Glossary
What is a VAST tag?
VAST — Video Ad Serving Template — is an IAB standard XML format that lets a video ad server tell a video player what ad to show, where to fetch it, and what to report back.
The problem VAST solves
Video players and video ad servers are built by different companies with no relationship. Without a shared format, every player would need bespoke integration with every ad server.
VAST is that shared format. The ad server returns an XML document; any VAST-compliant player can read it and render the ad correctly. It is unglamorous plumbing and it is the reason programmatic video works across an ecosystem of thousands of players.
What a VAST response contains
- Media files at several bitrates and formats, so the player can pick one appropriate to the device and connection.
- Duration and skip rules, including when a skip control may appear.
- Tracking URLs for each event — start, quartiles, complete, click, error — fired by the player as they occur.
- Click-through destination.
- Companion assets, where a banner accompanies the video.
- Wrappers, which point at another VAST document elsewhere.
How VAST breaks
Wrapper chains. A VAST response can point to another VAST response, which points to another. Each hop adds latency and a failure point, and chains of four or five are common in the wild. Timeouts here are a leading cause of unfilled video slots.
Version mismatch. A player supporting an older version silently ignores features from a newer one, so measurement events go unfired and the advertiser sees under-delivery that is really under-reporting.
Missing media formats. If the response carries no file the device can play, the slot fails.
Tracking that fires from the player, which means an ad that failed to render can still report events depending on implementation.
Relevance to AI surfaces
Limited today, and worth understanding as a precedent rather than as current practice. Video does not appear inside conversational answers in any volume, and the formats that matter are text and structured content.
What VAST demonstrates is why a shared serving standard matters at all: without one, the market fragments into bilateral integrations and only the largest participants are reachable. That is precisely the argument being made for AdCP on conversational surfaces.
Common questions
What is the difference between VAST and VPAID?
VAST describes the ad and how to track it. VPAID allowed executable code to run inside the player for interactivity, which created security and performance problems and has largely been replaced by safer alternatives.
How many wrapper redirects are acceptable?
As few as possible. Most players enforce a limit and a timeout; chains beyond three or four hops materially increase the chance of an unfilled slot.
More in ad formats and creative
The shapes an ad takes, and where it sits.