VAST Error 601: Companion creative dimensions do not fit

The companion creative dimensions do not fit within the companion display area. In operations, this usually shows up when the returned companion asset is larger than the available slot or layout.

This is the kind of issue that is easiest to solve when the investigation starts from the right evidence path. Creative Size Validator is usually the fastest first step because it confirms whether the response, wrapper chain, or media selection behaves the way the player expects.

This is one of the most straightforward companion issues because the physical mismatch is the root problem.

The most common causes are oversized companion creative, slot dimensions that changed without updating trafficking, and layout assumptions that do not match the real page or app surface. Teams often treat the visible error as the root problem, but in practice the real issue may sit earlier in the wrapper path, in the creative package, or in the environment assumptions around the request.

That is why it helps to compare a failing request against a known-good example. Once you know whether the difference is structural, timing-related, or environment-specific, escalation becomes much faster and cleaner.

Creative Size Validator should be the first stop for this class of issue. From there, move into VAST Inspector and VAST Error Code Explainer to narrow the failure further.

A useful investigation should end with concrete evidence: the failing tag or asset, the exact point where the player behavior diverged, and the partner or configuration change most likely responsible. That turns a generic player code into an actionable support ticket or launch fix.

Compare the companion dimensions to the real slot size and update the asset or the trafficking rules so the surface and creative match.