Nexus Browser MCP

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nexus-browser-mcp

A browser-automation MCP server with event-driven, deterministic snapshots.

Built on Playwright. Drives a browser for LLMs through the Accessibility Tree — navigate, click, type, read, fill forms, manage tabs. Key differences from alternatives (e.g. Playwright MCP):

1. Deterministic snapshots: no fixed-interval sleep guessing. A MutationObserver records the last DOM mutation and the browser's own requestAnimationFrame loop decides when the page has been quiet for STABLE_WINDOW_MS (default 800ms) before extracting a snapshot — eliminating "captured mid-animation" races.
2. Built-in governance gates: HITL rules (e.g. clicking "pay/confirm" requires human approval), browser_evaluate disabled by default with unconditional confirmation, JSONL audit log (sensitive-parameter redaction + per-call in/out character metering, so token cost can be reconciled).
3. Multi-task isolation: one MCP connection (session) can host multiple independent task_ids, each with its own BrowserContext (no login-state cross-contamination). Idle tasks are reclaimed by TTL; on next use they're rebuilt and the last page is restored automatically.
4. Death observability + self-healing: if a tab or the whole browser is closed externally or crashes, the next call rebuilds it automatically (a persistent profile keeps your login state) and prepends a [state change] notice telling the agent exactly what was restored and what was lost — no raw Playwright exceptions leak through.
5. Developer observability: every page records console messages, uncaught JS exceptions and network request metadata (method/URL/status/failure reason — never bodies) into capped ring buffers; browser_console / browser_errors / browser_network read them incrementally via a since cursor, so the agent can answer "why did nothing happen" instead of guessing.

Installation

pip install nexus-browser-mcp
# or
uvx nexus-browser-mcp

Two executable entry points are installed: nexus-browser-mcp and nexus-browser. A guaranteed fallback: python -m nexus_browser.server.

Requires playwright and its browser binary:

pip install playwright && playwright install chromium

Integrate (any MCP client)

opencode (~/.config/opencode/opencode.json):

{
  "mcp": {
    "browser": {
      "type": "local",
      "command": ["uvx", "nexus-browser-mcp"],
      "enabled": true
    }
  }
}

Claude Code (.mcp.json, project root):

{
  "mcpServers": {
    "browser": {
      "type": "stdio",
      "command": "uvx",
      "args": ["nexus-browser-mcp"]
    }
  }
}

Pi Coding Agent: reads standard MCP configuration — project .mcp.json or user-global ~/.config/mcp/mcp.json; stdio is the default transport:

{
  "mcpServers": {
    "browser": {
      "command": "uvx",
      "args": ["nexus-browser-mcp"]
    }
  }
}

See docs/INTEGRATE.md for details (Chinese).

Use your own browser (with login state)

By default, isolated mode launches Playwright's bundled Chromium without your cookies/login state. To use your own browser, pick one:

Option A — load your browser profile directly (recommended, simplest)

Use system Chrome with your everyday user data directory (cookies/login/bookmarks included):

BROWSER_CHANNEL=chrome
BROWSER_USER_DATA_DIR="C:\Users\<you>\AppData\Local\Google\Chrome\User Data"

Note: while running against your real User Data, the process owns the browser — launching your own Chrome concurrently will conflict. Prefer a copied profile or a dedicated --user-data-dir.

Recommended: a tool-dedicated profile (no conflict with your daily browser)

Use BROWSER_CHANNEL=chrome plus a dedicated user data dir (e.g. C:\Users\<you>\.nexus-browser\chrome-profile):

BROWSER_CHANNEL=chrome
BROWSER_USER_DATA_DIR="C:\Users\<you>\.nexus-browser\chrome-profile"

On first use, log in to target sites once in the dedicated Chrome window that pops up when the agent calls a browser tool. Cookies persist in that profile forever after — the agent carries login state while staying fully isolated from your daily browser.

Option B — attach to a running Chrome via CDP

Start chrome --remote-debugging-port=9222 first, then set BROWSER_MODE=cdp.

If the CDP connection fails, the server now fails loudly (no silent fallback to a fresh browser) and tells you to start the debug-port browser first.

In CDP (or persistent-profile) mode the agent can also take over tabs you already have open: browser_list_pages shows them under "external tabs", and browser_adopt_page(ext_index) pulls one into the task (snapshot/click/read all work from then on). Adoption always requires confirmed=true — it hands the agent full read/write access to that page, including its login state.

Configuration (environment variables)

Every option can be overridden via BROWSER_-prefixed env vars:

评测/全功能使用：两个高危能力默认关闭（安全优先）。需要 browser_evaluate 时设 BROWSER_ALLOW_JS_EXECUTION=true，需要 browser_network_body 时设 BROWSER_ALLOW_NETWORK_BODY=true——冷启动跑基准/评测不开它们，对应子任务会被拒（这是设计，不是故障）。

Tools