browser-harness MCP

One MCP server named browser-harness for your real browser — plus a Gmail send flow.

• Core browser control over CDP (new_tab, page_info, cdp, js, …) via the browser-harness daemon — no second CDP layer.

• Gmail flow: open login → compose → attach PDF → send → verify in Sent.

Single file: server.py. Stdio transport.

Layout

browser-harness-mcp/
  server.py        # the MCP server (37 tools)
  SKILL.md         # agent workflow (single-tab default, memory, plans)
  pyproject.toml   # package + deps (backend included)
  README.md        # this file
  LICENSE        # MIT

Related MCP server: chrome-mcp

Install — ONE command

pip install git+https://github.com/vaibhxvvy/browser-harness-mcp

That's it. The browser backend (browser-harness, mcp, pillow) installs automatically in the same command — there is nothing else to install, no second MCP, no separate doctor CLI. Then check health from inside any MCP client with the browser_doctor tool.

From a local clone (contributors):

pip install -e .

Needs Chrome/Brave/Edge with remote debugging (port 9222). The daemon auto-starts on first tool call.

MCP config — ONE entry

After the install command (no python path needed):

{
  "mcpServers": {
    "browser-harness": { "command": "browser-harness-mcp" }
  }
}

Local-dev alternative (run from source without installing):

{
  "mcpServers": {
    "browser-harness": {
      "command": "python",
      "args": ["<clone-path>/browser-harness-mcp/server.py"]
    }
  }
}

Only ONE mcpServers entry — this server is the whole thing.

Tools

Core browser (32):

Gmail flow (5, all in the current tab):

Every tool returns JSON text. Failures return {"error": "..."} — never raises.

Parallel tabs (explicit only)

Default is ONE tab: single-tab tasks must never open extra tabs — navigate the current tab.

Only when the request is genuinely multi-task ("do these things at the same time"):

browser_task_tabs(urls=[...]) → [{index, targetId, url}]
browser_switch_tab(target_A) → act on A → browser_switch_tab(target_B) → act on B

Every tool call runs atomically under a server-side lock, so switched work can't interleave mid-flight. Execution across tabs is sequential, not simultaneous — the backend holds a single session, and true concurrency would land actions in wrong tabs.

Usage (Gmail end-to-end)

gmail_open_login → sign in once in the debug Chrome window
gmail_compose(compose_url="https://mail.google.com/mail/?view=cm&to=someone@example.com")
gmail_attach(pdf_path="/path/to/resume.pdf")
gmail_click_send() → {"sent": false, "needs_confirm": true, "preview": {...}}
gmail_click_send(confirm=True) → {"sent": true}
gmail_check_sent(recipient="someone@example.com", subject="Application for Prompt Engineer")

Usage (low-level)

browser_new_tab("https://mail.google.com/")
browser_page_info()
browser_js("() => document.title")
browser_screenshot()

Notes

• Backend (browser_harness daemon) is a declared dependency — the ONE install command pulls it, no separate install.

• Dead browser fails fast (~1s) instead of hanging 30s.

• Vision is native: screenshot/see return image blocks the driving model sees directly (~1–2k tokens a shot, capped at 1280px). BH_VISION_API_KEY is only a fallback for text-only drivers.

• Tool calls are lock-atomic: explicit multi-tab work can't interleave.

• Read-only tools retry once on transient IPC blips; clicking/typing/sending never retry (no double side effects).

• gmail_click_send sends nothing without confirm=True — first call returns a draft preview.

• This package owns the tab marker (🥸, applied on tab-attach tools, legacy horse stripped once). Backend marking is disabled via BH_TAB_MARKER=0; reload an old daemon once to pick that up.

• If the backend is ever missing, tools return {"error": "...run ONE command..."} instead of crashing.

• stdout is redirected to stderr inside tools so MCP stdio never corrupts.

• NaN/Infinity/bytes/dates are normalized before JSON serialization.

• Gmail Send clicks the AX Send button, falls back to div[role=button] JS click.

License

MIT — see LICENSE.