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Under VAST 4.0, a response cannot contain both a root-level <Error> and a root-level <Ad>. The specification makes those choices mutually exclusive: return an ad using <Ad>, or represent a no-ad response using one or more <Error> elements. A root-level error is optional in the no-ad case.
What the VAST 4.0 rule means
The top-level element is <VAST>, whose version attribute identifies the response version. In the VAST 4.0 root-element description, the allowed child elements are <Error> and <Ad>, with an explicit restriction that either may be provided, but not both. See the IAB Technology Laboratory VAST 4.0 specification (updated April 2016).
| Response state | Root-level element | Meaning |
|---|---|---|
| Ad returned | <Ad> |
The response contains an ad; do not also put a root-level <Error> in the same response. |
| No ad returned | One or more <Error> elements, or no child element |
The response represents no ad. The error element is optional; when included, its URI is used to notify the server that no ad was returned. |
How to represent a no-ad response
For a no-ad response, the specification describes a document containing the root <VAST> element and one or more root-level <Error> elements. The player uses the URI in an included error element to report that no ad was returned. Multiple error elements may be used to notify multiple parties. The error element is optional, as section 3.2.1 of the VAST 4.0 specification states.
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<VAST version="4.0">
<Error><![CDATA[https://adserver.example/noad.gif]]></Error>
</VAST>The URI shown is an illustrative example, not a verified live endpoint. An ad-bearing response uses a root-level <Ad> instead of a root-level <Error>.
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Do not confuse root-level errors with ad-level error reporting
The restriction here concerns the children directly under <VAST>. VAST also includes error reporting associated with ads. An error element in that context is not the same response choice as a root-level no-ad error. When debugging, check the element’s position in the XML tree before deciding whether this rule applies.
How to troubleshoot a response containing both
- Inspect the root children. Look directly beneath
<VAST>for an<Error>and an<Ad>. Do not treat an error element nested elsewhere as the same issue. - Correct the response state. If an ad is being returned, keep the root-level
<Ad>and remove the root-level<Error>. If no ad is being returned, use the no-ad form and include an error URI only if needed. - Validate against the schema for the emitted version. The IAB VAST 4.x schema repository identifies a 4.0.7 schema. Validate against the version your integration actually emits rather than assuming every VAST 4.x payload uses the same schema.
- Test with the target player. Schema validation can catch structural problems, but testing the corrected response in the player helps confirm that the integration handles it as intended.
IAB Tech Lab’s VAST overview describes VAST as an XML schema for serving ads to digital video players. Its Video Tech FAQs identifies VAST error 101 as a schema validation error and lists impermissible element or attribute combinations among possible causes. A validation failure can have causes beyond this particular root-level conflict.
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What this rule cannot diagnose on its own
Finding both root-level elements establishes a conflict with the VAST 4.0 rule, but it does not show why the response was constructed that way or identify the responsible component. Without the actual XML, the version emitted, the ad server, the player, and runtime logs, the specific production cause and implementation-specific fix remain undetermined.
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