VAST XML is how video ads get delivered on the web and on TV. It's a 20-year-old format with a 200-page spec, and broken tags are incredibly common. Most get caught in QA or post-campaign, not while writing the code. The US video ad market is around $30B a year and a lot of that leaks through errors that are honestly pretty easy to prevent.

The VS Code extension shows squiggles on the offending tag as you type, hover tooltips with what's wrong and how to fix it, and a Problems panel with file and line. 108 rules covering the full IAB VAST spec from 2.0 to 4.3.

It's on both the VS Code Marketplace and Open VSX so it works in VS Code, Cursor, Windsurf, Kiro, VSCodium, Gitpod, and Theia. The validation runs locally via WASM, no network calls.

GitHub: https://github.com/aleksUIX/vastlint VS Code Marketplace: https://marketplace.visualstudio.com/items?itemName=aleksUIX... Open VSX: https://open-vsx.org/extension/aleksUIX/vastlint