Prebid vs Open Bidding vs Amazon TAM and APS: How the Three Demand Paths Differ

The three paths differ first in where the competition happens and who controls it.

Prebid. The publisher loads Prebid.js on the page, or calls Prebid Server. Prebid asks each chosen bidder for a price, waits until a timeout, then passes the best bids to the ad server as key-values. Prebid can also act as the final decision layer if the publisher wants (Prebid.org).

Open Bidding. The page calls Google Ad Manager with the normal Google tag. Ad Manager sends bid requests to the exchanges in each eligible yield group, they run their own auctions, and their best bids compete with AdX and direct line items (Google Ad Manager Help). Google sets the reserve price and sends it to all bidders.

Amazon TAM and UAM. The page makes one call to Amazon's server, which gathers bids from Amazon and other buyers. The winning bids return to the page and are passed to the ad server (APS). TAM needs Amazon's multi-slot tag and a contract with each demand partner. UAM is invitation-only and aimed at web publishers on Google Ad Manager (APS UAM).

People search "prebid vs header bidding" as if they were rivals. They are not. Header bidding is the method; Prebid is the most widely used open-source software for it, maintained by Prebid.org (Prebid.org). Other wrappers exist, and some commercial ones are built on Prebid code.

The suite has three parts. Prebid.js runs in the browser. Prebid Server runs the same auction on a server, which suits apps, AMP pages and streaming TV. Prebid Mobile is the app SDK (Prebid.org). Prebid Server ships as source code in Go and Java, with no configuration tools from the open-source project. Most publishers use a hosted version from a member company (Prebid Server overview). Running it yourself is covered in the Prebid Server self-hosting guide.

The bidder directory lists more than 700 entries, including aliases (Prebid.org bidders, accessed 9 November 2026). Prebid's own guidance is that client-side setups "typically use 5 to 15 demand partners", with more on the server (Prebid.org).

Amazon Publisher Services (APS) sells publisher tools under the A9.com company. Its two header bidding products differ in who holds the buyer contracts (APS, accessed 9 November 2026).

Transparent Ad Marketplace (TAM) is open to publishers that integrate the multi-slot APS tag. The publisher contracts with each demand partner, and Amazon charges bidders a 2.5% service fee. APS says publishers can see "the record of each auction", including each bidder, bid and winner (company claim) (APS).

Unified Ad Marketplace (UAM) is invitation-only. It is "designed for web publishers who directly or exclusively represent their site" and use Google Ad Manager. Amazon brings the buyers, which include OpenX, PubMatic, Magnite and Index Exchange (APS UAM). UAM's fee is not published.

The Prebid adapter. APS demand is now reachable through Prebid with the bidder code aps. APS is listed as a Prebid.org member, supports the SupplyChain object and deals, and has IAB Global Vendor List ID 793 (Prebid.org). The Prebid Server version needs an APS account and is in limited beta. For publishers this means Amazon demand can sit inside a Prebid auction instead of a separate tag.

Each path has its own fee, and the exchange fee still applies on top. The table lists what is public today.

Worked example. Suppose an exchange's best bid is a $4.00 CPM. Through Open Bidding at the 2019 typical rate of 5%, Google keeps $0.20 per thousand and the publisher is paid $3.80 (CMA, Appendix R). Through TAM, the 2.5% fee falls on the bidder, which is $0.10 per thousand at the same price (APS).

A bidder that prices the TAM fee in may bid lower, so the publisher still bears some of it. Through Prebid, no path fee applies, but a hosted Prebid Server contract may add one. These are our calculations on published rates (APS); the current Open Bidding rate is not public.

The SSP fee is the larger number in every case. The industry studies on it are summarised in the ad tech tax explained.

What you can see differs by path.

1. Prebid keeps the auction in the publisher's hands. Analytics adapters send every bid, timeout and win to the publisher's own analytics (Prebid.org). The code is public, so the auction logic can be read.
2. Open Bidding reports aggregated impressions, revenue, bid requests and responses by exchange and yield group (Google Ad Manager Help). Google says no bidder sees another's bid before the auction ends, and the highest net bid wins (Google Ad Manager Help).
3. Amazon TAM offers auction-level records (company claim) (APS). UAM's reporting detail is not documented on its public page.

