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What is VAST Tag & Wrapper Chains?

Also known as: VAST tag, VAST wrapper, Inline VAST

Quick definition

A VAST tag is the URL a video player calls to fetch an XML ad response. The response is either Inline (it holds the media files and tracking URLs) or a Wrapper that points to another ad server's VAST, so one impression can pass through several redirects before a playable creative arrives.

How it works

  1. Player reaches an ad break and calls the VAST tag URL with macros (size, app bundle, consent, IFA)
  2. Ad server returns a Wrapper or Inline response
  3. Player follows each Wrapper's VASTAdTagURI and merges every hop's tracking pixels
  4. Inline response supplies MediaFile options; the player picks one that fits bitrate and codec
  5. Player fires impression, quartile, click and error events to every collected URL

Key points

Inline vs Wrapper
Inline carries the creative; Wrapper only adds tracking and forwards. Players usually cap wrapper depth (often around 5) to protect latency.
Macros
Placeholders such as [TIMESTAMP], [IFA] or [GDPRCONSENT] are replaced by the player before the request so buyers receive context.
Error codes
Standard codes (e.g. 303 no ad, 301 timeout, 403 unsupported media) explain why a fill failed and are vital for debugging fill rate.
Measurement
Verification scripts are declared via AdVerifications/OMID rather than the legacy VPAID executable format.

Benefits

  • Simple to traffic across any compliant player
  • Each party in the chain can add its own tracking

Trade-offs

  • Long wrapper chains add latency and timeouts
  • Tracking discrepancies multiply with every hop

FAQ

Why does my VAST tag show impressions but low fill?

Common causes are wrapper timeouts, unsupported media files for the device, empty responses (error 303) or consent signals missing from macros. Check the player's error pixels first.

Is VPAID still supported?

VPAID is deprecated in favour of VAST 4.x with OMID for verification and SIMID for interactivity; many CTV and mobile players reject it.

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Official sources

Last reviewed 2026-10-27. Explanations are written by AdTech Radar; consult the official specification for implementation.