WebMCP

Where the spec stands today

How WebMCP relates to MCP

The specification puts it directly: pages using WebMCP can be thought of as Model Context Protocol servers that implement tools in client-side script instead of on the backend. The vocabulary is shared, tools, schemas, parameters, but the deployment model is not.

Two ways to expose a tool

1. In JavaScript

Register a tool with a name, a natural language description, and a JSON Schema for its input. The handler runs your existing application code.

const controller = new AbortController();

await document.modelContext.registerTool({
  name: "add-todo",
  description: "Add a new item to the user's active todo list",
  inputSchema: {
    type: "object",
    properties: {
      text: { type: "string", description: "The text content of the todo item" }
    },
    required: ["text"]
  },
  async execute({ text }) {
    await addTodoItemToCollection(text);
    return {
      content: [
        { type: "text", text: `Added todo item: "${text}" successfully.` }
      ]
    };
  }
}, { signal: controller.signal });

2. In HTML, with no JavaScript

An existing form can be annotated directly. The browser builds the JSON Schema from the form controls, so a server rendered site can expose a working tool without shipping a line of script.

<form toolname="supportRequestTool"
      tooldescription="Submit a request for support."
      action="/submit">
  <input type="text" name="firstName">
  <select name="team" required
          toolparamdescription="Determines what team this request is routed to.">
    ...
  </select>
</form>

This is worth knowing before you audit anything. A scanner that only greps for JavaScript will report zero tools on a site that is exposing them perfectly well through markup.

What tends to break

• Registration moves behind a condition. Tools register at runtime. A refactor that puts the call behind a feature flag, a consent gate, or a lazy loaded bundle removes them without any error appearing anywhere.

• The schema drifts from the handler. A required field is added to the form but not to the declared input schema, so agents keep sending calls that your handler now rejects.

• The permission is withdrawn. Tool exposure is governed by the tools permission policy. A tightened header or an embedding change can disable registration entirely.

• The spec moves under you. The namespace already changed once. During an origin trial, more change is likely, and nothing in your build will tell you it happened.