VAST

VAST, the Video Ad Serving Template, is the IAB Tech Lab XML format that tells a video or audio player which ad file to play and which tracking URLs to call.

Before VAST, each video player needed custom code for each ad server. VAST gave them one schema. A player asks for an ad, receives a VAST document, picks the media file that suits its device and bandwidth, plays it, and calls the tracking URLs for impressions, quartiles, clicks and errors.

The specification has moved in steps (IAB Tech Lab, accessed 9 November 2026). VAST 4.0, released in January 2016, separated media files from verification and interactive code and added a mezzanine file for server-side insertion. VAST 4.1, from November 2018, added ad requests, Open Measurement verification and audio ads. VAST 4.2, from June 2019, added SIMID for interactivity in place of VPAID.

VAST 4.3 is the latest version. A 2024 CTV addendum adds support for the Ad Creative ID Framework, icons for EU Digital Services Act disclosures, and higher-resolution creative for large screens. VAST also works server-side. In SSAI, the stitching server reads the VAST response, inserts the ad into the stream and fires the trackers for the player.

Example: A streaming app plays a 15-second ad from a VAST 4.2 response. The response names the ad server, gives an impression URL, a duration of 00:00:15 and three MP4 files at different resolutions. The app picks the 1080p file on a TV and the 720p file on a phone, and calls the impression URL when the ad starts to play.

Related terms: VPAID, CSAI, SSAI, pod bidding, OM SDK.

Authoritative source: IAB Tech Lab, "Digital Video Ad Serving Template (VAST)" (accessed 9 November 2026).