VASTlint — VAST XML Validator

Inline linting for IAB VAST ad tags directly in VS Code. Supports VAST 2.0 through 4.4 with clean Problems entries, concise hovers, fix guidance, and direct docs links. Web validator and full documentation: vastlint.org

Features

• Inline squiggles — red (error), yellow (warning), blue (info) underlines on the offending XML tag
• Hover tooltips — concise issue summary, fix guidance, and a direct rule docs link
• Problems panel — clean issue titles with file, line, and column; click to jump directly to the tag
• Any file type — works in .xml, .html, .js, .ts, .json, templates — anywhere <VAST> appears
• Multi-block — validates every <VAST>...</VAST> block in a file independently
• Live as you type — re-validates 500 ms after you stop typing, and on every save
• CLI backend — uses the vastlint CLI binary when available, falls back to WASM in-process
• 232 rules across VAST 2.0–4.4 plus SIMID and OMID validation: required fields, schema structure, URLs, verification semantics, deprecations, and CTV/SSAI advisories

How it looks

Hover over a squiggled tag:

🔴 <InLine> is missing required <AdSystem>

✅ Fix: Add `<AdSystem>` inside `<InLine>`, e.g. `<AdSystem>My Ad Server</AdSystem>`.

Docs: vastlint.org/docs/rules/VAST-2.0-inline-adsystem

Settings

Example: silence HTTP warnings

// .vscode/settings.json
{
  "vastlint.ruleOverrides": {
    "VAST-2.0-mediafile-https": "off",
    "VAST-2.0-tracking-https": "off"
  }
}

Example: only show errors

{
  "vastlint.minSeverity": "error"
}

Rules

VASTlint checks 232 rules across:

• Required elements and attributes (VAST 2.0–4.4)
• Value formats (durations, URLs, enums)
• Schema conformance (unknown elements/attributes)
• Deprecation warnings (VPAID, Flash, Survey, conditionalAd)
• SIMID interactive creative validation (<InteractiveCreativeFile apiFramework="SIMID">)
• OMID AdVerification validation (<AdVerifications><Verification>, verificationNotExecuted, extension compatibility blocks)
• Security (HTTP vs HTTPS)
• CTV/SSAI best practices (Mezzanine, AdServingId)
• Structural issues (wrapper depth, ad sequence, duplicate impressions)
• VMAP 1.0 ad break playlists (<AdBreak> structure, timeOffset/breakType formats, inline <vmap:VASTAdData> VAST validation)
• DAAST 1.0 digital audio ads (<Category>, <DAASTAdTagURI>, <AdInteractions>, audio MediaFile attributes)

Full rule reference with examples and fix guidance: vastlint.org/docs/rules

Canonical rule catalog:

• RULES.md in this repo
• vastlint.org/docs/rules for the hosted per-rule pages

All 232 rules

VAST 2.0

VAST 3.0

VAST 4.0

VAST 4.1

VAST 4.2

VAST 4.3

VAST 4.4 rules (CTV Ad Portfolio)

VAST 4.4 is a working-group draft, not a published spec. The CTV Ad Portfolio signaling guidance it accompanies is final (2026-07-22), so rules derived from the guidance carry more weight than rules derived from the draft schema. Nothing here errors unless the markup is malformed under any version.

The NonLinear content model these rules cover is accepted on any VAST 4.x document, not only on version="4.4", because every example in IAB's final guidance ships it on a 4.2 tag.

SIMID rules

Requirements

No external tools needed - the validator runs entirely in-process via WebAssembly.

License

Apache-2.0 - see LICENSE