VAST Error 203: Video player expecting different size
The returned creative's dimensions or bitrate don't fit the placement — wrong resolution, aspect ratio, or file size class.
Error 203 is the size mismatch: the media files in the response don't match what the player requested — resolution too high for the device, wrong aspect ratio for the slot, or a bitrate outside the accepted range.
Players that can't find any acceptable rendition sometimes report 403 (no supported media file) instead; 203 is the trafficking-side variant of the same family.
Why error 203 happens
Single-rendition tags
The tag ships one media file (e.g. 1080p high-bitrate) with no alternatives, so constrained players — mobile, embedded, low-bandwidth — have nothing acceptable.
Aspect ratio mismatch
A vertical creative served to a 16:9 slot or vice versa. Some players reject rather than letterbox.
Width/height attributes wrong on MediaFile
The declared width/height don't match the actual encoded dimensions, so player-side selection logic picks nothing.
How to fix VAST error 203
- 1
Provide a rendition ladder
Include multiple <MediaFile> entries across resolutions and bitrates (e.g. 480p/720p/1080p) so every player finds a fit.
- 2
Verify declared vs actual dimensions
Inspect each media file and confirm the width, height, and bitrate attributes match the real encode.
- 3
Match creative aspect ratio to the placement
Traffic vertical assets to vertical slots explicitly instead of relying on player-side cropping.
Seeing error 203 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 203 — frequently asked questions
How many media file renditions should a VAST tag include?+
Three to five renditions covering low (≈500 kbps), medium (≈1500 kbps), and high (≈3000+ kbps) bitrates at common resolutions covers virtually all players. CTV specs often mandate specific ladders.
Is error 203 the same as 403?+
They overlap. 203 is reported as a trafficking expectation mismatch; 403 means the player scanned MediaFiles and found none it supports. Different players pick different codes for the same underlying problem.