VAST errors: look up the code and the XML fix
Short answer
If the player printed a numeric VAST error (100, 301, 401), start at VAST error codes. If you have the XML, paste it into the validator and follow the rule link. Spec fragments such as idValue, IFrameResource, or AdServingId belong on the rule page, not this index.
Paste the live tag, then look up the rule
If you only have a VAST URL, test it first. If wrappers are involved, inspect each hop. This index is the lookup after you have a rule ID or a player error.
This page is the VAST errors lookup for XML-side failures: 232 rules, searchable, each with the copyable fix. Use it when you know the error class but not the exact rule. Machine-readable catalog: /docs/rules.json.
VAST error lookups
Spec fragments go to the rule page
These queries should not land here. Each one has a dedicated rule URL.
Every rule vastlint enforces against the IAB VAST specification (versions 2.0 through 4.4) is listed below. Search by error, element, or rule ID.
error: ad will likely fail to render. warning: spec violation or risky practice. info: advisory.