Nullprint

nullprint-mcp

An MCP server for Nullprint, the multi-profile browser. It lets any MCP-capable AI agent (Claude Code, Claude Desktop, OpenAI Codex, Kimi, …) create browser profiles, launch them, run consistency check-ups and drive the pages inside a running profile.

The server is a thin layer over the local Nullprint daemon's HTTP API: every tool maps to one daemon endpoint. The daemon, the browser kernels and the fingerprint engine ship with the Nullprint desktop app and are not part of this package.

Requirements

• Nullprint desktop app installed and running (Windows or macOS), signed in. Download: https://nullprint.ai
• Python 3.11+ on the same machine. The easiest way to run the server is uv, which installs the package on first use.

Install

# Claude Code
claude mcp add nullprint -- uvx nullprint-mcp

# OpenAI Codex
codex mcp add nullprint -- uvx nullprint-mcp

Claude Desktop, Kimi and other clients that use an mcpServers config file:

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

Without uv: pip install nullprint-mcp, then use nullprint-mcp as the command (no args).

No port or key to configure: the server reads the running daemon's port and API key from ~/.nullprint/daemon.json, which the desktop app writes on start-up. To point at a daemon elsewhere, pass --daemon-url http://host:port (or --port 7801 for localhost) and set NULLPRINT_API_KEY. Resolution order, most explicit first:

1. --daemon-url / --port
2. NULLPRINT_DAEMON_URL
3. NULLPRINT_PORT
4. ~/.nullprint/daemon.json (NULLPRINT_HOME changes the directory)

If the daemon is not running the tools say so plainly instead of throwing a connection trace.

Tools

Notes that matter when writing prompts:

• session_* tools need an open session. They drive the warm session opened by profile_start; calling them on a profile without one returns 409. Selectors are CSS, not XPath or role/name; call session_snapshot first when you do not know a selector.
• Dialogs are left open, as in a normal browser. While an alert / confirm / prompt is open the page is blocked and session_* tools fail with 409 naming the dialog; answer it with session_dialog, then retry. A beforeunload caused by your own session_goto or click is accepted automatically.
• session_eval returns an envelope. A page-side JavaScript error comes back as data ({ok: false, error, error_type}), not as a tool failure; result follows the expression's type.
• profile_stop has two levels. A profile can be "running" as a warm session or as a window launched from the app. The tool tries the session first and falls back to the app-level stop, which is idempotent for idle profiles. It returns {name, stopped, closed}.
• profile_create does not hard-code a kernel. Leave engine empty to use the app's default; unknown values come back as a 422 listing the options.
• proxy_check failures are data. An unreachable proxy returns {ok: false, error}; only an unreachable daemon raises.
• profile_start returns cdp_endpoint (a ws:// DevTools address) for Chrome kernels with external debugging turned on in the app (off by default; Edit → Advanced → External debugging, needs a second verification, then reopen the profile), so external tools can attach. Otherwise it is null, with cdp_unavailable (reason code) and cdp_note saying why. A session driven from outside bypasses Nullprint's consistency guards: an outside tool can set a viewport, language or time zone that contradicts the profile's frozen fingerprint and make it detectable. Prefer the session_* tools; if you must attach, do not change anything the profile has not declared.

What is deliberately not exposed

Permanently deleting a profile (DELETE /trash/{name}) destroys its logins and cannot be undone. It is not a tool on purpose: emptying the recycle bin is a human action in the desktop app. profile_delete's description tells the agent to say so instead of working around it, and a test scans every tool's source to keep the purge endpoint out. Screenshot-to-disk and file-writing tools are left out for the same reason (local path safety).

Development

pip install -e ".[dev]"
pytest tests
nullprint-mcp --help

Tools live in nullprint_mcp/tools.py (the TOOLS tuple); server.py and __main__.py do not change when a tool is added. Nothing may be printed to stdout: stdout is the MCP channel.

License

MIT. Copyright (c) 2026 HENGCHENG LLC.

中文说明

这是 Nullprint（多身份指纹浏览器）的 MCP 服务：Claude Code、Claude Desktop、OpenAI Codex、Kimi 等任何支持 MCP 的 AI 都可以用它建身份、启动浏览器、做一致性体检、驱动身份里的网页。它只是本地 daemon HTTP 接口上的薄薄一层，每个工具对应 daemon 的一个端点；daemon、浏览器内核和指纹引擎随 Nullprint 桌面应用发布，不在本包内。

前提：本机装好并登录 Nullprint 桌面应用；本机有 Python 3.11+，推荐用 uv（首次运行自动装包）。

接入：

claude mcp add nullprint -- uvx nullprint-mcp      # Claude Code
codex mcp add nullprint -- uvx nullprint-mcp       # OpenAI Codex

Claude Desktop、Kimi 等走配置文件的客户端，在 mcpServers 里加 {"command": "uvx", "args": ["nullprint-mcp"]}。没有 uv 就 pip install nullprint-mcp，命令改为 nullprint-mcp。

不用配端口和密钥：服务自动读桌面应用写在 ~/.nullprint/daemon.json 里的端口和 API Key。要连别的机器上的 daemon，传 --daemon-url 并设 NULLPRINT_API_KEY。

18 个工具：profiles_list、profile_start、profile_stop、profile_create、profile_settings、profile_checkup、proxy_check、profile_delete、trash_list、profile_restore，以及 8 个会话工具 session_goto / snapshot / click_selector / fill_selector / press / eval / tabs / switch_tab（先 profile_start 再用）。彻底删除身份故意不做成工具：清空回收站只能在桌面应用里由人操作。