Common VAST Errors Advertisers Face And How To Fix Them

January 12, 2024
9 minutes

Programmatic video advertising is gaining momentum, and more brands are starting to realize the positive impact it can make on their overall growth.

However, it’s not always smooth sailing, at least when it comes to VAST.

Although it’s rare, VAST, a standard for serving and tracking video ads, can bring some advertising issues for both sides. One of the most common challenges that advertisers (and publishers) face is video player-related VAST error codes and the disruptions they may cause.

In this post, you’ll learn what is VAST, what are the most relevant VAST errors for advertisers, and how to resolve them to ensure the best video advertising experience.

What is VAST?

Video Ad Serving Template (VAST) scheme

VAST, or Video Ad Serving Template, is a video ad served in a video player on the publisher’s website.

In more technical terms, it’s an XML script developed by the Interactive Advertising Bureau (IAB) that creates a communication link between the ad server and the video player. 

By providing a standardized structure, VAST enables advertisers to serve ads across multiple ad inventories within various video ad players.

What is a VAST error?

A VAST error is a sign of failure between the ad server and the video player. There is no single reason for this, but often, it can be caused by wrong ad sizes, missing information, or other technical issues.

Put simply, a VAST error is how video players provide feedback to ad servers in case the ad cannot be served.

Besides enabling advertisers and publishers to troubleshoot occurring problems more easily, VAST errors also positively impact the development of video advertising technology over time. By collecting more detailed information about why their ads or specific creative couldn’t be served, ad servers can improve their systems to produce fewer errors. 

As for advertisers, learning about VAST errors can help improve their video ad quality since they can foresee possible failures before they happen.

There are two types of VAST errors:

  • Non-fatal
  • Fatal

If the video player reports a non-fatal error, chances are that the video ad will still be displayed. 

On the contrary, a fatal error means that the video will either request another ad or the ad will not be served.

IAB VAST error codes advertisers should be aware of

The latest version of VAST, VAST 4.3, was released for public comment by IAB in September 2022, containing all you need to know about the Video Ad Serving Template, including VAST error codes.

Currently, there are nearly 40 possible VAST errors, some of them more relevant to advertisers than others. Therefore, below you’ll find a detailed list of the most common ones that the buyer side can get, alongside their descriptions and ways to resolve them.

VAST error 301 – VAST redirect timeout reached

Category: Fatal

This error can occur when a VAST redirect tag does not return a response within the defined timeout specified in a wrapper element. By default, it’s 5 seconds.

The most likely causes for this can include invalid URI, unreachable or request timeout for a URI, security or other exceptions related to VAST URI requests, and wrong creative dimensions. It could also be due to poor website optimization and high latency connection (e.g., 3G).

How to fix it

Buyers should ensure that the VAST redirect URI is reachable and no protocol issues exist (e.g., responses hosted on HTTP and served to an HTTPS site).

For publishers, it’s recommended to increase the timeout limit of the player.

VAST error 302 – Wrapper limit reached

Category: Fatal

This error occurs when the wrapper limit, which is defined by the video player, is reached. 

The most likely causes for VAST error 302 can include the fact that too many wrapper responses are received (but there’s no inline response), a circular loop of network bounces (daisy chaining) exists, or caused by too many empty VAST responses.

How to fix it

Buyers should ensure that any VAST redirects don’t redirect to another network, while publishers need to increase the player’s wrapper limit.

VAST error 303 – Empty VAST response returned

Category: Fatal

Typically, this error occurs when no ads are returned in a VAST response after one or more wrappers, including empty responses from video fallback.

In third-party networks, the offered fill rate can be less than 100%, meaning that if a third party provides an 80% fill rate, the rest 20% portrays the chance of an error occurring. 

How to fix it

Buyers must ensure that bids are not returned with an empty VAST response and that empty responses are not returned by the advertisers or agency that hosts the creative.

Meanwhile, publishers must ensure that the error rate is in line with the fill rate of the third-party network.

VAST error 400 – General linear error

Category: Fatal

This fatal error occurs when the video player cannot display the linear ad. 

The most common causes for it include the MediaFile being an invalid video file (not the specified format) or the browser restricting autoplay with sound.

How to fix it

If this error occurs, buyers must ensure that the VAST response returns a valid creative.

Publishers should check player issues and allow it to detect restricted autoplay or alert the buyer.

VAST error 402 – Unable to download or timeout of MediaFile URI

Category: Fatal

Typically, VAST error 402 occurs due to the problem fetching or playing the MediaFile, e.g., when a creative takes too long to load. 

Several reasons can cause this:

  • The size of the creative is too big
  • Incompatibility with the platform
  • Website bandwidth issues or poor website implementation
  • A video auto-plays in a mobile environment, whereas it should be click-to-play (there are exceptions)
  • Showing video ads in an auto-play environment while the window is not in focus

How to fix it

The buyer should check that any CDN-hosted creatives are returned within the specified timeout, while the publisher needs to increase the video player’s timeout limit.

