Zen DevTools MCP

Unofficial Model Context Protocol server for automating Zen Browser via WebDriver BiDi (through Selenium WebDriver). Works with Claude Code, Claude Desktop, Cursor, Cline and other MCP clients.

Repository: https://github.com/simon-ami/zen-devtools-mcp

This is a Zen-first fork of Mozilla's Firefox DevTools MCP server. The public package, CLI, MCP server name, logs, profile paths, and management tools use Zen naming. The lower-level implementation still uses Selenium's Firefox/geckodriver APIs because Gecko automation is exposed through moz:firefoxOptions, Browser.FIREFOX, and moz:* WebDriver BiDi commands.

This project is maintained independently and is not affiliated with or endorsed by Zen Browser.

Note: This MCP server requires a local Zen Browser installation and cannot run on cloud hosting services. Use npx zen-devtools-mcp@latest to run locally, or use Docker with the provided Dockerfile.

Security

Browser MCP servers carry inherent risks. A few key practices:

• Use a dedicated Zen profile. Never run the server against your regular profile unless you intentionally use connect-existing mode. The agent has access to whatever the browser can reach, including cookies and saved sessions.
• Be cautious about which sites you visit. Pages can return content designed to manipulate the agent (prompt injection). Stick to sites you control or trust.
• Avoid enabling extra flags unless needed. --enable-script and --enable-privileged-context significantly expand what the agent can do.

See SECURITY.md for a full breakdown of risks and how to report vulnerabilities.

Requirements

• Node.js >= 20.19.0
• Zen Browser installed (auto-detected on macOS at /Applications/Zen.app/Contents/MacOS/zen, or pass --zen-path)

Install and use with Claude Code (npx)

Recommended: use npx so you always run the latest published version from npm.

Option A: Claude Code CLI

claude mcp add zen-devtools npx zen-devtools-mcp@latest

Pass options either as args or env vars. Examples:

# Headless + viewport via args
claude mcp add zen-devtools npx zen-devtools-mcp@latest -- --headless --viewport 1280x720

# Or via environment variables
claude mcp add zen-devtools npx zen-devtools-mcp@latest \
  --env START_URL=https://example.com \
  --env ZEN_HEADLESS=true

Option B: Edit Claude Code settings JSON

Add to your Claude Code config file:

• macOS: ~/Library/Application Support/Claude/Code/mcp_settings.json
• Linux: ~/.config/claude/code/mcp_settings.json
• Windows: %APPDATA%\Claude\Code\mcp_settings.json

{
  "mcpServers": {
    "zen-devtools": {
      "command": "npx",
      "args": ["-y", "zen-devtools-mcp@latest", "--headless", "--viewport", "1280x720"],
      "env": {
        "START_URL": "about:blank"
      }
    }
  }
}

Option C: Helper script (local dev build)

npm run setup
# Choose Claude Code; the script saves JSON to the right path

Try it with MCP Inspector

npx @modelcontextprotocol/inspector npx zen-devtools-mcp@latest --start-url https://example.com --headless

Then call tools like:

• list_pages, select_page, navigate_page
• take_snapshot then click_by_uid / fill_by_uid
• list_network_requests (always-on capture), get_network_request
• screenshot_page, list_console_messages

CLI options

You can pass flags or environment variables (names on the right):

• --zen-path — absolute path to Zen binary (ZEN_PATH)
• --headless — run without UI (ZEN_HEADLESS=true)
• --viewport 1280x720 — initial window size
• --profile-path — parent directory for a dedicated Zen MCP profile
• --auto-profile — use a persistent profile under ~/.zen-devtools-mcp; enabled by default (AUTO_PROFILE=false disables it)
• --zen-arg — extra Zen arguments (repeatable)
• --start-url — open this URL on start (START_URL)
• --accept-insecure-certs — ignore TLS errors (ACCEPT_INSECURE_CERTS=true)
• --connect-existing — attach to an already-running Zen instead of launching a new one (CONNECT_EXISTING=true)
• --marionette-port — Marionette port for connect-existing mode, default 2828 (MARIONETTE_PORT)
• --pref name=value — set Zen preference at startup via moz:firefoxOptions (repeatable)
• --enable-script — enable the evaluate_script tool (executes arbitrary JavaScript in the page context) and debugging tools (list scripts, inspect source, set logpoints). Debugging tools require Gecko 153+. (ENABLE_SCRIPT=true)
• --enable-privileged-context — enable privileged context tools: list/select privileged contexts, evaluate privileged scripts, get/set Zen prefs, and list extensions. Requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1 (ENABLE_PRIVILEGED_CONTEXT=true)
• --log-file — write MCP server logs to a file instead of stderr. Useful for debugging sessions with MCP clients that hide server output. Set DEBUG=* to also include verbose debug logs. Example: --log-file /tmp/zen-mcp.log

Useful preferences (--pref)

• remote.prefs.recommended=false. When Zen runs in automation, it applies Gecko RecommendedPreferences that modify browser behavior for testing. Set remote.prefs.recommended to false to skip those and have a configuration closer to a regular Zen instance.
• remote.log.level=Trace. Enable verbose WebDriver protocol logs in Zen. The MCP server will automatically pass the matching log level to geckodriver so both sides log at the same verbosity.
• app.update.disabledForTesting=false. Allow Zen to automatically download and apply updates. Note that updates may interrupt your session. Requires also setting remote.prefs.recommended=false.

