Understanding Multi-Format Support in Prebid: What Publishers Need to Know

Monetizing inventory efficiently has always been a balancing act for publishers. One challenge: how to serve diverse ad formats—banner, video, or native—without complicating your setup or losing auction competitiveness. This is where Prebid’s multi-format support becomes critical.

If you’re responsible for optimizing ad yield or troubleshooting header bidding setups, understanding exactly how multi-format works in Prebid can mean the difference between unrealized potential and best-in-class revenue strategies. Let’s demystify the technical mechanics and explore the real-world implications for your operations.

What Are Multi-Format Ad Units in Prebid?

In digital advertising, multi-format ad units are placements that can accept more than one creative type—such as banner, video, or native—within a single auction. Prebid.js and Prebid Server both allow publishers to configure ad units this way, aiming to match diversified demand sources and maximize fill rates.

For publishers, this means a single ad slot could serve a banner to one user, a video to the next, and even native content as needed. On paper, this flexibility sounds ideal for increasing competition and revenue within each impression.

How Multi-Format Impacts Header Bidding Flow

When you set up a multi-format ad unit in Prebid, bid requests are sent to demand partners with the supported formats declared. Not all adapters interpret this the same way, so the actual auction dynamics hinge on how each bid adapter responds:

– Adapters can decide which formats to bid on.
– Some optimize for the highest-yielding format, while others ignore multi-format units entirely.

For a real-world example: if you create a 300×250 unit that allows both banner and outstream video, some bidders may respond with only banner or only video, while others might send both.

Bid Adapter Behavior: What Publishers Need to Check

The ability of each demand partner (bid adapter) to handle multi-format requests is not a given. Prebid groups adapter behaviors into three categories, which you’ll see referenced in documentation. Understanding these categories can save hours of troubleshooting and stop lost revenue before it starts.

Adapter Response Modes

– Will-bid-on-any: The adapter looks at all eligible formats you’ve specified and may bid on one, more, or all of them. Typically, this maximizes competition and yield.
– Will-bid-on-one: The adapter picks a single format to respond with—for example, either banner or video, depending on its internal logic or your configuration. This may be less flexible but avoids compatibility issues.
– Will-not-bid: The adapter ignores ad units with multiple formats. If you add such a partner expecting additional demand, you may see zero response.

Example: Suppose Partner A is ‘will-bid-on-any’ and Partner B is ‘will-not-bid’. If both are running on a multi-format unit, only Partner A contributes bid responses, potentially skewing competition and affecting fill.

Prebid.js and Prebid Server: Configuration Realities

Both Prebid.js (for client-side header bidding) and Prebid Server (for server-side header bidding) support multi-format ad units, but the way you configure and troubleshoot them may differ.

Key Configuration Steps

– Define the ad unit in your Prebid setup with all intended formats (banner, video, native).
– Check each adapter’s documentation for their multi-format stance: don’t assume universal support.
– Ensure your ad server (e.g., Google Ad Manager) line items and creatives align with possible responses, to prevent mis-matches or empty slots.

Example: In Prebid.js, your ad unit config might look like:

{
code: ‘ad-slot-1’,
mediaTypes: {
banner: {…},
video: {…},
native: {…}
},
bids: […]
}

If Prebid Server is in play, ensure your server config also reflects all desired formats, and check that endpoint demand partners support those combinations.

Common Publisher Pitfalls and Troubleshooting Tips

Multi-format support increases auction flexibility, but without careful setup, it can introduce confusion and missed revenue opportunities. Many issues stem from mistaken assumptions about adapter support or from misaligned ad server and Prebid configurations.

Troubleshooting Scenarios

– Ad slot serving blank/no fill: Commonly caused by at least one adapter not supporting the multi-format setup, so always test each partner.
– Low response rates: Review adapter docs and logs to ensure each partner is capable of responding with your chosen formats.
– Creative misalignment: Ensure your ad server can properly handle and render each format that might return, or risk serving errors or blank ads.

Example: You enable both banner and native, but your ad server has only banner creatives configured. Native bids will return, but they won’t render—leading to wasted impressions.

What this means for publishers

For publishers, the way you configure and manage multi-format ad units has direct operational impact. You need to know which demand partners actually support your format combinations, or you risk underfilling inventory and degrading auction pressure. Tracking and testing adapter support is as critical as the technical setup itself. Mistakes waste impressions, confuse ad ops teams, and can quietly lower revenue without obvious errors appearing in basic reports.

Practical takeaway

When deploying multi-format ad units, audit each demand partner’s ability to process and bid on your chosen formats. Don’t assume all adapters behave the same—mixing ‘will-bid-on-any’ and ‘will-not-bid’ partners can unbalance the auction. Run controlled tests in your environment: set up monitoring for response rates and confirm your ad server can support and render every possible format.

Document your adapter settings and revisit them whenever adding new partners or changing formats. This approach maximizes fill, ensures accurate reporting, and reduces day-to-day troubleshooting effort. Put operational rigor behind your multi-format strategy to turn technical flexibility into meaningful yield gains.