, if a bid request comes from a seller not listed in ads.txt, the AI system can reject it. You can measure the effectiveness by tracking the percentage of bid requests that pass the ads.txt check. A higher pass rate indicates better compliance. You can also check the file's format and ensure it is not empty or malformed.
04Common mistakes
- Forgetting to update ads.txt when you change ad networks or seller IDs.
- Using the wrong seller ID, often because you copy it from a different account.
- Placing the file in a subdirectory instead of the root of the domain.
- Including resellers without verifying that they are actually authorized.
- Ignoring the tag ID field when it is required by your ad partner.
- Not checking for typos in the domain or the file name.
05Limits
ads.txt only covers programmatic ad inventory sold through exchanges and networks. It does not apply to direct sold campaigns, private marketplace deals, or ads served outside the open web. It also does not guarantee that a listed seller is trustworthy; it only confirms that the publisher claims to have a relationship. ads.txt is often confused with app-ads.txt, which is the same concept but for mobile apps. Another confusion is with robots.txt, which controls crawler access, not ad sellers. In AI-search, ads.txt is not a ranking factor and does not affect organic search results. It only matters for advertising operations. Also, ads.txt does not prevent all types of ad fraud; it only addresses domain spoofing and unauthorized reselling.
06Worked example
Suppose you run a news site and you sell ads through Google AdSense and a reseller called AdNetworkX. Your ads.txt file might look like this:google.com, pub-1234567890, DIRECT, f08c47fec0942fa0adnetworkx.com, 987654321, RESELLER
When an AI-powered ad buyer sees a bid request from AdNetworkX, it checks your ads.txt. If the seller ID matches and the relationship is RESELLER, the buyer can accept the bid. If AdNetworkX is not listed, the buyer rejects the bid, preventing a spoofed impression.
Frequently asked questions
How is ads.txt different from sellers.json?
ads.txt is a file that publishers place on their own domain to declare authorized sellers of their inventory, while sellers.json is a file that ad exchanges publish to list the sellers they work with. ads.txt verifies the publisher's side, and sellers.json verifies the exchange's side. They work together to create a chain of trust.
Should I create an ads.txt file even if I only sell direct ad inventory?
Yes, you should create one if you sell any programmatic inventory, even if it's direct through a private marketplace. ads.txt helps prevent unauthorized sellers from reselling your inventory, which can happen even in direct deals. If you only sell directly without any programmatic component, you may not need it, but it's still a good practice to declare your authorized sellers.
Who is responsible for creating and updating the ads.txt file?
The publisher is responsible for creating and hosting the ads.txt file on their domain. The file must be placed at the root of the domain, typically at /ads.txt. It should be updated whenever you add or remove an authorized seller, such as an ad exchange or network.
Does ads.txt actually prevent ad fraud?
ads.txt helps reduce but does not eliminate ad fraud. It prevents unauthorized sellers from claiming your inventory, which blocks a common form of domain spoofing. However, it doesn't address other fraud types like bot traffic or click fraud, so it's one tool in a broader fraud prevention strategy.
What happens if my ads.txt file is missing or incorrect?
If your ads.txt is missing or incorrect, buyers may refuse to bid on your inventory, or they may pay less because they can't verify the sellers. You might notice a drop in fill rates or revenue. In severe cases, your inventory could be blocked entirely by exchanges that require a valid ads.txt.
How long does it take for ads.txt changes to take effect?
Changes to ads.txt typically take effect within a few hours to a day, but it can vary depending on how often buyers and exchanges crawl the file. Most systems check ads.txt regularly, often every few hours. You should monitor your bid requests after making changes to see if they are being accepted.
Asked out loud
spoken, not typedThe same term in the words somebody uses speaking to an assistant rather than typing into a box — written from the situation, which is why each one carries the situation it came from.
It's ads.txt. You need to create a text file with your authorized sellers and host it at the root of your domain. This tells buyers who is allowed to sell your inventory.
Tell them it's ads.txt. If the file is missing or wrong, buyers can't verify your sellers, so they skip your inventory. Fix the file and the issue should clear up.
It's ads.txt. You can check by visiting yourdomain.com/ads.txt on your phone. If it's not there, that's the problem.