One detail matters for brand safety. Ad Manager's protections, such as blocking rules, "do not apply to Open Bidding partners by default"; publishers must turn on creative filtering per partner (Google Ad Manager Help).

There is no neutral, current benchmark that measures all three paths on the same pages. What exists are the vendors' own descriptions.

• Google says Open Bidding "reduces the latency historically associated with header bidding", because it needs no extra page code (company claim) (Google Ad Manager Help).
• Amazon says TAM handles bidding from the server after one ad call (company claim) (APS).
• Prebid recommends fewer client-side bidders, moving bidders to Prebid Server, a firm timeout and A/B tests of each setting (Prebid.org).

The honest test is your own. Run each setup on a share of traffic and compare revenue per thousand page views, not just CPM.

Google's help page named 36 Open Bidding partners when we checked it on 9 November 2026 (Google Ad Manager Help). They are listed below in alphabetical order.

The page adds: "Some exchanges participate in Open Bidding, but have requested not to be listed." It also limits some partners by region, such as Improve Digital, Magnite Streaming and Nativo. A publisher needs its own contract with any partner before Google can connect it.

On 2 September 2026 Judge Brinkema ordered conduct remedies instead of a sale of AdX; the opinion was unsealed on 16 September (AdExchanger, 16 September 2026). Alphabet said on 28 October 2026 that final judgment had not been entered and that it plans to appeal once it is (Alphabet 10-Q, Q3 2026). The timetable below runs from an Effective Date 60 days after entry.

PubMatic put the engineering work for the Prebid integration at roughly a year to 15 months (PPC Land, 6 October 2026). We found no remedy aimed at Open Bidding itself in the published summaries. The publisher-side detail is in Google ad tech remedies explained, and the move to a rival ad server is in switch ad servers without losing AdX demand.

These criteria follow from the documented mechanics. Many publishers run more than one path.

Prebid fits when you have engineers or a managed partner, want every bid in your own data, and plan to use AdX inside Prebid once the remedies take effect.

Open Bidding fits when you want extra exchanges without page code and accept Google's share and aggregated reporting. It also suits app publishers already on Google's SDK.

Amazon TAM fits when you want Amazon's demand plus your own buyer contracts through one server call, with a published fee. UAM fits when you are invited and prefer Amazon to hold the buyer contracts.

The Google Ad Manager vendor profile covers the ad server these paths plug into. The wider series on running your own sell-side stack is in the publisher independence hub.

1. Prebid.org, "Introduction to Prebid" (accessed 9 November 2026). https://docs.prebid.org/overview/intro.html
2. Prebid.org, "What is Prebid.org" (accessed 9 November 2026). https://docs.prebid.org/overview/what-is-prebid-org.html
3. Prebid.org, "Prebid Server overview" (accessed 9 November 2026). https://docs.prebid.org/prebid-server/overview/prebid-server-overview.html
4. Prebid.org, "APS" bidder adapter documentation (accessed 9 November 2026). https://docs.prebid.org/dev-docs/bidders/aps.html
5. Prebid.org, bidder directory (accessed 9 November 2026). https://docs.prebid.org/dev-docs/bidders.html
6. Google, Ad Manager Help, "Introduction to Open Bidding" (accessed 9 November 2026). https://support.google.com/admanager/answer/7128453?hl=en
7. Google, Ad Manager Help, "How Open Bidding works" (accessed 9 November 2026). https://support.google.com/admanager/answer/7128958?hl=en
8. Google, Ad Manager Help, "Open Bidding payments" (accessed 9 November 2026). https://support.google.com/admanager/answer/9752368?hl=en
9. Amazon Publisher Services, "Transparent Ad Marketplace" (accessed 9 November 2026). https://aps.amazon.com/aps/transparent-ad-marketplace/
10. Amazon Publisher Services, "Unified Ad Marketplace" (accessed 9 November 2026). https://aps.amazon.com/aps/unified-ad-marketplace/