MCP Browser Extension

MCP Browser Extension

[CI] [npm] [license]

Website and docs: mcp-browser-extension.com · Quickstart · All 43 tools · Install the extension from the Chrome Web Store

Let Claude use the Chrome you are already logged into. Not a fresh automated browser that greets every site as a stranger — your Chrome, with your sessions, your cookies, your 2FA already done. If you can see a page in your browser, your agent can read it, without logging in again and without pasting credentials anywhere.

Most browser MCP servers launch their own Chromium and hand your agent a signed-out window. MCP Browser Extension does the opposite: an MV3 extension dials into a localhost WebSocket server and drives the browser you already have open, through chrome.scripting/chrome.tabs. Works with Claude Code, Claude Desktop, and any other MCP host.

MCP Browser Extension has two parts: the extension, and a small MCP server you run with npx (npm package and command: mcp-browser-extension). The extension comes from the Chrome Web Store or loaded unpacked.

This build is extension-only. It never launches or attaches a Chromium of its own, so the extension is required, not optional — without it, no tool can run. The CDP flags (--cdp-fallback, --no-cdp-fallback, --cdp-endpoint, --prefer) are still accepted for back-compat but are ignored.

Full design: docs/BLUEPRINT.md — architecture, wire protocol, the complete tool surface, the extension manifest, the security model, and the phased build plan.

Use Claude in your signed-in Chrome

You are already signed in to Gmail, GitHub, your analytics dashboards, the admin panel, the CRM. MCP Browser Extension lets Claude (Claude Code, Claude Desktop, or any other MCP host) work in those same tabs: no re-login, no second 2FA prompt, no password or cookie in a config file. The extension runs inside your normal Chrome, so a page the agent opens is the page you would see.

• Gmail — read a thread or search results in a tab you already have open.
• GitHub — go through PRs, issues and settings pages as yourself, private repositories included.
• Dashboards — pull numbers from analytics, billing or admin screens that have no API, or whose API you never set up.

Other tools drive a signed-in Chrome too (the hangwin/mcp-chrome extension, for one), so the question is what you get on top of the session:

• Website checks with explicit Chrome permission. Local suspicious-site checks are on by default. After one Chrome permission grant, ordinary sites work automatically and flagged sites need Allow. Named-site approval remains available; --no-site-grants enforces the server's fixed allowlist. Reads and actions are checked in the extension. Mutations, eval, downloads and uploads are separate opt-ins.
• Password values are never returned, and --redact scrubs tokens, API keys and JWTs out of page reads.
• An audit trail: every call is logged with the URL, the allow/deny verdict, duration and bytes returned.
• Recent activity in Chrome: the popup shows the latest five activity rows (a tab's open/close pair shares one row); Settings shows up to 100 with outcome filters. This session history contains website names and app keys, without page text, entered values or full URLs.
• Stop access: disconnect all apps from the popup or Settings. The stop survives restarts; Resume connection requires fresh approval for each app. A command already running may finish, and completed actions cannot be undone.
• It stays out of your tab: the agent opens its own tabs in the background and works there, so the page you are on is never navigated, clicked or pulled out of focus. batch reads many of them at once in one call.
• Several Chrome profiles: load the extension in your work and personal profiles and switch between them with profile_use.
• auth_check: when a session does expire, the agent gets an [AUTH_REQUIRED] signal instead of a confusing timeout, so it can stop and ask you to sign in again. MCP Browser Extension never holds credentials or signs in for you.

Setup is two pieces: the MCP server (npx, below) and the extension, which you can install from the Chrome Web Store or load from the folder the server unpacks.

Quickstart

Up and running in one paste

Hand this to your AI agent (Claude Code, Cursor, Windsurf, anything MCP) and it installs the server, wires it into the client, and walks you through installing the extension, verifying your email, and pairing Chrome:

Set up MCP Browser Extension on this machine by fetching and followinghttps://raw.githubusercontent.com/Mehmoodqureshi/mcp-browser-extension/main/SETUP.mdexactly, step by step. Work autonomously and verify each step.

Prefer to read before you run an agent on your machine? SETUP.md is the exact file the agent follows. The manual steps are below.

1. Register the MCP server with your host.

claude mcp add mcp-browser-extension -s user -- \  npx -y mcp-browser-extension@latest \  --allow-domain example.com --enable-mutations --persist-token

{  "mcpServers": {    "mcp-browser-extension": {      "command": "npx",      "args": ["-y", "mcp-browser-extension@latest",               "--allow-domain", "example.com", "--enable-mutations",               "--persist-token"]    }  }}

Website access requires app approval and Chrome permission. Suspicious-site checks are on by default; click Allow website access once in the extension. For a fixed named-site policy, use --no-site-grants --allow-domain <glob>. Mutations, downloads, uploads and eval stay off until enabled with --enable-mutations, --enable-downloads, --enable-uploads or --unsafe-enable-eval.

