A VAST 4.0 response cannot contain both a root-level <Error> and a root-level <Ad>. The specification makes those choices mutually exclusive: return an ad response with <Ad>, or a no-ad response that may include one or more <Error> elements.
What the VAST 4.0 root rule requires
The topmost XML element is <VAST>, and its version attribute identifies the response version. In the root-element description, the VAST 4.0 specification permits <Error> and <Ad> as subelements, then states that either may be provided, but not both. A document such as <VAST><Error>…</Error><Ad>…</Ad></VAST> violates that rule. IAB Tech Lab’s VAST 4.0 specification, updated April 2016, is the primary source for this requirement.
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Choose the response that matches what happened
| Response state | Root-level content | Meaning |
|---|---|---|
| An ad is being returned | <VAST> with one or more <Ad> elements, and no root-level <Error> |
The response contains ad content. |
| No ad is being returned | <VAST> with one or more <Error> elements, and no root-level <Ad> |
The response reports that no ad was returned. |
For the no-ad case, section 3.2.1 describes a response containing only the root <VAST> element and one or more <Error> elements. The Error element is optional. If included, the player must use its URI to notify the server that no ad was returned; multiple Error elements can notify multiple parties. The VAST 4.0 specification gives this behavior in section 3.2.1.
Example no-ad response
<VAST version="4.0">
<Error><![CDATA[https://adserver.example/noad.gif]]></Error>
</VAST>The URI here is illustrative, not a verified endpoint. An ad-bearing response instead uses root-level <Ad> elements without a root-level <Error>.
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Do not confuse root-level errors with ad-level error reporting
This rule concerns the choice directly under <VAST>. VAST also defines error reporting associated with ads. The existence of an ad-level error mechanism does not make it valid to add a root-level <Error> alongside a root-level <Ad>; check the element’s location before interpreting its role.
How to troubleshoot a response containing both
- Inspect the XML root. Confirm the document’s
<VAST>version and identify whether<Error>and<Ad>are both direct children. Errors elsewhere in the document are a different question. - Correct response construction. If the server returned an ad, remove the root-level
<Error>. If it returned no ad, omit root-level<Ad>; include an Error URI only if the implementation needs to report the no-ad outcome. - Validate against the schema version used by the integration. IAB Tech Lab’s schema repository identifies a VAST 4.0.7 schema, so select the schema corresponding to the version your payload declares and your player supports. IAB Tech Lab’s VAST 4.x schema repository provides the version reference.
- Test in the target player. Confirm that the corrected response parses and that the intended ad or no-ad behavior occurs. A schema check can identify structural problems, but it does not by itself establish the cause of a production incident.
The IAB Tech Lab FAQ describes VAST error 101 as a schema validation error and lists impermissible element or attribute combinations among possible causes. That makes validation a useful debugging step, but the combined root elements alone do not identify which component generated the malformed response or what implementation-specific change is needed. IAB Tech Lab’s Video Tech FAQs covers the error-code guidance.
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References
- IAB Tech Lab, VAST 4.0 specification (updated April 2016)
- IAB Tech Lab, Video Tech FAQs
- IAB Tech Lab, VAST 4.x schema repository
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