VAST error 403 – Response declared unsupported MIME types for all MediaFiles

Category: Fatal

Suppose you get VAST error 403 (which is more common on mobile). In that case, it indicates that there have been unsupported MIME (Multipurpose Internet Mail Extensions) types for all available MediaFiles (like Flash on mobile or WebM on iOS). 

It can mean that the wrong creative attempted to play. 

How to fix it

Buyers should ensure that all returned creatives respect the video format in the bid request. 

From the publisher's side, the creative format or technology requested should be supported by the video player’s device and platform.

VAST error 408 – Rejected ad in VAST response

Category: Fatal

VAST error 408 occurs when the ad returned in the VAST response is rejected.

How to fix it

Publishers should alert the buyer, while buyers themselves should ensure that only approved creatives are returned in the VAST response or advertisers/agencies that host the creatives know about rejections.

VAST error 600 – General CompanionAds error

Category: Non-fatal

VAST error 600 is potentially non-fatal and can be caused due to the publisher’s inability to display a companion ad. The reason for this is unknown.

How to fix it

If this error occurs, publishers should either alert the buyer or check for ad-serving issues, while the buyer should ensure that the VAST response returns a valid creative.

VAST error 601 – Companion creative dimensions did not align with companion display area

Category: Non-fatal

This non-fatal VAST error can occur when the companion creative dimension does not fit within the companion display area.

How to fix it

The buyer should alert the publisher, while the publisher should ensure that the requested companion ad size fits within the dimension of the ad unit from which the request is made.

VAST error 602 – Unable to display required companion

Category: Non-fatal

The VAST error 602 will likely occur when the publisher cannot render the companion ad creative returned in the VAST response.

How to fix it

Publishers should alert the buyer and check for ad-serving issues, while the buyer should ensure that the VAST response returns a valid creative.

VAST error 603 – Unable to fetch CompanionAds/Companion resource

Category: Non-fatal

This error can be caused when the buyer’s response returns a companion ad that fails to return a valid creative.

How to fix it

While the publisher should alert the buyer, the latter should ensure that valid creatives are returned in the companion ad response.

VAST error 604 – Could not find Companion resource with supported type

Category: Non-fatal

If the publisher requests an unsupported creative type for the companion ad slot, it can cause VAST error 604 to occur.

How to fix it

In this case, the buyer needs to alert the publisher. Meanwhile, the publisher should ensure that specified creative types are supported for the companion ad slot.

VAST error 901 – General VPAID error

Category: Fatal

VAST error 901 falls under the category of fatal VAST errors and can be caused by any of the following:

  • SSP VPAID wrapper not buying.
  • SSP VPAID wrapper timeout.
  • VPAID creatives required insecure mode.
  • An Authorized Buyers “IMA Adapter” tag is used with the IMA SDK, and the VPAID ad is returned.

How to fix it

Buyers should ensure that:

  • Any VAST responses that return HTML5 VPAID wrappers from IAS/DV/MOAT include an MP4 asset.
  • The wrapper is not aborting playback.
  • You don’t bid on impressions without a determined advertiser or buyer.
  • You don’t attempt to initiate a client-side resale.

Publishers should make sure that any VPAID creative requests are supported for devices and players.

All IAB VAST error codes

All IAB VAST error codes

Best practices to follow

All of these errors are highly technical and often, advertisers simply don’t have the power to prevent video ad serving issues once they occur.

But what advertisers can do is to do their best before their video ads are delivered to video players across different placements and strive for the highest quality ads.

Some things to consider:

  • User experience. Most users will watch your ad without sound, and even some ad placements will serve them like that. Therefore, think about how you can make your video suitable for different environments and take user experience into consideration.
  • Length. Keep it short and show only what’s important. Ensure that your video is no longer than 10-15 seconds (depending on the environment; for in-game ads, the best length is no more than 6 seconds).
  • Brand awareness. Ensure that your brand logo appears within the first second of the video. 
  • Design. Avoid busy design – keep it minimal and don’t overdo it with text.
  • Ask for advice. Collaborate with your DSP and other partners to improve your VAST video performance and prevent possible issues and errors.


To get the best out of VAST videos, it’s crucial to understand the issues that may occur from time to time and learn how to avoid them. 

Understanding all potential ad-serving problems can help you create better ads and optimize your video advertising performance for the better.

If you need assistance serving VAST videos to your audience, contact the Eskimi team or book a demo for a smooth start.

Philip is the CMO of Eskimi. When he’s not busy growing the Eskimi brand, he spends his time with family and playing ping pong.
Philip is the CMO of Eskimi. When he’s not busy growing the Eskimi brand, he spends his time with family and playing ping pong.
Share this article

Level Up Your Advertising with Eskimi

  • Reach 96% of Open Web
  • 2,500+ Targeting Options
  • 100% Managed or Self-Service
  • In-House Creative Studio Team
  • Display, Video, In-Game & CTV
  • #1 Rated DSP on G2

Create Your Next Ad Campaign With Us

Suggested reading