Email verification is on by default. For a trusted, local unattended setup, you can opt out with --no-email-verification (or set MCP_BROWSER_EXTENSION_EMAIL_VERIFICATION=0). This skips the email prompt and the Firebase access service, so no email or device grant is sent there. The local MCP client still needs approval inside Chrome before using the extension, and stays within the configured domain and capability policy. Treat every process that can launch or reach this local server as trusted. This option is rejected with --tunnel and does not turn off telemetry or the npm update check.

--enable-uploads permits upload_file (setting local file(s) on a page's file <input>). It is off by default because sending local files to a page is an exfiltration risk; it is also gated by the destination-domain allowlist. Pair it with --uploads-dir <path> to restrict uploads to files inside that directory (.. traversal is blocked) — strongly recommended for unattended use.

Keep transport pairing across restarts. Both examples above include --persist-token, which is what makes the pairing survive a restart — drop it if you'd rather have the stricter default described next.

Without --persist-token a fresh token is minted every boot (the secure default), which means re-pairing the extension on each restart. With it, the token is stored 0600 at ~/.mcp-browser-extension/token and reused; the extension's keepalive auto-reconnects with no manual step. MCP_BROWSER_EXTENSION_TOKEN pins the token explicitly (and is never written to disk). This transport pairing does not grant an app access: every new app session has a separate key and needs Chrome approval.

2. Install the extension — required; the server can drive nothing without it.

Install MCP Browser Extension from the Chrome Web Store. One click, no Developer mode, and Chrome keeps it updated for you. This is the way to install it.

3. Verify your email. In your MCP client, ask the agent to call chrome_status. On this first browser tool call, the client shows an email prompt. Enter an address you can access, then open the sign-in link sent to that address and click Verify this device. The waiting tool reports Email verified and continues.

If the tool returns ACCESS_PENDING, ask the agent to call chrome_status again to keep waiting on the same link. You do not need another email or a verification code.

If your client suppresses the prompt (for example, in YOLO/bypass mode), does not support MCP elicitation, or the dialog is dismissed, the server automatically opens a temporary local verification form in your browser. The tool also returns ACCESS_INPUT_REQUIRED with the link, so you can open it yourself if necessary on the computer running the MCP server. Enter your email in the browser form, and open the sign-in link from your inbox. Click Verify this device, then call chrome_status to continue. The local form expires after 10 minutes; call the tool again for a fresh one. Opening the form does not send an email, and access remains blocked until the email link and device grant are verified. Installing the extension alone does not activate access.

4. Click Connect if Chrome is not already paired. A fresh install opens the extension's Settings with a Connect button (it is in the toolbar popup too). Click it, allow the one permission Chrome asks for, and you are paired: the server registers a small helper with Chrome every time it starts, and the button asks it for the port and token. Chrome lets only this extension talk to that helper. If the server ever changes its token, the extension fetches the new one by itself. Nothing to copy or paste.

5. Approve the app connection in Chrome. Run chrome_status, then open the extension popup's App connections section. Compare the key fingerprint with the one in the tool result and click Approve for 8 hours. App names are supplied by the app; the key identifies this particular session. Retry the tool after approving. A question-mark badge and the popup show pending requests, even if the client hides its prompts. Approve, deny, revoke and restore connections directly in the popup or in Settings.

Each app session has its own signing key, held only in that server session's memory. Approval applies to this Chrome profile for eight hours. Restarting the app's MCP connection creates a new key and needs a new approval. Remove access blocks further commands from that key; an action already running may finish. YOLO, bypass-permission mode and --no-email-verification cannot skip this extension check.

6. Allow website access once. Suspicious-site checks are on by default. In the popup or Settings, Allow website access asks Chrome for HTTP/HTTPS access. After you accept, ordinary sites work without an Allow request. Local checks pause flagged addresses and risky login forms and show their reasons under Waiting for Allow. Allow approves only the displayed reasons for that exact origin; new reasons ask again. Blocked websites lets you block or restore any site. Both the extension and connected server must be updated; --no-site-grants disables this option.

These checks run locally with no external checking service. They can miss scams and flag legitimate sites. Form checks run after a page loads, before app reads or actions; they do not prevent page scripts from running or detect all malware. App approval, Stop access and the configured tool permissions still apply.

