mcp-fw 0.2.3

MCP Firewall Proxy — policy-based tool access control using NAIL effects

mcp-fw

MCP servers can read your files, run processes, and access the network — all without asking.

mcp-fw is a firewall proxy that sits between Claude Desktop and MCP servers. It uses NAIL effect labels to control exactly what each server can do.

Claude Desktop  ←→  mcp-fw (proxy)  ←→  MCP Server
                     ↑
                  policy.yaml
                  allow: [FS, IO]
                  deny:  [NET]

Why

MCP servers are powerful — but there's no built-in permission model. A filesystem server could silently make network calls. An "everything" server could spawn processes.

mcp-fw enforces boundaries:

Effect	What it controls	Example
FS	File read/write	Read a file, list directory
IO	General I/O	stdin/stdout, echo
NET	Network access	HTTP requests, DNS
PROC	Process execution	Spawn subprocess, exec
TIME	Time/clock access	Get current time
RAND	Randomness	Generate random numbers

Quick Start

pip install mcp-fw

1. Write a policy

# policy.yaml
servers:
  filesystem:
    command: npx
    args: ["@modelcontextprotocol/server-filesystem", "/tmp"]
    allow: [FS, IO]
    deny: [NET]

This lets the filesystem server read/write files (FS, IO) but blocks all network access (NET).

2. Point Claude Desktop to the proxy

In claude_desktop_config.json:

{
  "mcpServers": {
    "filesystem-fw": {
      "command": "mcp-fw",
      "args": ["--config", "/path/to/policy.yaml", "--server", "filesystem"]
    }
  }
}

That's it. Claude Desktop now talks to mcp-fw, which filters tools before forwarding to the real server.

3. See what happened

$ mcp-fw --config policy.yaml --server filesystem --verbose
2025-01-15 10:00:01 [INFO] Filtered 5 → 3 tools (allowed effects: ['FS', 'IO'])
2025-01-15 10:00:05 [WARNING] Blocked tool call: fetch_url

Menubar App (macOS)

For a GUI experience:

pip install "mcp-fw[menubar]"
mcp-fw-menubar --config policy.yaml

A [FW] icon appears in your menubar:

[FW]
├── mcp-fw v0.1.0
├── ────────
├── ● filesystem
│   ├── Status: Running
│   ├── ────────
│   ├── [x] FS
│   ├── [x] IO
│   ├── [ ] NET          ← toggle effects live
│   ├── [ ] PROC
│   └── ...
├── ○ everything
├── ────────
├── Edit Policy YAML
├── View Logs...
├── ────────
├── Sync to Claude Desktop
└── Quit

• Toggle effects with checkboxes — changes are written to policy.yaml instantly
• Sync to Claude Desktop generates claude_desktop_config.json entries automatically
• View Logs opens a live log viewer
• Process monitor shows ● running / ○ stopped status per server

How It Works

1. Claude calls list_tools()
   → mcp-fw forwards to the real server
   → NAIL annotates each tool with effect labels
   → mcp-fw filters out tools that violate the policy
   → Claude only sees allowed tools

2. Claude calls a tool
   → mcp-fw checks if it's in the allowed set
   → Allowed: forward to server
   → Blocked: return error

The key insight: tools aren't just allowed or denied by name — they're classified by what they do (filesystem, network, process, etc.) using NAIL's effect system. This means mcp-fw can enforce policies on servers it has never seen before.

Policy Reference

servers:
  my-server:
    command: npx                    # server command
    args: ["@org/server", "/tmp"]   # command arguments
    allow: [FS, IO]                 # permitted effects (empty = all)
    deny: [NET]                     # blocked effects (overrides allow)
    tool_overrides:                 # per-tool effect corrections
      safe_fetch: [IO]             # override NAIL's auto-detection

Rules:

• allow: [] (empty) = all effects permitted, then deny subtracts
• allow: [FS, IO] = only these effects, then deny subtracts
• deny always wins over allow
• tool_overrides lets you correct NAIL's automatic effect detection for specific tools

License

MIT