Understanding Prebid.js MultiBid: Unlocking Multiple Bids per Bidder for Publishers

Header bidding has transformed programmatic advertising by improving competition and transparency, but most publishers still see only a single bid per bidder hitting their ad server. This default behavior can limit revenue opportunities and flexibility in how different formats or fallback strategies are managed.

The Prebid.js MultiBid module addresses this gap directly. By allowing publishers to configure multiple bids per bidder, it unlocks better yield management, granular format support, and smarter fallback mechanisms—all with the control to tune for your specific needs. Here’s what publishers need to know to make MultiBid work for them.

What Is the Prebid.js MultiBid Module?

The MultiBid module is an optional Prebid.js feature that enables publishers to forward more than one bid per bidder to their ad server for each ad unit. By default, Prebid will only pass the highest bid per bidder, leaving potentially valuable bids behind. With MultiBid, you can capture multiple price points, formats, or fallback options from a single bidder and put them to work in your ad server’s competition.

Why Multiple Bids Matter in Practice

Multiple bids per bidder let you:
– Compete both banner and outstream video bids from a single SSP.
– Provide backup bids for fallback scenarios (such as video failover).
– Allow more nuanced targeting logic in your ad server—if primary bids are blocked by targeting, alternative bids can still win.
For example, if a bidder can deliver both a video and a banner bid for the same ad slot, MultiBid makes it possible for both bids to participate. If your primary (highest) bid is blocked due to targeting or advertiser restrictions, the next-highest eligible bid from the same SSP can still be considered.

How MultiBid Works: Configuration and Ad Server Targeting

Implementing MultiBid starts with the right configuration. Publishers define exactly which bidders are allowed to submit multiple bids, how many to permit, and how these extra bids are labeled and targeted. This control is key for operational success and troubleshooting.

Configuring MultiBid in Prebid.js

MultiBid configuration is handled via setConfig, with options to specify bidders individually or as groups. Each entry requires a ‘bidder’ (or ‘bidders’ array), the maximum number of bids allowed (maxBids), and an optional custom prefix (targetBiddercodePrefix) for how these bids appear in ad server targeting.

Example:
pbjs.setConfig({
multibid: [
{bidder: “bidderA”, maxBids: 2, targetBiddercodePrefix: “bidA”},
{bidder: “bidderB”, maxBids: 3, targetBiddercodePrefix: “bidB”},
{bidders: [“bidderC”, “bidderD”], maxBids: 3}
]
});

Without a targetBiddercodePrefix, extra bids are cached for fallback but not sent to the ad server as unique keys.

Impact on Ad Server Setup and Line Items

With MultiBid, you are not required to create additional line items for extra bids unless you want to optimize for cases where the main bid might be blocked or skipped. For example:
– If you have complex blocking or custom targeting rules, setting up line items for secondary and tertiary bids (like hb_pb_bidB2, hb_pb_bidB3) can help prevent missed revenue.
– If your demand partners favor different formats (e.g., video and banner), MultiBid helps ensure all relevant bids are eligible for each slot, provided your ad server line items and creatives are set up to match the more granular targeting keys.

Common Pitfalls and Implementation Tips

Adopting MultiBid isn’t complex, but several areas deserve close attention to avoid predictable snags. Even seasoned ad ops teams can miss nuances if they treat MultiBid as a plug-and-play upgrade.

Module Build and Compatibility

The MultiBid module is not bundled with Prebid.js by default. You must include it explicitly in your build process (e.g., gulp build –modules=multibid,otherModules). Omitting this step will result in no change in behavior—even with config present.

Bid Adapter Behavior and Bid Caching

Some bid adapters inherently return multiple bids per request. MultiBid controls how many of these get processed and surfaced. Be sure bidders you want to support are explicitly listed in your config; otherwise, any additional bids could be ignored or not passed beyond the cache.

Also, if you use useBidCache, secondary bids are available for internal fallback, but only those with the right MultiBid targeting setup will reach the ad server.

What this means for publishers

MultiBid offers publishers much greater control over bid flow and competition, unlocking incremental revenue by making secondary and tertiary bids from the same bidder eligible for the auction. This is especially valuable in environments with diverse formats or sophisticated targeting rules—whether for fallback video, outstream, or formats with varying advertiser exclusions. For ad ops teams, it introduces new opportunities for optimization but makes configuration discipline and ad server setup more critical to avoid leaving money on the table.

Practical takeaway

To benefit from MultiBid, ensure your Prebid.js build includes the MultiBid module and that you’ve thoughtfully configured which bidders and how many bids per bidder you want to allow. Audit your ad server setup, including line items and targeting keys, to ensure extra bids are eligible to compete and not inadvertently blocked.

Monitor bidder performance and bid distribution with reporting—secondary and fallback bids can be meaningful revenue drivers if surfaced correctly. Use MultiBid’s controls deliberately to support your auction strategy, not just as a catch-all. Start small (select key bidders, limited maxBids) and expand as you validate the impact in real-world operations.