VAST Error 407: Mezzanine is being downloaded
The mezzanine exists but wasn't transcoded yet — the ad can't play this break, but likely will in a few minutes.
SSAI platforms fetch and transcode mezzanine files on first encounter with a creative. Until that finishes, the ad can't be stitched, and the slot reports 407. It's a first-serve warm-up cost, not a defect.
Persistent 407s for the same creative, though, mean the transcode pipeline is failing or the file is too slow to fetch.
Why error 407 happens
First impression of a new creative
The stitcher just met this ad and kicked off ingestion. Expected once per creative per platform.
Slow mezzanine host
Multi-gigabyte masters served from slow origins take long enough to fetch that repeat 407s pile up.
Transcode pipeline failures
If ingestion silently fails, every retry re-reports 407 indefinitely for that creative.
How to fix VAST error 407
- 1
Pre-warm new creatives
Where the platform supports it, submit creatives for ingestion before the campaign goes live so the first real impression finds them ready.
- 2
Watch for repeat offenders
One-off 407s per creative are normal; the same creative reporting 407 for hours means ingestion is stuck — check the mezzanine URL and the platform's asset status.
- 3
Host mezzanines on fast storage
Serve masters from CDN-backed storage so multi-GB fetches complete quickly.
Seeing error 407 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 407 — frequently asked questions
Is a 407 a lost impression?+
That specific slot, yes — the stitcher fills it with other demand. The creative usually becomes servable minutes later once transcoding completes.