VAST Error 100: XML parsing error

The VAST response could not be parsed as valid XML by the player or SDK. In operations, this usually shows up when the returned markup is malformed, empty, encoded incorrectly, or not actually XML at all.

This is the kind of issue that is easiest to solve when the investigation starts from the right evidence path. VAST Inspector is usually the fastest first step because it confirms whether the response, wrapper chain, or media selection behaves the way the player expects.

It often appears before any meaningful ad logic runs, which is why teams need to validate the response body itself instead of assuming the issue is media or timeout related.

The most common causes are unclosed XML tags, invalid characters or bad encoding, and a URL that resolves to HTML instead of VAST. Teams often treat the visible error as the root problem, but in practice the real issue may sit earlier in the wrapper path, in the creative package, or in the environment assumptions around the request.

That is why it helps to compare a failing request against a known-good example. Once you know whether the difference is structural, timing-related, or environment-specific, escalation becomes much faster and cleaner.

VAST Inspector should be the first stop for this class of issue. From there, move into XML Beautifier and Redirect Chain Analyzer to narrow the failure further.

A useful investigation should end with concrete evidence: the failing tag or asset, the exact point where the player behavior diverged, and the partner or configuration change most likely responsible. That turns a generic player code into an actionable support ticket or launch fix.

Validate the raw XML first, confirm the endpoint really returns VAST, and trace any redirects if the response content does not match the expected tag.