What Is Supply Path Optimization (SPO)?
A supply path is every platform and intermediary an ad placement passes through on its way from the publisher to the buyer. This article explains supply paths, how SPO works, three standards for checking them, and what Google’s ad tech ruling means.
Key Takeaways
What Is a Supply Path?
A supply path is every platform and intermediary an ad placement passes through on its way from the publisher to the buyer. Think of an ad placement as a box of fruit. A farm (the publisher) can sell directly to you, or the box can pass through several wholesalers first. The more hands it passes through, the more fees get added, and the harder it is to know where the fruit came from.
Ads work the same way. A banner on a news site’s homepage might be bought through two different paths:
The placement is the same, but Path B has two extra steps.[ For a full introduction to each role, see our earlier article: Decoding the AI-driven digital advertising landscape: Unraveling the roles of DSPs, DMPs, and SSPs ]
What Is Supply Path Optimization (SPO)?
Supply path optimization (SPO) is how buyers decide which supply paths to use: which to keep, which to consolidate, and which to stop using. The decision is based on transparency, whether the seller is authorized, traffic quality, efficiency, and measurable performance. Going back to the fruit analogy, SPO is choosing your suppliers. Is this wholesaler worth working with? Is there a more direct source?
SPO is not the same as finding the cheapest path. If Path A has a lower CPM (cost per thousand impressions) than Path B, that does not make A the better path. Likewise, a path should not be dropped just because its CPM is higher. What matters is whether the path delivers the inventory you want, with enough transparency to justify buying through it.
Why Do Supply Paths Affect Cost and Quality?
Every extra step adds fees and uncertainty. Long paths usually cause three problems:
Have Supply Paths Improved in the Past Two Years?
They have improved, but waste has not disappeared. ANA’s August 2025 benchmark report found that the median share of spend going to MFA sites fell to 0.8%, down from 15% two years earlier. The median number of SSPs fell from 19 to 17, and the number of active domains dropped from 53,799 to 28,958. The same report noted that about $26.8 billion in global media value is still lost each year to inefficiencies.
How Do You Tell If a Supply Path Is Good?
Start with three public standards, then confirm with real performance data. IAB Tech Lab frames supply chain transparency around three questions: who is allowed to sell, who the seller is, and how the request reached the buyer. They map to the standards below.
Standard | Published by | Question it answers | In plain terms |
|---|---|---|---|
ads.txt | Publisher | Who is authorized to sell my placements? | The publisher’s list of authorized sellers |
sellers.json | SSP or exchange | Who are the sellers that sell through me? | The SSP’s list of sellers |
SupplyChain object (schain) | Attached to the bid request | Which parties sold or resold this request? | The route record of the placement |
If all three line up, it only means the path is authorized and transparent. You still need to check real data, including placement content, impression quality, and performance, to confirm it meets your expectations.
What Changed Recently? The Google Ad Tech Ruling and IAB Tech Lab Best Practices
Two developments in September 2026 both relate to supply chain transparency.
The first is the Google ad tech antitrust case. AdX is the ad exchange Google operates, and DFP is the Google ad server publishers use to manage their placements. The two had been found to be unlawfully tied together. On September 2, the court decided not to require Google to sell AdX and chose behavioral remedies instead. The full opinion was made public in mid-September. It includes the following:
According to the opinion’s timeline, AdX must be opened to outside ad servers within 12 months, and connected to Prebid within 12 to 15 months. Google has said it disagrees with the liability ruling and plans to appeal, so details may still change.
The second is IAB Tech Lab’s Programmatic Governance Council, which released Programmatic Best Practices version 1.0 on September 16. It asks participants to keep their ads.txt and sellers.json files up to date. The public comment period closed on October 16.
The direction is the same in both cases: who is selling, how they sell, and how many hands are involved are all being asked to be made clear. For buyers, checking the path before buying matters more than it used to.
What Can Agencies and Publishers Do?
Agencies
Publishers
Conclusion
Programmatic advertising is about more than bidding. It is also about where the traffic comes from. As court rulings and industry standards continue to move forward, supply paths will come up more and more often. Rather than checking after something goes wrong, make “how did this placement get here?” a standing question before you buy.
Frequently Asked Questions
Q1: What is the difference between ads.txt and sellers.json?
A1: ads.txt is published by the publisher and lists who is authorized to sell its placements. sellers.json is published by an SSP or exchange and lists the sellers that sell through it. Buyers compare the two to confirm that a seller’s identity and authorization match.
Q2: How can a publisher check its ads.txt file?
A2: ads.txt sits in the root directory of the website. Add /ads.txt to the end of the site’s URL to view it. Check that every seller listed is still an active partner, that there are no unfamiliar entries, and that the list matches the sellers.json files published by your SSP partners.
Q3: What is a supply path?
A3: A supply path is every platform and intermediary an ad placement passes through on its way from the publisher to the buyer. The same placement can reach buyers through different paths, and the more steps involved, the higher the cost and uncertainty tend to be.
Q4: What does SPO mean?
A4: SPO stands for supply path optimization. It is how buyers decide which supply paths to use for buying ads, based on transparency, authorization, quality, and performance, not just price.
If you would like to learn more about how programmatic ads are bought and where traffic comes from, feel free to contact us.
