@acertijosapps/electron-viewer

Live viewer for Electron apps in the browser, plus an MCP server so an AI agent can drive them. Built for the browser pane of AI coding tools (such as Claude Code): you and the agent watch and control the same app window, in real time.

• Live video: Page.startScreencast from the Chrome DevTools Protocol (CDP) over WebSocket. It keeps streaming while the app window is covered or minimized.

• Real input: mouse move and hover, click, double click, right click, drag, wheel and full keyboard passthrough (including ⌘C/⌘V/⌘A on macOS, IME and dictation).

• Every app and window: launch your project with electron . from the viewer, or attach to any app started with --remote-debugging-port. Apps on ports 9222–9231 are found automatically.

• Main process too: launched apps also expose their main process (Node inspector), so the agent can run code there, resize the real window, open DevTools and capture the native frame.

• Native frame capture: title bar, context menus and dialogs on top of the window.

• MCP server: the same actions as tools, plus JavaScript in the renderer and main process, the accessibility tree with click-ready coordinates, and console logs.

• Nothing to add to your app: no preload, no dependency, no code changes.

Requirements

• Node.js 18 or later.

• An Electron app. To launch it from the viewer, the project must have electron in its node_modules (npm install --save-dev electron); if its binary hasn't been downloaded yet, the viewer downloads it on the first launch.

• Works on macOS, Linux and Windows.

Related MCP server: Electron MCP Server

Quick start

From your Electron project folder:

npx @acertijosapps/electron-viewer

Open http://localhost:8094 and click Launch. The viewer runs electron . on free debugging ports (CDP from 9222, Node inspector from 9229). Deep link to a window: ?window=<target id>.

If your app is started by another tool (Electron Forge, electron-vite, a custom script), pass the flags yourself and the viewer finds it:

electron --remote-debugging-port=9222 --inspect=9229 .

--inspect is optional; without it you get everything except the main-process features (evaluate_main, frame capture, real window resizing and in-app DevTools). To attach on another port, use the form in the viewer or --attach.

electron-viewer [options]        Start the viewer
electron-viewer mcp [options]    Start the MCP server on stdio

      --project <dir>      Project to launch with "electron ." (default: current folder)
  -p, --port <port>        Viewer port (default 8094, env PORT)
      --host <host>        Interface to bind (default 127.0.0.1, env HOST)
      --attach <port>      Attach at startup to an app on this CDP port
      --inspect <port>     With --attach: the app's main process inspector port
      --viewer-url <url>   Viewer the MCP talks to (default http://localhost:8094)

Other environment variables: MAX_DIM (longest side of the streamed frames, default 1920), QUALITY (JPEG quality, default 80), SCREENSHOT_MAX (longest side of MCP screenshots, default 1024), CDP_PORT / INSPECT_PORT (first ports to scan and launch on, default 9222 / 9229) and ELECTRON_VIEWER_STATE_DIR (where launched and attached apps are remembered, default ~/.cache/electron-viewer).

Use it with an AI agent (MCP)

Claude Code:

claude mcp add electron-viewer -- npx -y @acertijosapps/electron-viewer mcp

Any other MCP client:

{
  "mcpServers": {
    "electron-viewer": {
      "command": "npx",
      "args": ["-y", "@acertijosapps/electron-viewer", "mcp"]
    }
  }
}

Keep the viewer running (npx @acertijosapps/electron-viewer) to watch what the agent does. Without it, the MCP drives the app over CDP directly. Apps launched by either one are visible to the other.

Coordinates are normalized to 0–1 over the content screenshot (x = pixel_x / image_width), so they hold at any window size. window is optional when only one window is open; it takes the target id from list_windows or part of the window title.

In the Claude Code browser pane

Add the viewer to .claude/launch.json and open it with preview_start electron-viewer:

{
  "version": "0.0.1",
  "configurations": [
    {
      "name": "electron-viewer",
      "runtimeExecutable": "npx",
      "runtimeArgs": ["-y", "@acertijosapps/electron-viewer"],
      "port": 8094,
      "autoPort": false
    }
  ]
}

Native frame capture

The frame button (and screenshot with frame: true) captures the screen area of the window from the main process with desktopCapturer: the title bar, native context menus and dialogs on top of the window show up, at a few frames per second. Anything else covering the window shows up too, and the macOS menu bar at the top of the screen is not included. In frame mode, clicks on the content still work; the title bar and native menus can't be clicked from the viewer.

On macOS the app needs the Screen Recording permission (System Settings › Privacy & Security › Screen & System Audio Recording). For an app run with electron ., that is the Electron.app inside the project's node_modules/electron/dist; the error message gives the exact path. Restart the app after allowing it.

HTTP API

The MCP uses the viewer's API; you can call it too. window (target id or part of the title) goes in the query string or the body and is optional with a single window. The server only listens on 127.0.0.1, only accepts a local Host and origin, and POST requests require content-type: application/json, so other websites can't drive your apps.

How it works

• Everything goes through CDP with ws: no Playwright, no Puppeteer, nothing injected into the app. The renderer is reached through --remote-debugging-port (one page target per window) and the main process through the Node inspector (--inspect), where the console API provides require.

• One screencast session per window, shared by every viewer tab and stopped when the last one closes. The last frame is kept for late joiners. Focus and page lifecycle are emulated while someone is watching, so a covered or minimized window keeps painting.

• Viewer input is queued in order and mouse moves are coalesced to one per animation frame. A hidden textarea holds the focus: keys are sent with Input.dispatchKeyEvent, and text that arrives without key events (IME, dictation) with Input.insertText. On macOS, ⌘A, ⌘C, ⌘V, ⌘X and ⌘Z are sent as editing commands, because the native menu that normally handles them never sees CDP events.

• Each process keeps one connection per window and per main process, which also collects console messages and exceptions (the last 1000 per app).

• Launched and attached apps are remembered in ~/.cache/electron-viewer, so the viewer and the MCP see the same apps. Launched apps write their output to a log file there and keep running if the viewer or MCP exits.

Limitations

• CDP only sees web content. Native UI (title bar, menus, dialogs, tray, notifications) is visible only in frame mode and can't be clicked; use evaluate_main to drive it from the main process.

• Synchronous dialogs (dialog.showMessageBoxSync) block the main process: evaluate_main and frame capture wait until the dialog is closed.

• Each WebContentsView is a separate CDP target, so it is listed as its own window. <webview> tags are not listed.

• No audio.

• Frame mode runs at a few frames per second and, on macOS, needs the Screen Recording permission.

• Apps that build before starting (Forge, electron-vite) can't be launched from the viewer; start them with the debugging flags and they are picked up.

Author

Made by AcertijosApps. See the rest of our libraries at acertijosapps.com/librerias.

Questions or proposals: acertijos.contacto@gmail.com or open an issue.

License

MIT