Imported from reason-machines/mcp-skills (skills/mcpsnoop-mcp-debugging/SKILL.md). Install upstream with npx skills add reason-machines/mcp-skills --skill mcpsnoop-mcp-debugging. Copyright stays with the author.

mcpsnoop MCP Debugging Skill

Skill by ara.so — MCP Skills collection.

mcpsnoop is a transparent proxy for Model Context Protocol (MCP) that sits between your AI client (Claude Desktop, Cursor, Claude Code) and your MCP servers. Unlike the official MCP Inspector which connects as a separate client, mcpsnoop intercepts the actual traffic between your real client and server, showing every JSON-RPC frame live in your terminal.

Installation

Using Go

go install github.com/kerlenton/mcpsnoop/cmd/mcpsnoop@latest

Using Homebrew

brew tap kerlenton/mcpsnoop
brew install mcpsnoop

Pre-built Binaries

Download from the Releases page for your platform.

Core Concepts

mcpsnoop operates in two modes:

1. Shim mode (mcpsnoop -- <server-command>): Acts as a transparent proxy between client and server
2. UI mode (mcpsnoop): Displays the captured traffic in an interactive TUI

The shim and UI communicate via a well-known socket and on-disk logs, so they can start in any order.

Configuration

Stdio Transport (Most Common)

Wrap your server command in your MCP client's configuration file. For Claude Desktop, edit ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "my-server": {
      "command": "mcpsnoop",
      "args": ["--", "node", "build/index.js"]
    }
  }
}

Everything after -- is your normal server launch command.

Examples:

Python server:

{
  "mcpServers": {
    "weather": {
      "command": "mcpsnoop",
      "args": ["--", "python", "-m", "weather_server"]
    }
  }
}

TypeScript server with npx:

{
  "mcpServers": {
    "filesystem": {
      "command": "mcpsnoop",
      "args": ["--", "npx", "-y", "@modelcontextprotocol/server-filesystem", "/path/to/allowed"]
    }
  }
}

Compiled binary:

{
  "mcpServers": {
    "custom": {
      "command": "mcpsnoop",
      "args": ["--", "/usr/local/bin/my-mcp-server", "--port", "8080"]
    }
  }
}

HTTP/SSE Transport

For servers using streamable HTTP, run mcpsnoop as a reverse proxy:

mcpsnoop http --target http://localhost:3000/mcp --listen :7000

Then point your client to http://localhost:7000.

Key Commands

CLI Commands

TUI Keybindings

Filtering Traffic

Press / in the TUI to filter frames. Combine tokens with spaces (ANDed):

Filter Syntax

tool:search status:slow           # Slow calls to search tool
dir:s2c kind:req                   # Server-to-client requests
method:tools/call id:123           # Specific method and ID
status:error tool:database         # Failed database tool calls
kind:notif                         # All notifications
dir:c2s status:pending             # Pending client requests

Available Filters

• tool: Filter by tool name (e.g., tool:search, tool:write_file)
• method: JSON-RPC method (e.g., method:tools/call, method:initialize)
• id: Request/response ID
• kind: req (request), resp (response), notif (notification)
• dir: c2s (client-to-server), s2c (server-to-client)
• status: ok, error, slow, pending

Plain text searches match method, tool name, ID, and payload content.

Common Patterns

Debugging a Tool That Isn't Being Called

1. Configure mcpsnoop wrapper in your client
2. Restart the client
3. Open mcpsnoop TUI: mcpsnoop
4. Press c to view capabilities
5. Verify your tool is listed in server capabilities
6. Check if client accepted the capability
7. Use filter: /tool:your_tool_name
8. Attempt to use the tool in your client
9. Look for PENDING status (indicates hung call) or absence of call

Inspecting Capability Negotiation

# Start your client with mcpsnoop wrapper
# Open TUI
mcpsnoop

# Press 'c' to view capabilities screen
# Navigate between:
# - Client capabilities (what the AI client supports)
# - Server capabilities (what your server advertises)
# - Handshake details

Replaying a Tool Call

# In TUI, navigate to a tools/call request
# Press 'r' to replay
# mcpsnoop spawns a fresh isolated server instance
# Executes the exact same call
# Shows the new response

This is useful for:

• Testing fixes without restarting your client
• Comparing responses after code changes
• Debugging non-deterministic behavior

Monitoring Slow Tools

# In TUI
/status:slow

# Shows only slow calls (threshold configurable)
# Each frame shows elapsed time
# PENDING calls show live timer

Debugging Server Stderr

Server stderr appears in the stream with special highlighting:

[STDERR] Warning: database connection slow
[STDERR] Error: timeout connecting to external API