Choose Use per-site approval or Turn off to use named-site access instead. With automatic access off, the popup shows a Website access request. Click Allow, then accept Chrome's website permission prompt if shown. This single flow grants that named website in both Chrome and the server's allowlist. Allowing a website lets approved apps read and take actions there, including sending messages, deleting information and making purchases, subject to the configured tool permissions. There is no separate extension approval for each action. Website access is remembered until removed and shared by approved app sessions; each app still needs its own unexpired connection approval. Removing the website also removes its Chrome permission. Existing server flags do not grant Chrome access, and an upgrade may ask you to approve websites again.

Cookies and downloads ask separately under Extra permissions when needed. They start off, and their buttons also let you remove permission. Downloads are fetched without browser cookies, redirects are refused, and files are limited to 50 MiB. Links that require a signed-in browser session or redirect may not work. Proxy authentication and IP lookups request permission from Settings. Chrome does not support making debugger or proxy optional; those remain required, and debugger commands still check website permission before they run.

Update both the extension and server before using client approval. An older extension cannot enforce it, and the updated server refuses browser commands until the extension is updated. See client approval details.

Where to see the badge: it sits on the extension's icon in Chrome's toolbar, not on the chrome://extensions page. Chrome hides new extensions behind the puzzle-piece button at the right of the address bar, so click that, find MCP Browser Extension, and click the pin next to it once; the icon then stays in the toolbar. Hover it for the status in words.

Manual fallback (the helper is turned off with --no-native-host, or Connect reports a problem): run npx -y mcp-browser-extension@latest --print-pairing, open the extension's Settings, and paste the port + token from ~/.mcp-browser-extension/handshake.json.

Extension missing after restarting Chrome

Run the read-only diagnostic to see the actual installation type, missing files, Chrome disable reasons, and any conflicting extension registrations:

npx -y mcp-browser-extension@latest extension doctor

Older unpacked copies could acquire a new ID when an update added the Web Store public key to their manifest. Chrome then retained both IDs for one folder and skipped that folder on its next startup. Updates now preserve existing IDs. For an installation already affected by this exact conflict:

npx -y mcp-browser-extension@latest extension repair# Review the preview, fully quit Chrome and other Chromium browsers, then:npx -y mcp-browser-extension@latest extension repair --apply

The repair backs up the original preferences under ~/.mcp-browser-extension/extension-repair-backups/, removes only the confirmed obsolete registration, and preserves the current extension and its saved settings. It refuses to write while a Chromium browser is running. Reopen Chrome afterwards. Use --user-data-dir <directory> for a custom browser data location, or extension doctor --json for a diagnostic report. Reports contain extension paths and registration status, but no browsing history, cookies or pairing tokens.

A Web Store installation with no matching stale-ID conflict is left unchanged. If the diagnostic finds missing files, a disabled Store copy, or no registration, that needs its own diagnosis; this command does not assume the cause or reset the browser profile. When moving from an unpacked copy to the Store version, remove the unpacked copy first, then install from the Store.

Running more than one session

Every MCP host session (each Claude terminal, tab or window) starts its own MCP Browser Extension server, and they all share your Chrome at once. The first one to start owns the bridge port and the extension connections — the hub. Each later session finds the port held by a live MCP Browser Extension server and joins it as a peer: its tool calls are relayed through the hub to the same browsers, so every session keeps working side by side. Nobody is disconnected.

When the hub's session ends, its peers race for the port; one takes it over (with the same token, so the extension re-pairs by itself within a few seconds) and the rest join the new hub. A call that was in flight at that moment fails once with EXTENSION_DISCONNECTED and is retried automatically when it is safe to repeat.

Peers authenticate to the local transport with the pairing token from the 0600 handshake file. That token alone cannot authorize browser commands: each session must sign its own commands with a key approved inside Chrome. A server too old to share the port is replaced as before: it is verified to be an MCP Browser Extension server, then stopped. Anything that isn't a verified MCP Browser Extension server is never touched — a port held by some other program is reported, never killed.

Sessions share one browser, so they also share its tabs: two sessions driving the same tab at the same moment will step on each other. Give each session its own tabs (tab_new), or its own Chrome profile (below).

Each session can also drive several browsers at once: load the extension in each Chrome profile and they all pair to the same server, each under its own profile name. Tools act on the active profile — pick it with --profile <name> at startup or the profile_use tool at runtime.

Naming is automatic. Chrome won't tell an extension which profile it runs in, so each install keeps a random id and the server names it: the first browser is default, the next profile-2, then profile-3, and so on. Names are stored in ~/.mcp-browser-extension/profiles.json, so a browser keeps its name across restarts. chrome_status lists every paired browser (with its active tab as a hint), and profile_rename gives one a friendly name (profile-2 → work). To pin a name yourself instead, type it into the extension's Options → Profile; that always wins.

