VAST Error 101: VAST schema validation error
The XML parsed, but the document violates the VAST schema — required elements are missing, misplaced, or misspelled.
Error 101 means the response is well-formed XML but not valid VAST. The player found the document readable yet structurally wrong: a missing <Ad> child, an <Impression> in the wrong place, a required attribute like the VAST version missing, or elements from one VAST version mixed into another.
Strict players validate against the schema of the version declared in the root element. A tag that declares version="4.0" but uses VAST 2.0-only structures fails here even if a lenient player would play it.
Why error 101 happens
Missing required elements or attributes
Every VAST version mandates a minimum structure — e.g. an InLine ad needs <AdSystem>, <AdTitle>, and <Creatives>. Hand-built or templated XML often drops one.
Declared version doesn't match the structure
The root element says version="4.2" but the body uses elements that were removed or renamed since 3.0, or vice versa. Players validate against the declared version.
Elements in the wrong order or nesting
The VAST schema enforces element order in several places. An <Error> pixel placed after <Creatives>, or a <MediaFile> outside <MediaFiles>, is a schema violation.
Custom or vendor-specific elements without an extension
Anything non-standard must live inside <Extensions>. Vendor elements injected directly into the ad body break validation on strict players.
How to fix VAST error 101
- 1
Validate the tag against the declared VAST version
Run the XML through a VAST validator, not just an XML checker. It will list each schema violation with the element path, so you fix the actual structure.
- 2
Match the version attribute to the real structure
If the ad server assembles VAST 3.0 bodies, declare 3.0. Don't bump the version number to satisfy an SSP requirement without migrating the structure.
- 3
Move non-standard elements into <Extensions>
Vendor data belongs in the extensions block. Everything outside it must be schema-defined for the declared version.
- 4
Test against the strictest player in your chain
Some players ignore schema issues; others (and most SSP scanners) do not. Validate against the strictest consumer so the tag survives everywhere.
Seeing error 101 on a live tag?
Paste the tag URL or XML — the validator resolves the full wrapper chain, validates the schema, downloads every media file, and shows exactly which stage fails.
Validate your VAST tagVAST error 101 — frequently asked questions
The tag plays fine in some players — why does one SSP reject it with 101?+
Player leniency varies widely. Many players skip schema validation entirely, while SSP quality scanners validate strictly. A tag can play in your test player and still be rejected upstream.
What's the difference between VAST error 100 and 101?+
100 means the document isn't readable XML at all. 101 means it's readable XML but breaks the VAST schema rules — wrong elements, wrong order, or missing required parts.
Does declaring a higher VAST version make my tag more compatible?+
No — the opposite. Declaring 4.2 while serving 2.0-style XML triggers 101 on strict players. Declare the version your structure actually conforms to.