VAST Error 400: General linear error
The player couldn't display the linear ad for a reason it can't classify more precisely — the catch-all for video playback failures.
Error 400 is the linear family's general bucket: the ad resolved, a media file was selected, and then playback failed somewhere the player couldn't pin to a specific code — decoder errors, DRM issues, rendering failures, or player-internal exceptions.
Players differ widely in how much they classify. Some report almost everything media-related as 400; others reserve it for genuinely unclassifiable failures. Read it as "video playback broke, inspect the creative."
Why error 400 happens
Codec or container the decoder can't handle
The file's declared MIME type looked acceptable, but the actual encode (profile, level, pixel format) exceeds what the device's decoder supports.
Player-internal errors mid-playback
Buffer stalls, decoder resets, or SDK exceptions after playback started often surface as a generic 400.
Corrupt media file
The file downloads fine but is truncated or has broken headers, failing when the decoder opens it.
How to fix VAST error 400
- 1
Download and inspect the actual media file
Validate the tag with media inspection: check the real codec, profile, container, and whether the file plays in a reference player.
- 2
Encode to the broadest baseline
H.264 baseline/main profile in MP4 plays essentially everywhere. Exotic profiles, HEVC without fallback, or unusual containers cause device-specific 400s.
- 3
Segment the error by device
If 400 clusters on one device family or app version, it's a decoder capability issue there — provide a compatible rendition rather than chasing a global bug.
Seeing error 400 on a live tag?
Paste the tag URL or XML — the validator resolves the full wrapper chain, validates the schema, downloads every media file, and shows exactly which stage fails.
Validate your VAST tagVAST error 400 — frequently asked questions
How do I narrow down a generic 400?+
Cross-reference with device data and inspect the media file itself. If specific codes (401–405) also appear for the same creative, they usually name the real problem — 400 is what less-precise players report for the same failure.
Is HTTP status 400 related to VAST error 400?+
No. VAST error codes are defined by the VAST spec and share numbers with HTTP statuses coincidentally. VAST 400 is a linear playback failure, not a bad request.