AOAdOps Toolkit

VAST error code

403 — Could not find a supported media file

What the spec says

The player examined the media files offered and could not use any of them.

What actually causes it

  • No MP4 rendition — still the most broadly supported container across environments.
  • Only WebM or another format the environment does not support.
  • A VPAID creative served where VPAID is not executed, which now includes most CTV.
  • MediaFile elements missing the type attribute, so the player cannot determine playability at all.

How to fix it

Always include an H.264 MP4 rendition, and make sure every MediaFile declares type, width and height. If the tag is VPAID, expect this code from any environment that has withdrawn VPAID support and plan a SIMID or plain-video fallback.

To see whether a tag would hit this before it runs, paste it into the VAST validator. It reads the XML and reports the conditions that map to this code.

Related codes

Code definitions from the IAB Tech Lab VAST 4.3 specification. Causes and fixes are practitioner guidance, last verified July 2026.