VAST error codes / General
VAST error 900: Undefined error
Short answer: An error the player could not classify.
Debug this tag now
If you only have a live URL, test it first. If the failure is in the wrapper chain, inspect each hop. Validate resolved XML when you already have the response.
What your player reported
900· Undefined Error.
Wording from the IAB VAST 4.3 specification error table. Players may shorten or reformat it, but the code is the same.
What it means
A fallback the player fires when it cannot map a failure to any specific code. It tells you something broke, not what.
Common causes
- Player-specific failure with no matching code
- A bug or unexpected state in the player
How to fix it
Reproduce with the raw tag in the validator and the inspector, and a 900 usually resolves into a concrete structural problem once you see the XML.
VAST error codes are runtime buckets emitted by the player. They tell you where the failure happened, not which line of XML caused it. Paste the raw tag into the validator to map this code onto the exact element and rule.
Related error codes
Debug this tag now
Validate the XML, follow the wrapper chain, or check it in CI before launch:
# CLI: exits non-zero on errors, ideal for pipelines
vastlint check creative.xml