VAST

Video Ad Serving Template — IAB Tech Lab spec.

VAST (Video Ad Serving Template) is an XML schema published by the IAB Tech Lab that describes how a video ad and its tracking events are delivered to a player. A VAST response tells the player which media file to fetch, what tracking pixels to fire on impression, quartile, completion, click, and error, and may wrap another VAST response one or more levels deep.

The current major version is VAST 4.3, which separates media-file delivery from interactivity (handled by SIMID) and viewability measurement (handled by OMID). Earlier versions combined these concerns into VPAID, which is deprecated and unsupported on most CTV environments.

What this is not

VAST is not a player, not a creative format, and not an ad server. A VAST tag is a delivery contract — the player is responsible for parsing it, fetching the media, and firing the tracking events. A tag that conforms to the schema can still fail in production for reasons like CORS, mixed content, or wrapper-chain timeouts.