WebMCP: declare a site's agent tools, and gate the ones that spend money

The Stack

Steps

1. 1

   Declare the tools and validate the manifest

   List the tools your site offers an agent, each with an input schema and a declared return, and mark the sensitive ones. CI checks every tool has a real schema and every sensitive tool requires a confirmation click, so a purchase or delete can never run unattended. Wiring the tools into the page and running them is fenced.

   cat > webmcp.json <<'JSON'
   {
     "tools": [
       {
         "name": "search_flights",
         "description": "Search available flights for a route and date",
         "sensitive": false,
         "input": { "origin": "string", "destination": "string", "date": "string" },
         "returns": "a list of flight options with prices"
       },
       {
         "name": "select_flight",
         "description": "Select a flight by id",
         "sensitive": false,
         "input": { "flight_id": "string" },
         "returns": "the selected flight held for checkout"
       },
       {
         "name": "checkout",
         "description": "Pay for the held flight",
         "sensitive": true,
         "requires_confirmation": true,
         "input": { "payment_token": "string" },
         "returns": "an order confirmation number"
       }
     ]
   }
   JSON
   node -e '
   const fs = require("fs");
   const c = JSON.parse(fs.readFileSync("webmcp.json", "utf8"));
   function bad(m) { console.error("BAD: " + m); process.exit(1); }
   const tools = c.tools || [];
   if (tools.length < 1) bad("declare at least one tool");
   const sensitive = [];
   for (const t of tools) {
     if (!t.name) bad("each tool needs a name");
     if (!t.input || typeof t.input !== "object" || Object.keys(t.input).length < 1) bad("tool " + t.name + " needs an input schema with at least one field");
     if (!t.returns || String(t.returns).length < 3) bad("tool " + t.name + " must declare what it returns");
     if (t.sensitive === true) {
       if (t.requires_confirmation !== true) bad("sensitive tool " + t.name + " must set requires_confirmation: true (money or irreversible needs a human click)");
       sensitive.push(t.name);
     }
   }
   console.log("webmcp OK: " + tools.length + " tool(s) declared with input+return schemas; " + sensitive.length + " sensitive tool(s) all require confirmation (" + sensitive.join(", ") + ")");
   '

2. 2

   Wire the tools into the page (the actuation step, not checked by CI)

   Register the tools with navigator.modelContext (or add toolname/tooldescription attributes to existing forms as a progressive enhancement), and test them in the Chrome 149 origin trial with the WebMCP extension. Keep a human in the loop on sensitive steps. The live actuation is fenced.