VAST error code
101 — VAST schema validation error
What the spec says
The document is well-formed XML but does not conform to the VAST schema. A required element is missing, or an element appears where the schema does not allow it.
What actually causes it
- A required element is absent — commonly AdSystem, AdTitle, Impression, or Duration on a Linear creative.
- Duration in the wrong format. The schema requires HH:MM:SS; a bare number of seconds fails.
- Elements in an order or nesting the schema does not permit, often from a hand-assembled tag.
- A MediaFile with no type attribute, so the player cannot determine playability.
How to fix it
Run the tag through a validator and check which required element is missing. This is the most common code produced by hand-built or template-generated tags, and it is nearly always a single absent element rather than anything structural.
To see whether a tag would hit this before it runs, paste it into the VAST validator. It reads the XML and reports the conditions that map to this code.
Related codes
Code definitions from the IAB Tech Lab VAST 4.3 specification. Causes and fixes are practitioner guidance, last verified July 2026.