Connect to existing Zen

Use --connect-existing to automate your real browsing session with cookies, logins, and open tabs intact:

# Start Zen with Marionette enabled
/Applications/Zen.app/Contents/MacOS/zen --marionette

# Run the MCP server
npx zen-devtools-mcp --connect-existing --marionette-port 2828

Or set marionette.enabled to true in about:config (or user.js) to enable Marionette on every launch.

BiDi-dependent features (console events, network events) are not available in connect-existing mode; all other features work normally.

Warning: Do not leave Marionette enabled during normal browsing. It sets navigator.webdriver = true and changes other browser fingerprint signals, which can trigger bot detection on sites protected by Cloudflare, Akamai, etc. Only enable Marionette when you need MCP automation, then restart Zen normally afterward.

For personal profiles, you may set remote.prefs.recommended=false in about:config before starting Zen with Marionette. This prevents Gecko's testing preferences from disabling features such as password autofill, but it also leaves saved credentials available to the automated session. A dedicated profile remains the recommended option.

Tool overview

• Pages: list/new/navigate/select/close
• Snapshot/UID: take/resolve/clear
• Input: click/hover/fill/drag/upload/form fill
• Network: list/get (ID-first, filters, always-on capture)
• Console: list/clear
• Screenshot: page/by uid (with optional saveTo for CLI environments)
• Script: evaluate_script
• Privileged Context: list/select privileged ("chrome") contexts, evaluate_privileged_script (requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1)
• WebExtension: install_extension, uninstall_extension, list_extensions (list requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1)
• Zen Management: get_zen_info, get_zen_output, restart_zen, set_zen_prefs, get_zen_prefs
• Profiler: profiler_is_active, profiler_start (preset or explicit config), profiler_stop (saves profile to downloads directory)
• Utilities: accept/dismiss dialog, history back/forward, set viewport

Screenshot optimization for Claude Code

When using screenshots in Claude Code CLI, the base64 image data can consume significant context. Use the saveTo parameter to save screenshots to disk instead:

screenshot_page({ saveTo: "/tmp/page.png" })
screenshot_by_uid({ uid: "abc123", saveTo: "/tmp/element.png" })

The file can then be viewed with Claude Code's Read tool without impacting context size.

Local development

npm install
npm run build

# Run with Inspector against local build
npx @modelcontextprotocol/inspector node dist/index.js --headless --viewport 1280x720

# Or run in dev with hot reload
npm run inspector:dev

See CONTRIBUTING.md for more details on local development, testing, and CI.

Troubleshooting

• Zen not found: pass --zen-path "/Applications/Zen.app/Contents/MacOS/zen" (macOS) or the correct path on your OS.
• First run is slow: Selenium sets up the BiDi session; subsequent runs are faster.
• Stale UIDs after navigation: take a fresh snapshot (take_snapshot) before using UID tools.
• Saved-password suggestions stopped after using --connect-existing: Marionette's recommended automation preferences may leave signon.autofillForms and signon.rememberSignons set to false after the session ends. Stop the MCP server, restart Zen without Marionette, and reset both preferences in about:config.
• Windows 10: Error during discovery for MCP server 'zen-devtools': MCP error -32000: Connection closed

  • Solution 1 Wrap with cmd /c (details):

    "mcpServers": {
      "zen-devtools": {
        "command": "cmd",
        "args": ["/c", "npx", "-y", "zen-devtools-mcp@latest"]
      }
    }
    

  • Solution 2 Use the absolute path to npx (adjust extension: .cmd, .bat, .exe, or .ps1, to match your setup):

    "mcpServers": {
      "zen-devtools": {
        "command": "C:\\nvm4w\\nodejs\\npx.ps1",
        "args": ["-y", "zen-devtools-mcp@latest"]
      }
    }
    

Versioning

• Pre-1.0 API: versions start at 0.x. Use @latest with npx for the newest release.
• This fork uses its own version line rather than upstream Firefox DevTools MCP version numbers.

Contributing

See CONTRIBUTING.md for how to file issues, run tests, and work on the project locally.

Author

Maintained independently by Simon Benedict. Based on Mozilla's Firefox DevTools MCP server.

License

Licensed under either of MIT or Apache 2.0 at your option.