MCP validate_vast Passes the XML the Agent Pasted. The Live Tag URL Still Has Wrappers

Description and use

Agents validate pasted VAST XML but ignore the live tag URL, causing VAST errors. Adding a separate URL validation and wrapper‑chain check resolves the issue.

Problem

A buyer agent validates pasted VAST XML with MCP validate_vast, but trafficking fails because the live tag URL contains wrappers that were not validated.

Outcome

By separating paste validation from URL validation and explicitly checking wrapper chains, trafficking failures due to unseen wrappers are prevented.

Workflow

• 1. Agent receives VAST XML or URL from ticket.
• 2. If XML only, agent asks for serving URI; if URL present, proceed to URL validation.
• 3. Open VAST tag tester with live URL to fetch current response and preview creative.
• 4. If wrappers detected, run VAST inspector hop‑by‑hop to identify chain breaks.
• 5. In agent pipeline, choose tool: validate_vast_url or inspect_vast(url) for URLs; validate_vast(xml) only for in‑memory XML and do not mark trafficking ready.
• 6. CLI/CI run vastlint validate on files and vastlint inspect <url> for chains.
• 7. Use rule reference and validate VAST XML guide to decide on paste vs URL validation and wrapper depth.

Evidence

Agents wired only to validate_vast inherit that ceiling even when the user says "validate this tag."

Source

I treat paste validation and URL validation as separate gates.

Source

When the tester shows wrappers, I run the same URL through the VAST inspector hop by hop.

Source