Without --port, each server binds an ephemeral port (no conflict ever), but the port changes every boot — so you'd re-pair the extension each time. Pin --port plus --persist-token for a pair-once setup.

Windows

WSL2 is not required — native Windows works. One config change is, though: on Windows npx is npx.cmd, a batch shim, and MCP hosts spawn the server without a shell, which cannot execute a .cmd. So "command": "npx" fails to start. Wrap it in cmd /c:

{  "mcpServers": {    "mcp-browser-extension": {      "command": "cmd",      "args": ["/c", "npx", "-y", "mcp-browser-extension@latest",               "--allow-domain", "example.com", "--enable-mutations",               "--persist-token"]    }  }}

Or from Claude Code: claude mcp add mcp-browser-extension -- cmd /c npx -y mcp-browser-extension@latest --allow-domain example.com

For normal use, install the extension from the Chrome Web Store as described above, then click Connect. Do not use Load unpacked as part of the normal Windows setup. The unpacked developer build has the same extension ID as the Store build, so loading it can replace the Store installation; Chrome may then disable the non-Store copy. If you are developing mcp-browser-extension, load %USERPROFILE%\mcp-browser-extension in a separate Chrome profile.

The tools cover tabs, navigation, interaction (click/type/press/hover/ scroll/select_option), reads (get_text/get_html/screenshot/eval/wait_for), an accessibility snapshot (interactive elements with stable refs the model can target instead of guessing CSS selectors), session access (get_cookies/storage), helpers (extract_links/read_as_markdown/fill_form/download_file/upload_file), and chrome_status. upload_file sets local file(s) on a file <input> without the OS dialog (requires --enable-uploads).

click/type accept trusted: true for real OS-level input (works on React/Vue controlled inputs); interactions auto-wait for the target to appear.

Use it from anywhere — --tunnel

Drive the Chrome on your computer from claude.ai, the Claude mobile app, ChatGPT, Claude Code on another machine, or an agent running on a server.

brew install cloudflared            # once (Windows: winget install --id Cloudflare.cloudflared)npx -y mcp-browser-extension@latest --tunnel --allow-domain example.com --enable-mutations

It prints a connector URL like https://<random>.trycloudflare.com/mcp/<key>:

• claude.ai / Claude mobile: Settings > Connectors > Add custom connector, paste the URL.
• ChatGPT (web): Plugins > Add > Add custom MCP server. Choose Server URL, paste the complete connector URL (including /mcp/<key>), and choose No authentication. Create and install the personal plugin, then select it with @ in a new chat. Keep the tunnel running.
• Claude Code elsewhere / agents: use <url>/mcp with Authorization: Bearer <key>; the exact claude mcp add line is printed too.

The key (32 random bytes, kept in ~/.mcp-browser-extension/remote-key) guards every request; a wrong or missing key gets a bare 404. Anyone holding the URL can drive your browser within the allowed sites, so keep it private and keep the allowlist tight. --rotate-remote-key cuts every client off. The quick-tunnel URL changes on each start; --http alone serves on 127.0.0.1:38018 (--http-port) for your own tunnel (ngrok, Tailscale Funnel, a reverse proxy). The server runs until Ctrl-C.

A permanent address and phone pairing — --shared-relay (invite only)

npx -y mcp-browser-extension@latest --shared-relay --allow-domain example.com --enable-mutations

--shared-relay links this computer to the shared relay on mcp-browser-extension.com and gives it a permanent private address, https://mcp-browser-extension.com/remote/c/<id>, that also serves phone pairing (Phone access in the extension popup). It needs a verified email that has been invited; until then the server logs that the email is not invited and retries every ten minutes. The relay only carries bytes: the remote key, app approval and site approval are still checked on your computer. Each invited computer is limited to 240 requests a minute and 2 GiB a day. --relay <url> links to a relay server you run yourself instead.

Proxy per Chrome profile — proxy_status

Chrome applies a proxy to a whole profile, never one tab, so keep a Chrome profile per IP: set the proxy on the extension's Settings page in that profile, name the profile there, and switch with profile_use. A dead proxy fails pages rather than falling back to your real IP. proxy_status reports the exit IP and country; the agent cannot change the proxy or read its password.

