VAST Error 403: No supported media file found
The player scanned every MediaFile in the ad and none matched the codecs, MIME types, or delivery methods it supports.
Error 403 fires at selection time, before any download: the player compared each <MediaFile> declaration — MIME type, codec, delivery method, dimensions — against its own capabilities and found zero acceptable candidates.
The classic historical case is VPAID-only tags on inventory that doesn't allow VPAID; the modern one is HEVC/WebM-only tags hitting devices that only decode H.264, or declared MIME types that don't match the actual files.
Why error 403 happens
Codec not supported by the device
Tags shipping only HEVC, VP9, or AV1 renditions fail on the huge installed base that only decodes H.264.
VPAID or interactive-only creatives
If every MediaFile is apiFramework="VPAID" and the placement forbids executable ads, nothing is selectable.
Wrong MIME type declarations
A file declared as video/webm that is actually MP4 (or vice versa) gets filtered out by declaration before the player ever fetches it.
Streaming-only delivery on progressive-only players
Tags offering only HLS/DASH manifests fail on players that require progressive MP4, and vice versa.
How to fix VAST error 403
- 1
List what the tag declares vs what the player accepts
Validate the tag and read each MediaFile's type, codec, and apiFramework. Compare against the placement's published specs — the gap is usually obvious.
- 2
Always include a plain H.264 MP4 rendition
One progressive H.264/AAC MP4 makes the tag playable virtually everywhere, whatever else the tag also offers.
- 3
Verify MIME declarations against real files
Download each file and confirm the container matches the declared type. Fix declarations at the ad server.
- 4
Replace VPAID-only setups
Pair interactive units with a standard video fallback, or migrate interactivity to SIMID/OMID, so non-VPAID inventory still plays the ad.
Seeing error 403 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 403 — frequently asked questions
What's the safest media file setup for maximum compatibility?+
Progressive H.264/AAC MP4s at three bitrates (≈500/1500/3000 kbps), correctly declared as video/mp4. Add HLS, WebM, or interactive layers on top for players that use them.
How is 403 different from 405?+
403 happens before download — no file even looked acceptable on paper. 405 happens after selection — a file looked fine, was fetched, and then failed to display.