VAST errors: build a useful diagnosis before replacing the video

Replacing a video file will not fix an ad tag that never returns a usable response. Yet “the video is broken” often reaches the creative team before anyone checks where playback failed.

VAST is the format used to describe video ad delivery instructions, media resources, and tracking. A VAST response is not the video itself. It may point to another response through a wrapper before the player reaches the media file.

That distinction makes the first diagnostic question straightforward: did the player fail to receive instructions, follow them, load the asset, or play it?

Read the error in context

IAB Tech Lab’s video technical FAQ explains common VAST error codes and possible causes. A few useful examples are:

Code	What it points to
100	A response that cannot be parsed as XML
301	A wrapper URL request or timeout problem
302	The player’s wrapper limit was reached
401	A referenced media file could not be found
402	The media file did not load within the allowed time
403	No suitable supported media file was available

A code narrows the investigation. It does not prove which company caused the failure. For example, a media timeout can involve the asset, connection, delivery infrastructure, or the surrounding implementation.

Use the current player documentation alongside the VAST specification. Avoid assuming that every player supports every feature described by a standard.

Capture one failed attempt properly

Record the placement, creative ID, device, app or browser version, test time and time zone, error code, and the last successful step. Where authorized, collect the response and relevant network timing.

Keep credentials, user identifiers, and private targeting information out of shared logs. Use the partner’s approved secure channel if the evidence contains sensitive request data.

A desktop preview may not reproduce a failure on a television or mobile app. Test the environment where the problem occurs. If only one device family fails, that is useful evidence to preserve.

Separate three common cases

The instructions are invalid. Inspect the returned response and its declared version. A URL returning an HTML error page can be reachable while still being unusable as VAST. Escalate the actual response, not just a screenshot of an empty player.

The chain does not resolve. Trace the permitted wrapper sequence and identify the repeated or failing hop. Check the player’s supported depth and time limits. Adding another wrapper is unlikely to help a chain that is already too long.

The media does not play. Confirm that the selected file exists and that its actual encoding is compatible with the player. File extension alone is insufficient. Check available renditions, duration, delivery timing, and the platform’s requirements.

The creative QA guide covers broader asset and destination checks. Video diagnosis also needs the response and playback evidence.

Compare failures with a denominator

“Five thousand errors” is not a rate. Record the corresponding attempts for the same placement and time window, and establish whether multiple errors can be logged for one attempt.

Do not automatically add every error-code row and call the total failed impressions. The logs may describe different stages or retries.

After the correction, repeat the failing case and compare a representative delivery sample. Confirm playback and the expected measurement events. Keep the evidence that links the original symptom to the fix, so the next escalation starts with a known failure pattern rather than another replacement file.