VAST Error 601: Companion dimensions don't fit
No companion creative matches the size of any available companion slot on the page.
The tag offers companions in certain sizes (e.g. 300×250); the page offers slots in others (e.g. 728×90 only). No match, no render, error 601.
Why error 601 happens
Single-size companion against varied slots
Tags shipping only 300×250 fail on pages whose companion zone is a leaderboard, and vice versa.
Responsive layouts with dynamic slot sizes
The slot's rendered size at match time differs from its configured size on smaller screens.
How to fix VAST error 601
- 1
Ship the standard size set
Provide 300×250 plus 728×90 (and 300×60 for compact layouts) to cover typical companion zones.
- 2
Align slot declarations with reality
Ensure the sizes the page registers with the SDK match what the layout actually renders.
Seeing error 601 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 601 — frequently asked questions
Which companion size fills most often?+
300×250 is by far the most commonly implemented companion slot, with 728×90 second. Shipping both covers the large majority of integrations.