VAST error codes / Linear and media errors
VAST error 401: Media file not found
Short answer: The player could not fetch the MediaFile at its URI.
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
401· File not found. Unable to find Linear/MediaFile from URI.
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
The media URL did not resolve to a playable file.
Common causes
- A broken or expired media URL
- HTTP media URL blocked on an HTTPS page (mixed content)
- A CDN 404 or access-control failure
How to fix it
Confirm the `<MediaFile>` URL is reachable over HTTPS and returns the media bytes, not an error page.
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 vastlint rules
- VAST-2.0-mediafile-https: <MediaFile> URL uses HTTP instead of HTTPS
- VAST-2.0-linear-mediafiles: <Linear> must contain <MediaFiles> with at least one <MediaFile>
Related error codes
- VAST error 400: General linear error
- VAST error 402: Media file timeout
- VAST error 403: No supported media file
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