Filter to see only stderr: /kind:notif

Real-World Examples

Example 1: Debugging a File System Server

{
  "mcpServers": {
    "filesystem": {
      "command": "mcpsnoop",
      "args": [
        "--",
        "npx",
        "-y",
        "@modelcontextprotocol/server-filesystem",
        "/Users/dev/projects"
      ]
    }
  }
}

In TUI:

# Filter to file operations
/tool:read_file

# Or see all filesystem tools
/method:tools/call

# Check which directory was authorized
# Press 'c' for capabilities, look for "roots"

Example 2: Monitoring a Database Tool

{
  "mcpServers": {
    "database": {
      "command": "mcpsnoop",
      "args": ["--", "python", "db_server.py"],
      "env": {
        "DATABASE_URL": "postgresql://localhost/mydb"
      }
    }
  }
}

In TUI:

# See only database queries
/tool:query

# Find slow queries
/status:slow tool:query

# Inspect a specific query
# Navigate with j/k, press enter to see full JSON
# Press 'y' to copy query details

Example 3: HTTP Server Debugging

# Terminal 1: Start your HTTP MCP server
python http_server.py --port 3000

# Terminal 2: Start mcpsnoop proxy
mcpsnoop http --target http://localhost:3000/mcp --listen :7000

# Terminal 3: View traffic
mcpsnoop

# Configure client to connect to http://localhost:7000

Example 4: Multi-Server Setup

{
  "mcpServers": {
    "weather": {
      "command": "mcpsnoop",
      "args": ["--", "python", "weather_server.py"]
    },
    "database": {
      "command": "mcpsnoop",
      "args": ["--", "node", "db-server.js"]
    },
    "filesystem": {
      "command": "mcpsnoop",
      "args": ["--", "npx", "-y", "@modelcontextprotocol/server-filesystem", "/home/user"]
    }
  }
}

The TUI shows all servers in separate sessions. Use :sessions command to switch between them.

Troubleshooting

UI Shows "No sessions"

Cause: Shim hasn't started or client hasn't launched the server yet.

Solution:

1. Verify mcpsnoop is in your client config
2. Restart your AI client completely
3. Trigger an MCP action to start the server
4. Wait a few seconds and refresh UI

Tool Call Hangs (Shows PENDING)

Symptoms: Request shows PENDING with increasing timer, no response.

Debug:

1. Check server stderr in the frame list
2. Press enter on the PENDING frame to see full request
3. Look for resource locks or external API timeouts
4. Press r to replay with debug logging

Server Not Starting

Check:

# Test server command directly
node build/index.js  # Or your actual command

# Check mcpsnoop can find the command
which node
which mcpsnoop

# Look at client logs
# Claude Desktop: ~/Library/Logs/Claude/

Capabilities Mismatch

Symptoms: Client doesn't see tool server advertises.

Debug:

1. Press c in TUI
2. Compare server capabilities vs client capabilities
3. Check server's initialize response
4. Verify tool schema matches MCP spec

Frames Not Appearing in TUI

Check:

1. Socket connection: ls -la ~/.config/mcpsnoop/ (Linux/macOS)
2. Log directory: Check for .jsonl files
3. Permissions: Ensure shim can write logs
4. Try mcpsnoop demo to verify TUI works

Replay Fails

Possible causes:

• Server has state dependencies
• External resources unavailable
• Non-idempotent operations

Solution: Check stderr in replay output and compare request parameters.

Advanced Usage

Custom Log Directory

mcpsnoop --log-dir /custom/path

Filtering in Shim Mode

# Only log specific tools (reduces disk usage)
mcpsnoop --filter tool:search -- python server.py

Exporting Sessions

# Sessions are stored as JSONL
cat ~/.config/mcpsnoop/sessions/*.jsonl | jq .

Performance Considerations

• mcpsnoop adds minimal latency (microseconds for copying frames)
• Disk I/O is async and buffered
• Large payloads (>1MB) may slow down TUI rendering
• Use filters to reduce noise in busy systems

Integration with Development Workflow

During Development

# Terminal 1: Your editor/IDE
code .

# Terminal 2: mcpsnoop TUI
mcpsnoop

# Terminal 3: Test client or manual testing
# Make changes to server code
# Restart client or use replay feature to test immediately

Testing Tools

# Capture a successful tool call
# Press 'y' to copy JSON
# Paste into your test suite as expected output

# Replay with modified server code
# Press 'r' repeatedly while iterating

CI/CD Integration

mcpsnoop logs are structured JSONL, suitable for automated testing:

# Run test, capture logs
mcpsnoop --log-dir ./test-logs -- python server.py &
# Run your MCP client test
# Parse logs
jq 'select(.kind == "response" and .status == "error")' test-logs/*.jsonl

Resources

• MCP Specification
• mcpsnoop GitHub
• MCP Inspector (alternative tool)
• Demo Walkthrough