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How WebMCP runs one workflow

WebMCP lets a web page offer its own tools to an AI agent inside your browser. The same three checks from the MCP walkthrough run here, but every calculation stays in your browser tab and no AINumbers server takes part in the calls.

Goal of this animation

Show how a browser agent uses tools that web pages register themselves, running the same three-step workflow as the MCP page with every calculation kept in your tabs.

🔒 No personal dataThe tool calls run inside your browser and send nothing anywhere. Every value shown is synthetic.
☁ Our server is Cloudflare“AINumbers server” means Cloudflare today. It delivers the page files and the tool list once and never sees a tool call.
Why every result here is green
● green · the check passed and the step may go ahead● amber · held, flagged for review or waiting on something● red · refused, or a mismatch was caught
ALL GREEN ON PURPOSEThe bot in this example is genuine, so the three pages answer ACCEPT, VALID and TRUSTED.
WHAT A NO WOULD LOOK LIKEA forged signature would make art-129 answer REFUSE, a bad directory INVALID and a mismatched card UNTRUSTED. The page computes the refusal in your tab exactly as it computes a pass, and the agent gets it back with a fingerprint.
STILL LOCALA refusal sends nothing anywhere either. The verdict stays in the tab until the agent reads it.
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What the page does

Each AINumbers page that supports WebMCP runs a few lines like these when it loads. If the browser has no WebMCP, nothing is registered and the page works as usual.

const mc = document.modelContext ?? navigator.modelContext;
mc.registerTool({
  name: 'verify_webbotauth_signature',
  description: 'Verify an RFC 9421 HTTP Message Signature…',
  inputSchema: { … },
  annotations: { readOnlyHint: true },
  execute: params => /* run this page's own calculation */
});

Excerpt from art-129's page. The other two pages register validate_signature_directory and validate_signature_agent_card the same way.

Good to know

status
WebMCP is an early browser feature. It works in Chrome 149 and later with a testing flag, or on sites in its origin trial. Some page registrations are still being hardened.
the agent
The agent's language model may run in the cloud, depending on which agent you use. That choice is yours. The tool calculations always run in the page.
by hand
The same three pages work without any agent: type the inputs and press the button.

Workflow agent-identity-verification from chaingraph.json. Tool names, verdicts (ACCEPT, VALID, TRUSTED) and the tool list file come from the pages and .well-known/webmcp.json. The chat text is illustrative, the input values are placeholders and the fingerprints are demo SHA-256 values. Compare with How MCP runs one workflow.

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