BrowserAttach

Drive the Chrome / Edge tabs you already have open and already signed in from an MCP client. BrowserAttach connects to the browser through a locally paired extension, keeps your existing sign-in state, and needs neither a remote debugging port nor a separate automation browser.

npm · Usage guide · Architecture · Agent integration · Test report · Issues · Contributing

What it does

• Discover multiple browser profiles, pick one precisely, claim a tab exclusively, and release it when done.

• Control existing ordinary file:// tabs after enabling BrowserAttach's Allow access to file URLs setting in the browser's extension details; use the same discovery, claim, observation and input tools.

• Read the accessibility tree, query names/values/nearby field context, inspect field values and supported editor models; click, fill and type through refs or strict CSS/XPath, HTML label, placeholder and AX role/name locators. Type can also target the currently focused field explicitly.

• Native pointer hover, stepped drags and conditional waits; select HTML options without opening a native popup.

• Navigate and go back/forward, open or close tabs, handle JavaScript dialogs, upload local files, and capture the whole page or a single element.

• Evaluate JavaScript expressions in the main page; read console, page-error and network lifecycle logs captured after claiming; opt in to bounded, redacted request/response details when XHR/fetch needs verification.

• Read one HTTP(S) URL in a temporary background tab with the profile's sign-in, returning rendered text, current DOM, or complete JSON with the main-document HTTP status when observed.

• Standard stdio MCP with 23 tools. The Hub listens on loopback only and pairs with the extension through a random key.

Related MCP server: chrome-in-harness

Requirements

• Node.js 22 or newer, plus npm.

• Chrome / Edge 125 or newer with permission to load unpacked extensions. Latest stable is recommended.

• An MCP client that speaks stdio MCP and can find Node/npm on PATH.

Quick start

1. Install and pair

npm install --global @evan_zhong/browser-attach
browser-attach setup

setup prints a stable extension directory and starts the local Hub. The directory is usually:

• Windows: %USERPROFILE%\.browser-attach\extension

• macOS / Linux: ~/.local/share/browser-attach/extension

Open chrome://extensions or edge://extensions, enable developer mode, choose "Load unpacked" and load that directory. Load it once for every browser profile you want to expose; the extension options page lets you set a recognizable profile name.

Or without a global install:

npx --yes --package=@evan_zhong/browser-attach@latest browser-attach setup

2. Configure the MCP client

Merge the following into your client's MCP configuration, or see mcp-config.json:

{
  "mcpServers": {
    "browser_attach": {
      "command": "npx",
      "args": ["--yes", "--package=@evan_zhong/browser-attach@latest", "browser-attach", "mcp"]
    }
  }
}

On Windows clients that require a full command extension, use npx.cmd. With a global install you can also configure "command": "browser-attach", "args": ["mcp"]. Restart the MCP client to discover the tools.

@latest selects the npm package at launch. For a pair already running 0.17 or newer, startup coordinates an idle Hub upgrade, synchronizes the paired extension directory and asks older connected extensions to reload themselves. First-time extension loading and the first upgrade from a pre-0.17 pair still require one manual extension reload; a pre-0.17 Hub must be stopped once before the new MCP can start. If another MCP connection is using the Hub, close it and reconnect to retry the upgrade.

Assembly steps for supported hosts are in agent integration; other clients use the stdio entry above.

3. Start driving

1. browser_discover(kind="browsers") → browser_discover(kind="tabs") to confirm the browser, tab title and URL.

2. browser_claim_tab with the tab_id from discover(kind="tabs") (optional expected_url / expected_title exact guards) to take control and receive this connection's tab_handle.

3. browser_snapshot to get element refs, then click, fill or run other actions.

4. Observe again after each action, and finish with browser_tab_control(action="release").

When titles change easily, claim can omit expected_title; expected_url is optional too. Click/fill can instead use a selector that is re-resolved on every call. Native element clicks scroll into view automatically; full-page coordinate clicks use coordinate_space: "document". Evaluation results continue from the cache through cursor without re-running the script. Design trade-offs: architecture notes.

Claiming and ordinary input never activate a tab or window; call browser_tab_control(action="activate") explicitly when foreground rendering is needed. When an outcome is unknown, observe again first and avoid duplicate submissions. Full parameters, pagination, uploads and input semantics: usage guide.

Tools

Local data and upgrades

Override it explicitly with the absolute path BROWSER_ATTACH_HOME. The Windows default avoids AppData, which packaged hosts may virtualize; no other host's private cache is searched or merged automatically. The directory holds the pairing configuration, Hub logs and the stable extension copy, and depends on neither the clone location, the client working directory nor the npm cache. If you already use a custom directory, keep using the same environment variable or --config instead of accidentally creating a second pairing under the new default.

