Agent Board

A local tabbed HTML viewer the Cursor agent can drive over MCP.

You keep http://127.0.0.1:4747 open in the browser. The agent opens, updates, reads, and closes tabs there instead of writing throwaway .html files into a workspace.

How it works

• A small daemon serves the board on 127.0.0.1:4747 and remembers tabs in %LOCALAPPDATA%\agent-board.

• Cursor talks to a stdio MCP process. That process starts the daemon if needed, then calls the local HTTP API.

• The browser page stays connected over a WebSocket, so new pages appear as tabs immediately.

The MCP process can come and go with Cursor. The daemon stays up so the board does not reset when a chat ends.

Related MCP server: antigravity-browser-bridge

Setup on a new PC

dist/ is not in git, and Cursor only sees the MCP server and skill if they are registered on that machine. After clone:

cd C:\path\to\agent-board
npm install
npm run build

Then two copies outside the repo:

1. MCP — add this to %USERPROFILE%\.cursor\mcp.json, with the args path pointing at this clone:

"agent-board": {
  "command": "node",
  "args": ["C:\\path\\to\\agent-board\\dist\\index.js"]
}

2. Skill — copy .cursor\skills\agent-board to %USERPROFILE%\.cursor\skills\agent-board. A project skill only applies in this repo; the personal copy is what agents use from every other workspace. A junction stays in sync with git:

mklink /J %USERPROFILE%\.cursor\skills\agent-board C:\path\to\agent-board\.cursor\skills\agent-board

Reload MCP in Cursor after changing mcp.json. Then open http://127.0.0.1:4747 or ask the agent to present something visually.

npm start runs the daemon in the foreground. Cursor does not need this: the MCP server starts the daemon on first use. npm stop stops a running daemon.

Agent tools

Reuse the same key when updating a topic. Pass a full HTML document, or a fragment (it gets a readable dark template).

Images

board_show accepts local image files via assets (path strings, or { path, name }). The daemon copies them next to the tab and the page can reference them as asset:name:

<img src="asset:hero.png" alt="Hero">

png, jpg, gif, webp, svg, ico, and avif. 8 MB per file, 16 files / 32 MB per tab. These do not count toward the 2 MB HTML cap. Re-showing a key without assets keeps files already attached. Workspace-relative <img src> and file:// URLs do not work — tab pages are served from http://127.0.0.2 and cannot see the disk.

board_screenshot loads the tab's content page in a headless Chromium browser (Edge, Chrome, or Brave — not the board chrome) and returns an image. Pair it with board_show(..., background: true) so a design loop does not steal window focus. Default viewport is 1280×800; pass selector for one element or fullPage for a tall page.

Interactive pages

Every tab owns a JSON state object that lives in the daemon, not in the browser. The agent can read and write it whether or not the tab is focused, or the browser is even open. Pages get it as window.board, injected before any page script runs:

board.state                  // current state, readable synchronously on load
board.set({ todos })         // merge top-level keys, saved on a short debounce
board.signal("submitted")    // wake board_wait; flushes pending board.set first
board.onChange(render)       // agent or another viewer changed something
board.bind(el, "notes")      // two-way bind an input, textarea, or checkbox

A submit button can declare the same handshake without extra script: data-board-signal="submitted". The agent then calls board_wait with that signal name. board_show clears the last signal on the tab so a new wait does not instantly see the previous submit.

Interactive pages should use this instead of localStorage — all tab pages share one origin, so their localStorage collides, and the agent cannot see it.

board.bind is what makes text fields safe. It saves as you type (250 ms idle, 1 s ceiling), and when a remote change arrives for a field you are currently in, it leaves your caret and half-typed text alone, marks the field board-stale, and reconciles once you move on.

Writes merge at the top level, so the agent updating todos never disturbs the notes you are typing. To make in-progress form input completely off limits, keep it under a draft key — by convention the agent reads it but never writes it.

Conflicts

board_get_state returns a stateRevision. Passing it back as expectedRevision makes the write conditional: if the user changed the page in between, it is refused with 409 and the response carries their current state, so the agent can merge and retry. Agent writes without an expectedRevision are refused on a page that already has state, unless force is set. Writes from the page itself are never blocked — the person looking at the screen wins ties.

board_wait blocks until board.signal("name") (or data-board-signal="name") fires on that tab. It returns the signal plus the current state. Waiting for any state change would wake on every keystroke; the named signal is the handshake. After a successful wait, pass signal.revision as afterSignalRevision to wait for the next one without re-showing the page.

Data

Tabs persist in %LOCALAPPDATA%\agent-board\state.json across daemon and Cursor restarts, including each tab's state object (max 256 KB per tab). Image files live in %LOCALAPPDATA%\agent-board\assets\<tabId>\. Closing a tab moves it to the archive (up to 200, oldest dropped). Ctrl+Z restores whichever is newer: the most recently archived tab (no cap on how far back), or one of the last 5 tabs that were permanently deleted while still open.

The browser Clear button archives unpinned tabs. Pinned tabs stay until you archive or delete them. Shift+click a tab's × permanently deletes it (confirmation in the UI). Ctrl+S downloads the current page as HTML.

Port: 4747 (override with AGENT_BOARD_PORT). Bound to localhost only.

Tab pages load in an iframe from http://127.0.0.2:4747 so they can use localStorage without accessing the board chrome or API. Refresh the board after upgrading so the new iframe sandbox takes effect.