Free Online VAST Tag Validator
Use this free VAST tag validator when you already have the VAST XML and want a pure rule-by-rule compliance check. Drop a .xml file, paste raw XML, or type below. Results appear instantly and nothing is stored.
Need to test a VAST URL, fetch wrappers, or preview the creative? Try the VAST Tag Tester → · Need to debug redirects hop by hop? Open the VAST Inspector →
Drop a .xml file or URL · paste XML or a URL · auto-validates · ⌘↵ to validate now
Drop a .xml file or URL · paste XML or a URL · auto-validates · ⌘↵ to validate now
Full
rule coverage
6
VAST versions
<1ms
validation
0
data stored
Why trust this validator
vastlint is an independent open-source validator against the published IAB Tech Lab standards, not an official IAB Tech Lab tool. Where IAB publishes VAST XSD schemas, vastlint derives machine-verifiable structural rules from those schemas. Where the spec uses RFC 2119 normative prose, vastlint derives rules from that text. If you already run IAB XSD validation in CI, keep it as a baseline. vastlint complements it with prose-derived, XML, URI, media-type, and operational checks built for production CTV, streaming, SSP, DSP, SSAI, and ad-server workflows.
Published IAB XSD schemas first
Where IAB Tech Lab publishes VAST XSD schemas, vastlint derives machine-verifiable structural rules directly from those schemas.
RFC 2119 prose where schemas stop
When the VAST or SIMID specs use MUST, REQUIRED, SHALL, or SHOULD, vastlint derives rules from that normative prose. That is especially important for VAST 4.3, where no IAB XSD exists.
Baseline plus real-world rigor
Keep IAB XSD validation in CI if you already run it. vastlint complements it with prose-derived, XML, URI, media-type, and operational checks, but player, SDK, exchange, and business rules can still matter.
How this VAST tag validator works
- → Validates against IAB VAST 2.0, 3.0, 4.0, 4.1, 4.2, and 4.3 specs. Rules come from published IAB XSD schemas where IAB ships them and RFC 2119 normative prose where schemas stop. Version is detected automatically from the
versionattribute. - → Checks for missing required fields, malformed URLs, empty elements, deprecated features, VPAID, version mismatches, and more.
- → This is an independent validator against the published IAB standards, not an official IAB Tech Lab tool.
- → Your XML is never stored. The validation response is returned immediately and discarded.
- → Want validation in CI/CD? Use the vastlint CLI
brew install aleksUIX/tap/vastlint - → Need a REST API? vastlint is on RapidAPI: validate VAST tags with a single HTTP call. Subscribe free →
VAST tag validator FAQ
What is a VAST tag validator?
A VAST tag validator checks a VAST tag or VAST XML document against the IAB Tech Lab VAST specification. It catches missing required fields, malformed URLs, deprecated elements, and other compliance issues before launch.
How do I validate a VAST tag?
Paste raw VAST XML into the editor, upload a .xml file, or type directly in the validator. If you need to fetch a live VAST URL or preview the creative, use the VAST Tag Tester. If you need to inspect wrapper behavior hop by hop, use the VAST Inspector.
Which VAST versions does this validator support?
This VAST tag validator supports VAST 2.0, 3.0, 4.0, 4.1, 4.2, and 4.3. Version detection is automatic from the VAST version attribute.
Is vastlint an official IAB validator?
No. vastlint is an independent, open-source validator that checks VAST tags against published IAB Tech Lab specifications and schemas. It is not affiliated with or operated by IAB Tech Lab.
Does this validator use IAB XSD schemas?
Yes. Where IAB publishes VAST XSD schemas, vastlint derives machine-verifiable structural rules directly from those schemas. It then complements them with RFC 2119 prose-derived rules and other standards checks.
What happens for VAST 4.3 where no XSD exists?
IAB Tech Lab did not publish a VAST 4.3 XSD. For VAST 4.3, vastlint derives rules from the normative prose in the published VAST 4.3 specification.
Should I still keep IAB XSD validation in CI?
Yes, if you already run it. IAB XSD validation is a strong baseline. vastlint complements it with prose-derived rules, XML well-formedness, URI and media-type checks, and operational guidance that XSD alone does not cover.
Does passing vastlint guarantee playback everywhere?
No. Passing vastlint means the tag is structurally compliant with the published standards and related checks. Players, SDKs, exchanges, CTV environments, and business-specific rules can still impose additional requirements.
Is vastlint legitimate for production CTV, streaming, or ad-server workflows?
Yes. vastlint is self-hostable, signed, fuzz-tested, and designed for latency-sensitive CTV, streaming, SSP, DSP, SSAI, and ad-server workflows. The same core ships as a library, CLI, web validator, editor extension, and MCP server.
Which standards besides IAB VAST does it check?
Beyond IAB VAST and SIMID, vastlint checks W3C XML 1.0 well-formedness, RFC 3986 URI syntax, IANA Media Types, ISO 4217 currency codes, and Ad-ID formatting where applicable.
Keep going
Search visitors who land on the validator usually need one of three things next: a fix guide, a version explainer, an automation path, or a live wrapper-debugging flow. These pages cover the most common follow-up jobs.