A host assembly step that writes the client entry pins the absolute paths of the current Node, CLI and the actual pairing file and sets BROWSER_ATTACH_HOME; it works together with a custom --config. When the MCP launches the Hub it passes the same absolute pairing path and directory, so different launch environments cannot derive a second configuration. See agent integration.

npm install --global @evan_zhong/browser-attach

Then restart the MCP connection. With @latest, restarting MCP is sufficient for later upgrades once Hub and extension are on 0.17 or newer; setup and start use the same coordination when run explicitly. Upgrading from an older pair first requires browser-attach stop, a restart and one manual extension reload. Custom pairing paths, ports and extension directories are retained. Closed browser profiles update when they next connect. An older MCP package cannot downgrade a newer Hub or installed extension.

An upgrade waits for extension cleanup and confirms the new version after reload; cleanup or reload failures are reported instead of replaying page actions. Manual reload remains available on the extension options page for recovery. Hub and extension versions must match for page control. The default port is 47831; choose another port with setup --port when creating a pairing.

Architecture and boundaries

MCP client → Node.js stdio MCP → 127.0.0.1 Hub
                                  ↕ local pairing
                          Chrome / Edge MV3 extension
                                  ↓ chrome.debugger / CDP
                               existing HTTP(S) / authorized file:// tabs

Each MCP connection holds its own control and handles cannot be used across connections. Disconnecting or stopping debugging releases control. The client endpoint requires a Bearer key and rejects web page Origins; the extension verifies a fixed Origin, key and version. browser_evaluate can read and write data with the page's existing sign-in state; after it runs, snapshot pagination expires while refs to the original nodes still resolve; a timeout or error may already have had an effect and must not be replayed automatically. No generic CDP tool is exposed.

Logs are collected from claim time, survive navigation and are cleared on release; each kind keeps at most 500 entries and 1,048,576 JSON characters, and eviction reports data_loss. Network logs store lifecycle metadata only by default; an explicit capture reads bounded, redacted headers/bodies of subsequent XHR/fetch requests without changing caches or intercepting traffic. Full parameters and limits: usage guide.

The extension holds debugger permission and client actions can use existing site sign-in state; only connect clients you trust. Web page content must not be treated as user authorization. The pairing key and the generated config.generated.js must not be committed or published; the packaging check rejects packages that contain that file.

Chrome / Edge HTTP(S) pages and existing ordinary file:// tabs with extension file access enabled are supported. File access is checked during discovery, claim, operations and navigation, including history destinations. open_tab and fetch still accept HTTP(S) only. Built-in PDF viewer AX/content and screenshots are browser-dependent and are not guaranteed; browser internal pages and arbitrary extension pages remain outside scope. Download management, network interception and Firefox / WebKit are not supported. Cross-process iframes use recursive session routing covering snapshots, input, uploads and logs; clicks in perspective-transformed iframes are refused explicitly. Native pointer input requires an unsuspended tab in a non-minimized window. An unmodified left single click may target an inactive tab with trusted delivery verification; hover, drag and other click variants still require an active tab. Focus and document visibility are diagnostic only; when the tab is not ready it returns without dispatch and suggests tab_control(action=activate) instead of stealing focus. Background screenshots may time out because the browser pauses rendering; native menus and tab drags may also suppress input.

Local development

git clone https://github.com/EvannZhongg/BrowserAttach.git
cd BrowserAttach
npm ci
npm test
npm run test:package

Run the source directly and pair the extension in the repository:

node bin/browser-attach.mjs setup --extension-dir ./extension
node bin/browser-attach.mjs mcp

Development mode deliberately uses the repository extension path. For normal installs, use setup without --extension-dir so it keeps working after the repository moves. On Windows you can also run ./scripts/install.ps1.

Real-browser tests need a Chromium / Edge executable that allows loading extensions from the command line; the path is supplied by the developer:

npm run test:live -- --executable /path/to/chromium --family chromium
npm run test:interaction -- --executable /path/to/chromium --family chromium
npm run test:interaction -- --tab-indicators --executable /path/to/chromium --family chromium
npm run test:upgrade -- --executable /path/to/chromium

These tests use temporary profiles and local fixtures. Coverage and limits: test report; implementation and design trade-offs: architecture.

Contributing

Reproducible issues via Issues, or Pull Requests. Please include Node, OS, browser and extension versions, and remove pairing keys and private page data. See CONTRIBUTING.md for the process.

Released under the MIT License.