Driving several tabs at once — batch

batch runs many tool calls in one request — parallel (default) or serial (with optional stopOnError). Each sub-op goes through the same policy gate, rate limit, and error handling as a direct call (no bypass, no nesting). Use it to fan work out across tabs:

// open three product pages (background, so they don't fight for focus)…{ "name": "batch", "arguments": { "ops": [  { "tool": "tab_new", "args": { "url": "https://a.example/p" } },  { "tool": "tab_new", "args": { "url": "https://b.example/p" } },  { "tool": "tab_new", "args": { "url": "https://c.example/p" } }]}}
// …then read them all at once (wall-clock ≈ the slowest one, not the sum){ "name": "batch", "arguments": { "ops": [  { "tool": "get_text", "args": { "tabId": "<a tabId>" } },  { "tool": "get_text", "args": { "tabId": "<b tabId>" } },  { "tool": "get_text", "args": { "tabId": "<c tabId>" } }]}}

In parallel mode, tab-scoped ops must pass an explicit tabId — the active-tab default is unsafe under concurrency, so it's rejected rather than silently mis-routed. (tab_new, tabs_list, chrome_status are exempt.)

The agent works in its own tab, not yours. tab_new opens in the background and that tab becomes the agent's tab: calls without a tabId go there. navigate with no tab of its own yet opens one rather than replacing your page, and tab_new only reuses blank tabs it opened itself, never a New Tab page of yours. Before the agent opens anything, reads see your active tab, so "summarize this page" still works; to act on your page, the agent passes its tabId from tabs_list. Pass active: true to bring a tab to the front.

Reaching into iframes and shadow roots

A selector that "should" match but doesn't almost always means the element is somewhere your selector cannot reach: inside an <iframe> (checkout widgets, OAuth consent screens, embedded editors) or inside a web component's shadow root.

Shadow roots are handled for you — every selector and every ref now resolves through open shadow roots, so anything snapshot shows you is something you can click. (It used to show you elements no click could reach: the snapshot walked shadow roots, the actions did not.)

Frames are opt-in, because reaching into one is a decision:

frames_list {}                                  // what frames exist, and their URLsclick { "selector": "#pay", "allFrames": true } // find it in whichever frame has itget_text { "frameId": 7 }                       // pin one frame

Every frame is authorized against its own URL before anything runs in it, so an allowlisted page embedding a third-party iframe does not become a way to read that third party. Frames whose origin isn't on your allowlist are skipped.

Seeing why a page broke — console_logs, network_log, dialogs

Reading the DOM tells you what a page looks like after it failed, not why. With --enable-observers, an in-page hook records console output, uncaught errors, and fetch/XMLHttpRequest traffic, and intercepts native dialogs:

console_logs { "level": "exception" }    // the uncaught error behind a broken pagenetwork_log  { "failedOnly": true }      // the 500 behind the blank screendialogs      { "policy": "accept" }      // answer confirm() with true from here on

It is off by default and deliberately so: the hook patches console, fetch, XMLHttpRequest and the dialog functions on every allowlisted page in your real browser. When it's on, it is registered only for the domains on your allowlist, at document_start (so it catches load-time failures), and nothing it records leaves the page until a tool call reads it — through the same gate as any other page read.

Dialog interception is also a fix, not just an observation: alert/confirm/ beforeunload block the renderer, so a click that opened one used to hang every injected script until the command timed out and reported TIMEOUT with nothing to point at. With observers on, the dialog is answered (dismiss by default: confirm → false, prompt → null) and recorded.

What network_log sees: the requests page code makes — fetch and XMLHttpRequest, with method, URL, status and duration — plus Resource Timing entries (scripts, images, styles) when you ask for them. Not the document request, redirects, or headers. That is the cost of not holding a debugger session open on your browser.

Only what changed — snapshot { diff: true }

A snapshot is the most expensive read in the tool surface, and the loop that uses it most (snapshot → click → snapshot) re-sends a page that is mostly identical every time. Ask for the delta instead:

snapshot { "diff": true }                       // added / removed / changed onlyclick { "selector": "#save", "snapshotAfter": true }   // what the click changed

Nodes are matched across snapshots by role + accessible name, not by ref — refs renumber in document order on every snapshot, so diffing on them would report an unchanged button as removed-and-re-added the moment anything above it appears.

Remember a page and catch changes later — page_watch

Save a named accessibility checkpoint, then compare the current page against it in a later session. Checkpoints live under the active Chrome profile, so work and personal browser identities stay separate:

page_watch { "action": "create", "name": "Vendor pricing" }page_watch { "action": "compare", "watchId": "watch_…" }page_watch { "action": "list" }page_watch { "action": "delete", "watchId": "watch_…" }

The result shows added, removed and changed controls. It stores accessibility labels and control state, strips URL query strings, applies configured redaction, and never saves live form values. Creating or comparing a checkpoint still requires the page's domain to be allowed.

Targeting by role and name

Actions accept a locator instead of a CSS selector, so you don't need a snapshot first just to learn a ref:

click { "role": "button", "name": "Sign in" }type  { "role": "textbox", "name": "Email", "text": "[email protected]" }

Resolution is server-side and refuses to guess: an ambiguous locator fails with the candidates listed rather than acting on the first one (pass nth to pick).

