mcp-snoop

Transparent stdio interceptor for MCP JSON-RPC traffic.

Like strace, but for the Model Context Protocol. Wrap any MCP server and see every message in real time — tool calls, responses, errors. Zero dependencies.

$ mcp-snoop -- python3 my_server.py
mcp-snoop intercepting: python3 my_server.py
[03:14:22.847] →SERVER #1 initialize claude-desktop v1.0
[03:14:22.912] ←CLIENT #1 → server: my-server v0.1.0
[03:14:22.913] →SERVER notifications/initialized
[03:14:22.914] →SERVER #2 list tools
[03:14:22.916] ←CLIENT #2 → 8 tools
[03:14:23.201] →SERVER #3 call search_files(path="/tmp", pattern="*.py")
[03:14:23.847] ←CLIENT #3 → text: "['server.py', 'handler.py']"

mcp-snoop: 7 messages intercepted

Install

pip install mcp-snoop

Or run without installing:

uvx mcp-snoop -- python3 my_server.py

Usage

# Basic — logs all messages to stderr
mcp-snoop -- python3 my_server.py

# Verbose — print full JSON for each message
mcp-snoop --verbose -- node server.js

# Save trace to file
mcp-snoop --output trace.log -- uvx my-mcp-server

# No color (for CI/log files)
mcp-snoop --no-color -- python3 server.py

What you see

Each log line shows:

• timestamp — HH:MM:SS.mmm UTC
• direction — →SERVER (client→server) or ←CLIENT (server→client)
• message ID — #1, #2, etc. (pairs requests with responses)
• summary — parsed tool name, arguments, result preview

With --verbose, the full JSON follows.

Why

You're building an MCP server. Your agent keeps calling the wrong tool. Something's returning garbage. The MCP Inspector requires a browser. You want to script against the logs.

mcp-snoop gives you the raw protocol traffic, right in your terminal.

Use in Claude Desktop / cline / any MCP client

Replace your server command with mcp-snoop -- <your command>:

{
  "mcpServers": {
    "my-server": {
      "command": "mcp-snoop",
      "args": ["--output", "/tmp/trace.log", "--", "python3", "my_server.py"]
    }
  }
}

All traffic between the client and your server is now logged to /tmp/trace.log.

Relationship to other tools

Tool	What it does
agent-friend	Grades MCP schema quality (A+ to F)
mcp-patch	AST security scanner for MCP servers
mcp-pytest	pytest integration for testing MCP servers
mcp-snoop	Stdio interceptor — debug live protocol traffic

License

MIT