How to Validate Companion Ads in VAST Tags

Short answer

Companion validation in VAST starts with the XML. If CompanionAds is missing, a Companion has no StaticResource, IFrameResource, or HTMLResource, or width and height are absent, the banner slot never fills even when the video plays.

vastlint can catch empty companions and missing dimensions in the web validator and CLI. Slot matching still depends on the page. Ship 300x250 plus a large format, then paste the resolved tag. Most CTV players ignore companions.

What companion ads are doing in the tag

Companions are display creatives served alongside linear video. The page or app, not the video decoder, renders them. VAST describes the assets in CompanionAds. Each Companion must declare width and height so the host can match a slot.

required=all, any, or none on CompanionAds changes fill behavior. required=all can kill the video if no companion size matches. required=none keeps the video even when the banner misses.

What companion XML validation can catch before playback

Companion with no renderable resource.
Companion missing width or height.
required set to a value other than all, any, or none.
Unknown children under CompanionAds.
Wrapper hops that drop CompanionAds when followCompanions is false.

A companion block that should pass

xml

<CompanionAds required="any">  <Companion width="300" height="250">    <StaticResource creativeType="image/png">      <![CDATA[https://cdn.example.com/300x250.png]]>    </StaticResource>    <CompanionClickThrough>      <![CDATA[https://advertiser.example.com/landing]]>    </CompanionClickThrough>  </Companion>  <Companion width="728" height="90">    <StaticResource creativeType="image/png">      <![CDATA[https://cdn.example.com/728x90.png]]>    </StaticResource>  </Companion></CompanionAds>

<CompanionAds required="any">  <Companion width="300" height="250">    <StaticResource creativeType="image/png">      <![CDATA[https://cdn.example.com/300x250.png]]>    </StaticResource>    <CompanionClickThrough>      <![CDATA[https://advertiser.example.com/landing]]>    </CompanionClickThrough>  </Companion>  <Companion width="728" height="90">    <StaticResource creativeType="image/png">      <![CDATA[https://cdn.example.com/728x90.png]]>    </StaticResource>  </Companion></CompanionAds>

A companion that looks present but should be flagged

xml

<CompanionAds required="all">  <Companion>    <!-- no width, height, or resource -->  </Companion></CompanionAds>

<CompanionAds required="all">  <Companion>    <!-- no width, height, or resource -->  </Companion></CompanionAds>

What the companion rule set should complain about

VAST-2.0-companion-resource because the Companion is empty.
VAST-2.0-companion-dimensions because width or height is missing.
VAST-3.0-companion-required-attr because required is not all, any, or none.

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Which channel to use

The validator at vastlint.org/validate is the paste check for Companion structure.
The tester at vastlint.org/tester shows which companion assets survived a live fetch.
The inspector at vastlint.org/inspect is the wrapper chain tester when companions vanish after a hop.
Problem pages cover companion ads not showing and IAB sizes 300x250 and 728x90.

Use the CLI when companion XML should block launch

vastlint check companion-example.xml --format json vastlint check companion-example.xml --fail-on-warning

vastlint check companion-example.xml --format json vastlint check companion-example.xml --fail-on-warning

What companion XML validation cannot prove

A valid CompanionAds block is necessary. It does not prove the publisher page has a matching slot, or that the image CDN returned 200. Size mismatch is a trafficking problem, not a schema problem.

Validate the XML, ship more than one IAB size, then test the live URL. On CTV, treat a missing banner as expected unless the platform documents companion support.