Did the session expire? — auth_check and failOnAuthWall

Reusing a signed-in Chrome removes the login step, but a session cookie can still expire mid-run. Without a distinct signal the next step fails as SELECTOR_NOT_FOUND or TIMEOUT, and an eval harness scores the run as an agent failure when it was an auth failure. Every snapshot now carries an authWall verdict when the page looks like a sign-in wall, and there is a dedicated probe:

auth_check {}                                   // { authRequired, confidence, signals }auth_check { "failOnAuthWall": true }           // [AUTH_REQUIRED] error insteadnavigate  { "url": "https://app.example.com/dashboard", "failOnAuthWall": true }snapshot  { "failOnAuthWall": true }

For a harness, set it once instead of per call:

npx -y mcp-browser-extension@latest --allow-domain app.example.com --enable-mutations --fail-on-auth-wall

With the flag on, every step that can move the tab (navigate, click, type, select_option, press, fill_form, back, forward, reload) checks the page it landed on and fails with [AUTH_REQUIRED] if that page is a sign-in wall, and a wait_for that times out on such a page reports [AUTH_REQUIRED] instead of [TIMEOUT]. Each guarded step costs one extra snapshot round-trip; with the flag off the cost is zero. [AUTH_REQUIRED] is where a harness pauses for a human to sign in again in the same Chrome, then retries the step. MCP Browser Extension never re-authenticates on its own: it holds no credentials, by design.

Detection reads only what the snapshot already has: the URL (sign-in routes, identity-provider hosts such as accounts.google.com, login.microsoftonline.com, Okta, Auth0), the title, password fields, and sign-in controls. high confidence needs two independent cues (a password field plus a sign-in button, say); a lone password field or a bare /auth/... URL is medium. A header "Sign in" link on an ordinary page never counts. failOnAuthWall fires only on high, so a harness can bucket [AUTH_REQUIRED] separately from every other failure while a settings page with a "current password" field carries on.

Printing — print_pdf

print_pdf { "landscape": true }

Renders through Chrome's own print pipeline and saves to the task's results/ dir, returning the path and size. The bytes themselves are never returned — a PDF is megabytes of base64 no model can read.

Paying less per turn — --tools

Every MCP server you connect costs context before you ask it anything: the host sends the whole tool catalog to the model on every turn. MCP Browser Extension's 43 tools are 28 KB of JSON Schema, about 7.1k tokens, on each one.

Most runs need a handful of them. --tools advertises only those:

npx -y mcp-browser-extension@latest \  --allow-domain app.example.com --enable-mutations \  --tools tabs_list,tab_new,navigate,snapshot,click,type,get_text

That surface is 6.0 KB, ~1.5k tokens — a 78% cut against the full catalog, for a run that was never going to print a PDF or upload a file.

• Comma-separated and repeatable: --tools navigate,get_text --tools click.
• A tool left out is hidden from tools/list and refused if called — a batch op naming it fails the same way an unknown tool does. Hiding a tool is a real restriction, not a display filter. It is not a substitute for the policy gate, though: --tools eval still does nothing without --unsafe-enable-eval.
• An unknown name fails at startup and prints the catalog, so a typo can never quietly drop click from the surface.
• mcp-browser-extension --help prints the full catalog of 43 names to pick from.

Status

v0.5.0 — safe multi-tab concurrency. Adds the batch fan-out tool, makes parallel tab automation race-free (explicit-tabId guard; per-tab chrome.debugger serialization; collision-free tab_new), captures screenshots via chrome.debugger (a specific tab without stealing focus — plus true full-page and element capture), and focuses newly opened tabs by default. 111 automated tests + a gated headed extension smoke.

v0.2.0 — all six build phases complete and green. End-to-end working: npx mcp-browser-extension ⇄ bridge ⇄ extension ⇄ your real Chrome, with a Playwright CDP fallback. v0.2 adds the accessibility snapshot + element refs, auto-wait, cookies/storage/select_option, trusted input (chrome.debugger), a toolbar status badge, and a stable pairing token (--persist-token).

