VAST error code
100 — XML parsing error
What the spec says
The player could not parse the response as XML at all. Nothing further is attempted — the document is discarded before any ad logic runs.
What actually causes it
- The ad server returned an HTML error page instead of XML, often a 200-status page saying something went wrong.
- An unescaped ampersand or angle bracket in a tracking URL, which is invalid XML unless wrapped in CDATA.
- A macro that was never substituted, leaving a raw placeholder that breaks the document.
- Truncated response, usually a timeout partway through delivery.
How to fix it
Fetch the tag URL directly and look at what actually comes back. If it starts with a doctype or an HTML element rather than a VAST element, the ad server is erroring rather than serving. If it is XML, look for a bare ampersand in a URL — the fix is to wrap the URL in CDATA.
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.