What is WebMCP?

WebMCP is a proposed web standard, developed in the W3C Web Machine Learning Community Group, that lets a web page hand tools to the AI agent running in the visitor’s browser. Instead of reading pixels or raw HTML, the agent sees a list of named functions with descriptions and JSON Schemas, and calls them.

It borrows the idea of tools from the Model Context Protocol (MCP), but runs in the page, with the page’s own origin and the user’s own session, instead of on a server.

The API in one paragraph

agentprose follows the WebMCP Draft Community Group Report of 16 October 2026. The API lives on document.modelContext: a page calls registerTool(tool, { signal, exposedTo }) for each tool; agents (and the page itself) can list them with getTools() and call one with executeTool(). A tool is removed by aborting the AbortSignal it was registered with. The draft has no provideContext and no unregisterTool.

A tool, in WebMCP terms, looks like this:

document.modelContext.registerTool(
  {
    name: 'search_posts',
    description: 'Full-text search across every post on this site…',
    inputSchema: { type: 'object', properties: { query: { type: 'string' } }, required: ['query'] },
    annotations: { readOnlyHint: true, untrustedContentHint: true },
    execute: async ({ query }) => ({ /* results */ }),
  },
  { signal: controller.signal },
);

agentprose writes all of that for you: 14 tools, their descriptions and schemas, the annotations, and the code behind them.

Where it runs today

WebMCP is new. As of October 2026:

• Chrome / Chromium implements it behind the WebMCPTesting feature (chrome://flags/#enable-webmcp-testing), and sites can join the Chrome or Edge origin trial to switch it on for real visitors.
• Everywhere else, the open-source MCP-B project supplies document.modelContext with a polyfill, and its browser extension and local relay let agents such as Claude Desktop, Cursor and Claude Code reach the tools.

agentprose works with both: it registers natively when the browser has WebMCP, and otherwise loads the pinned, integrity-checked MCP-B polyfill after your page has loaded. The exact matrix, with dates and the browser builds we tested, is on Browser and agent support.

Note

WebMCP is a Community Group draft, not a W3C Recommendation. Names and details may still change; agentprose tracks the current draft and keeps a legacy path for older polyfills that expose navigator.modelContext.

Requirements worth knowing

• Secure contexts only. WebMCP exists on HTTPS pages and on localhost.
• Frames. An iframe that registers tools needs allow="tools"; to let a parent page see them, pass exposedTo (data-exposed-to).
• One name, one tool. If another script already registered a tool with the same name, that registration fails and the rest continue. data-prefix avoids collisions.

How agentprose fits

WebMCP is the plumbing. It doesn’t say which tools a blog should offer or how to build them. agentprose is that layer for content sites: it reads what your site already publishes, turns it into a well-described set of read-only tools, and registers them. Read Tools next.