• Phase 0 — Contracts & skeleton: shared/protocol.ts (wire contract), src/executor/types.ts (Executor interface), src/security/policy.ts (default-deny policy + capability gates), src/config.ts (CLI/env/policy resolution), build + test harness.
• Phase 1 — MCP server + StubExecutor: mcp/server.ts (clean-stdout stdio), mcp/tools.ts (28-tool catalog + never-throw dispatch + drift-check), validators/envelopes/helpers, ExecutorManager + StubExecutor, cli.ts. Point an MCP host at node dist/src/cli.js today.
• Phase 2 — WebSocket bridge + auth: bridge/server.ts (loopback WS, hello-token gate, welcome/unauthorized, displacement), bridge/auth.ts (per-boot 256-bit token, atomic-0600 handshake, SHA-256 timingSafeEqual), bridge/connection.ts (id-correlation, method-aware timeouts, backpressure, reject-all-on-close, heartbeat).
• Phase 3 — ExtensionExecutor + CdpExecutor + selection: executor/extension-executor.ts (Executor over the bridge), executor/cdp-executor.ts (Playwright connect/launch + lock recovery + tab resolution), executor/select.ts (extension-if-ping-responsive else CDP). CLI now starts the bridge, writes the 0600 handshake, and serves a real backend. Adds playwright.
• Phase 4 — MV3 extension: extension/ — manifest.json, sw/ws-client.ts (dial + hello/welcome + pong), sw/executor.ts (chrome.scripting/chrome.tabs command impls), sw/router.ts (never-throw + drift), sw/background.ts (top-level listeners + 25s keepalive/reconnect), options page (manual pairing), esbuild build → extension-dist/. Verified by a live --load-extension smoke (pair → navigate → get_text). Adds esbuild + @types/chrome.
• Phase 5 — Helpers, downloads, HITL: hardened download_file (shared/download.ts — path-traversal/dangerous-ext sanitize + size cap, wired into both backends), richer read_as_markdown, and a human-in-the-loop harness (hitl/ — npm run test:hitl [-- --include-mutating]) with pure, unit-tested gating. 50 automated tests.
• Phase 6 — Packaging & docs: files whitelist (ships dist/src, dist/shared, extension-dist, LICENSE, blueprint — not source/tests), prepack build, bin, quickstart + .mcp.json snippet. Verified by a tarball install smoke (npm pack → install → MCP tools/list).

Security posture (default)

Access requires consent. The server starts with an empty named-site allowlist; the extension's default suspicious-site mode requires an explicit Chrome host grant before ordinary sites work automatically. Eval, downloads and mutations stay off until enabled. Named-site policy remains available:

mcp-browser-extension --no-site-grants --allow-domain example.com --enable-mutationsmcp-browser-extension --policy ./policy.json          # see policy.example.jsonmcp-browser-extension --enable-observers              # console/network/dialog capture (patches page globals)mcp-browser-extension --redact                        # scrub secret-shaped strings out of page reads

Named-site mode: allowing a site without a restart. When a call is refused because its site is not allowed, the extension's toolbar icon shows a ?. Click it: the popup lists the blocked site with an Allow button, shows whether the site you are on is allowed (with its own Allow), and lists the allowed sites. The Options page (Settings in the popup) also has a box to allow any other site. An allowed site applies at once to every session sharing that Chrome, and is saved to ~/.mcp-browser-extension/allowed-sites.json so it survives restarts. Sites allowed there can be removed there; sites from --allow-domain stay until the flags change. Catch-all patterns do not grant access. Remove the retired --unsafe-all-domains option from older configurations before starting. The agent cannot approve a site itself: MCP Browser Extension never reads or drives the extension's own pages. Run with --no-site-grants to keep the allowlist exactly what the flags say.

What comes back is gated too. The allowlist decides which pages may be read; it says nothing about what is on them. A logged-in page routinely renders a session token into a script tag or an API key onto a settings screen.

• Cookie values are hidden by default. get_cookies returns each cookie's name, domain, path and flags with the value replaced by [hidden]. A session cookie can grant access to an account without its password. Pass --enable-cookie-values if a task really needs them.
• Password field values are always suppressed — in get_html, and in snapshot, where the field still appears (so you can type into it) flagged secret: true with no value. No flag, no opt-in: nobody wants those characters.
• --redact additionally scrubs secret-shaped strings — JWTs, AWS/GitHub/Slack/ Google keys, Bearer headers, private-key blocks — out of get_text, get_html, read_as_markdown and eval. It is opt-in because a pattern will eventually fire on something you actually wanted. --redact-pattern <regex> adds your own (and implies --redact); an invalid one fails at startup rather than silently never matching.
• Redaction runs before the output cap, so a truncated read cannot leak what a full one would have hidden.

Every call is recorded to the task's history.jsonl with the URL it touched, the policy verdict (allowed/denied), how long it took, how many bytes came back, and how many secrets were scrubbed — so "what did the agent do in my browser" has an answer after the fact.

The per-boot 256-bit token in ~/.mcp-browser-extension/handshake.json (mode 0600) is the only trust boundary; it is never written to stdout/stderr. On POSIX the mode is re-verified after every write and the server fails closed if the file ends up group/other-accessible. Windows has no such bits — chmod there only toggles the read-only attribute — so the check is skipped and the token's confidentiality rests on the per-user ACL of %USERPROFILE%\.mcp-browser-extension.

