Understanding Post-Bid with Prebid.js: A Practical Guide for Publishers

Ad monetization is full of technical options, each with its benefits and trade-offs. One approach that often gets overlooked is the Post-Bid setup with Prebid.js. For publishers with limited resources or unique workflow needs, Post-Bid can simplify the tech stack while maintaining access to multiple demand sources.

This guide explains what Post-Bid is, how it fits into your ad server workflow, and when it makes sense to consider it over full header bidding. You’ll also find concrete, implementation-focused advice tailored for publishers and ad operations teams.

What Is Post-Bid in Prebid.js?

Post-Bid is a method where Prebid.js is loaded within a specific ad creative, but only after the ad server has determined that the ‘post-bid’ line item should serve. Instead of bidders competing before the impression reaches the ad server (as in traditional header bidding), the auction happens after the ad server’s decision. This means only impressions routed to the post-bid line item enter the Prebid auction.

How Post-Bid Differs from Header Bidding and Mediation

– In header bidding, Prebid.js runs before the ad server, and all demand sources get a fair shot at each impression—often resulting in higher yields.
– In mediation, demand sources compete in a sequence (waterfall), often adding latency and complexity.
– In post-bid, all connected demand sources compete for the impression, but only after the ad server passes on direct-sold and prioritized programmatic deals.

Step-by-Step: The Post-Bid Auction Flow

Understanding the sequence helps both ad ops teams and technical leads make smarter implementation choices. Here’s how the process unfolds:

A Typical Post-Bid Workflow Example

1. A user visits the publisher’s page, sending an impression request to the ad server (like GAM).
2. The ad server sifts through its line items—direct-sold, exchange, network, and post-bid.
3. If the post-bid line item wins, its creative is delivered. Embedded in this creative is Prebid.js, which starts a local auction among configured demand partners.
4. The winning creative from this post-bid auction renders in the slot, delivering the ad.

Pros and Cons of Using Post-Bid

While Post-Bid offers workflow advantages, it comes with limitations that are crucial for publishers to weigh.

Key Benefits for Publishers

– No need for heavy engineering or page code changes—just insert a third-party tag into your ad server creative.
– Minimal impact on site latency because the auction happens after ad delivery starts.
– Simplifies the line-item setup compared to full header bidding.

Major Limitations to Consider

– Bid prices in the post-bid line item are typically fixed and based on past data; true dynamic competition with direct-sold or AdX demand isn’t possible.
– Running Prebid.js multiple times (one for each post-bid placement) can strain the browser and complicate reporting, as aggregated results mask partner-level performance.
– Less monetization potential than full pre-auction header bidding but generally more flexible than basic mediation.

Practical Implementation and Common Pitfalls

Getting Post-Bid right is straightforward, but mistakes can lead to lost revenue or tracking headaches.

Concrete Setup Tips

– Use an accurate historical CPM for your post-bid line items in the ad server; set it too high or low and you’ll either cannibalize other demand or lose yield.
– If you use multiple post-bid units on one page, be aware of potential performance degradation or user experience hiccups as each unit may load its own Prebid.js instance.
– For reporting, consider pairing with a dedicated analytics solution like Prebid Analytics or a third-party log collector since most ad server reports will only show combined revenue for the post-bid line item.

What this means for publishers

Switching to or testing Post-Bid can reduce the need for heavy engineering, allowing non-technical teams to run auctions with minimal site changes. It enables publishers with limited resources, legacy setups, or short-term demand tests to access multiple demand sources quickly. However, reduced transparency and the lack of real-time dynamic allocation mean that close monitoring of revenue, auction win rates, and page performance is still essential.

Practical takeaway

If you are working with constrained technical resources or need a simple way to onboard multiple demand partners, Post-Bid is an accessible solution worth considering. Just be conscious of the static nature of pricing and the need to track revenue at a more granular level outside your ad server.

Start with conservative CPMs, watch for browser load challenges on high-ad-density pages, and invest extra effort in reporting and analytics. For many publishers—especially those testing new demand or seeking operational simplicity—Post-Bid offers a low-friction entry point to the benefits of header bidding, even if it’s not as powerful as full pre-auction competition.