Let agents use your web app.

One small script, one Cloudflare Worker. Any framework.

1. Install

The client goes in your page, any framework. The Worker holds each session; deploy your own.

npm install github:acoyfellow/echo

git clone https://github.com/acoyfellow/echo && cd echo
npx wrangler secret put ECHO_SIGNING_SECRET
bun run deploy

2. Register

Name the functions agents may call and mark each read or mutation. Nothing else in the page is reachable.

import { createEcho } from '@acoyfellow/echo';

const echo = createEcho({ appId: 'todo', workerBase });

echo.register({
  listTodos: {
    effect: 'read',
    run: () => todos,
  },
  addTodo: {
    effect: 'mutation',
    run: ({ text }) => add(text),
  },
});

connectButton.onclick = () => echo.connect();
stopButton.onclick = () => echo.stop();

3. Connect, run, stop

The person presses Connect and gets one temporary MCP URL for any number of agents. Each agent sends a script that runs in a sandbox with no network; calls land in the tab one at a time. Stop ends every agent.

// echo_run { runId: 'r1', code: ... }
const todos = await app.listTodos({});

for (const todo of todos) {
  if (!todo.completed) {
    await app.setCompleted({
      id: todo.id,
      completed: true,
    });
  }
}

await app.addTodo({ text: 'Plan the next release' });

return app.listTodos({});

How it compares

Ways to let an agent act inside a web app today. A dash means it depends on how you build or configure it.

	Echo	MCP server	Browser automation	Browser extension	WebMCP
Works on the tab the person has open Live state, their login, what they see
Agent reaches only the functions you name Not the whole page, not the whole browser
No new backend API to build Reuse the functions your UI already calls
No install for the person No extension, no flags, any modern browser
One sandboxed script, many calls Fewer round trips, no network in the sandbox
Several agents at once, applied in order Calls queue in the tab, one at a time
The person can stop it in one click Revokes every agent's access immediately

Works on the tab the person has open Live state, their login, what they see

Echo

MCP server
Browser automation
Browser extension
WebMCP
Agent reaches only the functions you name Not the whole page, not the whole browser

Echo

MCP server
Browser automation
Browser extension
WebMCP
No new backend API to build Reuse the functions your UI already calls

Echo

MCP server
Browser automation
Browser extension
WebMCP
No install for the person No extension, no flags, any modern browser

Echo

MCP server
Browser automation
Browser extension
WebMCP
One sandboxed script, many calls Fewer round trips, no network in the sandbox

Echo

MCP server
Browser automation
Browser extension
WebMCP
Several agents at once, applied in order Calls queue in the tab, one at a time

Echo

MCP server
Browser automation
Browser extension
WebMCP
The person can stop it in one click Revokes every agent's access immediately

Echo

MCP server
Browser automation
Browser extension
WebMCP

Try it here

A real todo app with Echo added. Press Connect agent, copy the URL into any MCP client, and watch the list change.

Want two agents at once? The whiteboard lets several draw on one board.

Try it on other apps

all 17 examples →