Updates

You don't have to do anything to stay current. The setup snippets above use @latest, and on top of that an installed copy (npx or a global install) checks npm when it starts: if a newer version is published, it starts that version in its place, with the same arguments, so an MCP config written months ago still runs the newest release. The check gives up after 1.5 s when you are offline, and a copy run from a git checkout never updates itself. The new server also refreshes the unpacked extension in ~/mcp-browser-extension, which reloads itself; the Chrome Web Store copy updates through Chrome as usual. Turn it off with --no-auto-update or MCP_BROWSER_EXTENSION_AUTO_UPDATE=0.

Telemetry

The MCP Browser Extension server (not the extension) sends anonymous usage statistics to PostHog, so the project can see how many installs are active, which versions and platforms are in use, and which tools fail most. A notice is printed the first time it runs.

What is sent: a random install id (kept in ~/.mcp-browser-extension/telemetry.json), the server version, OS, CPU architecture and Node major version, whether the session owns the bridge port or shares it, how many browsers are paired, the paired extension's version and whether it is the store or an unpacked copy, and per-tool call and error counts with error codes. The first batch goes out a minute after the first tool call, then every 10 minutes. For PostHog's MCP Analytics view, each tool call is also sent as one $mcp_tool_call event carrying only the tool name and its group (reading, navigation...), its duration, whether it failed and the error code, the MCP client's name and version (such as claude-code), the MCP protocol revision, the AI model when the client names it in its own request metadata (Codex does), and a random session id that changes after 30 idle minutes. When a client connects, one event records that handshake and the names of the tools offered. Nothing an agent sends or reads: no arguments, results, error messages or intents. Batched every 15 seconds. One pair_check event a minute after startup says whether a browser paired and, if not, which of a few fixed reasons applies (no_extension, token_mismatch, version_mismatch, profile_mismatch), so setup problems can be fixed where they happen.

What is never sent: URLs, domains, tool arguments, page content, screenshots, cookies, profile names, tokens, file paths, or anything you type. Events are personless and GeoIP lookup is disabled.

The browser extension sends nothing — it only ever talks to 127.0.0.1.

Turn it off with any of:

MCP_BROWSER_EXTENSION_TELEMETRY=0     # or false / offDO_NOT_TRACK=1--no-telemetry             # server flag

Learn more

The full documentation, setup guides for each MCP client and the security model live on the website: MCP Browser Extension docs.

• What an MCP browser extension is, and when you need one
• Use Claude in your signed-in Chrome
• Give Claude Code browser access
• Give Cursor a real browser with MCP
• Let VS Code Copilot use your real Chrome
• Test a login flow end to end with an AI agent
• Check a release page through a browser MCP tunnel
• Chrome DevTools MCP vs Playwright MCP vs an MCP extension
• Best browser MCP servers in 2026
• How it compares with other browser MCP servers

Develop

npm installnpm run typecheck       # server/test sourcesnpm run typecheck:ext   # extension sources (@types/chrome)npm run build:ext       # bundle the extension → extension-dist/npm test                # builds, then runs node --test on dist/testRUN_EXT_SMOKE=1 node --test dist/test/extension-smoke.test.js   # live, headed

The extension

Published on the Chrome Web Store as MCP Browser Extension. Extension versions move only when extension/ changes, so the listed build can sit a release behind the npm package; the two negotiate capabilities on connect, so an older extension loses features rather than breaking.

For normal use, install from the Chrome Web Store above. The following is only for local development; do not load this unpacked copy in the same Chrome profile as the Store extension. On Windows, Chrome may disable a non-Store build.

extension/ builds (esbuild) to extension-dist/, loaded via chrome://extensions → Load unpacked → select ~/mcp-browser-extension, the mirror the server refreshes from extension-dist/ on every boot (loading extension-dist/ directly also works). It pairs itself from the pairing.json the server writes into that folder; the Options page paste of port + token from ~/.mcp-browser-extension/handshake.json (run npx -y mcp-browser-extension@latest --print-pairing to get the path) is only the fallback.

Reads/interaction use chrome.scripting/chrome.tabs — no "is being debugged" banner, CSP-safe reads (isolated world), testable under Playwright. chrome.debugger is used only where it's needed and worth it: trusted: true input (real OS-level events on React/Vue inputs) and screenshot (captures a specific tab without activating it — safe under parallel batch — with true full-page and element capture). Those ops show the debug banner while attached; the session lingers 1.5s after the last op so a burst of them attaches once. Screenshots are JPEG (quality 70) at CSS-pixel size by default — pass format: "png", quality, or scale to change that.