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WebMCP validator
Paste a web page that registers WebMCP tools: a Shopify store, or your own site. The validator reads the tool definitions that browser agents see, checks them against the WebMCP draft and Chrome's WebMCP guidance, and gives a grade from A to F with a fix for every finding. It never calls a tool. To see only which tools load, use the WebMCP checker.
Public pages only. The page opens in a headless browser that identifies itself as MCPCanary/1.0.
What the validator checks
Registration
Tools the page tried to register but the browser rejects: an empty name, an empty description, or a name that is already registered. registerTool() fails quietly, and agents never see the tool.
Tool definitions
Missing or thin descriptions, arguments without a type or a description, required names that do not exist, invalid tool names, and tools with no safety hint (readOnlyHint or consequentialHint).
Chrome's budgets
Chrome's WebMCP guidance: at most 500 characters per tool description, 150 per parameter description, and 30 per tool or parameter name. Longer definitions can run into agent guardrails. Also: more than 40 tools, a very large tool list, and no read-only tool.
The grade. The score starts at 100. An error takes 15 points and a warning 5; notes take nothing. One rule takes at most 20. A is 90 or more, B 75, C 60, D 40, and anything lower is F.
Questions
Can I change the WebMCP tools of my Shopify store?
Not Shopify's own tools: Shopify documents no theme hook for them. You can register more tools from your theme JavaScript with document.modelContext.registerTool(), for example a bundle builder or a size finder. The validator grades both kinds.
Does it change anything on my page?
No. It opens the page in a new, empty browser session, reads the registered tools, and closes the session. It calls no tool, so no cart or order changes.
Which WebMCP version does it follow?
The W3C Community Group draft as of November 2026: tools registered on document.modelContext with registerTool(). Pages that register only on the old navigator.modelContext show no tools.
How do I fix the findings?
Each finding has a fix. Under the report, “Fix it with your coding agent” puts all findings into one prompt for the agent that works on your theme or site code.
Is a valid page a working page?
No. The tools can be well described while the store search behind them fails. Use the WebMCP checker to see the tools now, and a monitor to call a read-only tool on a schedule and email you when it breaks.