VAST Tag Validator
Validate VAST 2.x, 3.x, and 4.x XML structure with version-aware checks for CTV deployments.
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<10ms
VAST decision latency
p99 under 15ms — product specification
7-tier
HouseholdID graph tiers
UID2 · PPID · ADID · DeviceID · ACR · IP/24 · fingerprint
Illustrative platform metrics · System status
Validation and inspection serve different purposes in a VAST debugging workflow. Inspection shows you what is inside a VAST document — the creatives, tracking events, media files, and wrapper chain. Validation checks whether what is inside the document conforms to the VAST specification for its declared version. A VAST 4.2 tag that passes validation has all required elements in the correct structure with correct attribute types. A VAST 4.2 tag that is inspectable but fails validation may serve in lenient players while silently breaking in strict CTV environments.
The IAB Tech Lab publishes version-specific VAST schemas. VAST 2.0 has a different required-element set from VAST 3.0, which differs again from VAST 4.x. Common validation failures include missing the VAST root attribute version (required in all versions), omitting the AdSystem element (required), using a non-numeric Duration format (HH:MM:SS is required, not plain seconds), specifying a mediaFileDelivery attribute value that isn't in the defined enum, and referencing non-existent event names in Tracking Event type attributes. Any of these causes a spec-compliant player to skip the ad and fire a VAST 400-series error.
Wrapper VAST tags require a different validation approach than inline VAST. A wrapper's required elements are: a VASTAdTagURI node (the next-hop URL), and optionally override elements for impression tracking. A wrapper that contains MediaFile nodes or Duration elements where the creative details should come from the inline ad — the wrapped tag — is structurally incorrect per spec. Some players tolerate this; others reject it. Validate wrapper and inline VAST tags separately and verify that required elements appear in the correct document type.
CTV DSPs and SSPs enforce VAST version requirements when accepting creative uploads. Most require VAST 4.0 or 4.2 to support ViewabilityVendor and Verification resources. If your creative was built for VAST 3.0, a validator will flag the missing 4.x elements, making it clear why the DSP's creative review system is rejecting the tag before you spend time on support tickets. Run the validator as the first step in troubleshooting any DSP creative rejection.
Frequently asked questions
What is the difference between VAST validation and VAST inspection?
Validation checks whether a VAST document's structure conforms to the IAB VAST specification for its declared version — required elements, correct attribute types, valid enum values. Inspection shows you what the document contains — creative details, tracking events, media files, wrapper chain. Use the validator first to confirm the tag is structurally correct, then use the inspector to audit the content and verify that tracking URLs, media files, and event types match your campaign setup.
Which VAST version should I use for CTV advertising?
VAST 4.2 is the current recommended version for CTV. It supports ViewabilityVendor (required for IAS, DoubleVerify, MOAT measurement), universal ad IDs, the updated Verification resource model, and InteractiveCreativeFile. VAST 4.0 is also acceptable for most CTV inventory. VAST 3.0 is widely supported but will fail on inventory that requires VAST 4.x for brand safety measurement. VAST 2.0 works only on legacy inventory and should not be used for new CTV campaigns.
My VAST tag passes validation but still fails to serve — why?
Validation confirms structural correctness per the VAST spec; it doesn't verify that external URLs resolve, that media files exist and match their declared attributes, or that your ad server returns a non-empty ad response. A structurally valid VAST tag can still fail to serve if the MediaFile URL returns a 404, the tracking pixel endpoints are unreachable, the wrapper chain times out, or the player's frequency cap or floor CPM logic prevents the auction from filling. Pair validation with the VAST Inspector's media file and tracking URL checks to cover the full pre-flight checklist.