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VAST Error Codes Explained: 100–900 and How to Fix Them

July 2, 2026 · 5 min read

A VAST error code tells you which stage of ad delivery failed: the XML, the wrapper chain, the media file, or the player. The fastest way to find the cause is to run the tag through a validator and read the report instead of guessing from the number alone.

Find the Cause Fast

Start here

Use VAST Ad Validator

Paste your tag URL or XML. It resolves the wrapper chain, validates the XML, downloads every media file, and shows exactly what is broken.

Validate your tag

Quick steps

How To Diagnose Any VAST Error

Copy the failing tag

Grab the VAST tag URL or the raw XML from your ad server or SSP.

Run it through the validator

Open VAST Ad Validator and paste the tag. It checks the chain, the XML, and the media files.

Match the finding to the code

Wrapper failures explain 3xx codes, media problems explain 4xx codes, XML issues explain 1xx codes.

Fix and re-run

Apply the fix on your ad server, then validate again to confirm the tag is clean.

The error codes at a glance

Code	Meaning	Usual cause
100	XML parsing error	Malformed XML — unclosed tags, bad encoding
101	Schema validation error	XML does not match the VAST spec version it declares
102	Version not supported	Player is older than the VAST version in the response
200	Trafficking error	Player got an ad type it was not expecting
201	Wrong linearity	Linear ad sent to a non-linear slot, or the reverse
300	Wrapper error	General failure while following a wrapper redirect
301	Wrapper timeout	A hop in the chain responded too slowly
302	Wrapper limit reached	Too many nested wrappers (usually more than 5)
303	Empty VAST response	The chain ended with no ad — a no-fill
400	General linear error	The video ad could not be displayed
401	Media file not found	The MediaFile URL returns 404 or is unreachable
402	Media file timeout	The file is too large or the CDN is too slow
403	No supported media file	No file matches the codecs or MIME types the player supports
405	Media file display problem	Codec, container, or CORS problem while playing
600	General companion error	Companion banner could not be shown
900	Undefined error	Anything the player could not classify

Which codes matter most

Most lost revenue comes from three of them:

303 — your demand path returned nothing. See our VAST error 303 guide.
301/302 — the wrapper chain is too slow or too deep. The validator shows every hop with its latency.
401/402/403/405 — the creative itself is broken. The validator downloads each media file and reports the real codec, size, and CORS status, so you can see why the player rejected it.