MCP Spy - Debug MCP Communication

Debug Model Context Protocol server integrations.

Overview

MCP Spy helps debug:

• Message traffic between Claude and MCP servers
• Latency issues
• Failed requests
• Protocol compliance

Traffic Analysis

View Recent Traffic

# Check MCP logs
tail -f ~/.claude/debug/mcp-*.log

# Or specific server
tail -f ~/.claude/debug/mcp-cm.log

Filter by Type

# Tool calls only
grep "tool_use" ~/.claude/debug/mcp-*.log

# Errors only
grep -i "error\|failed" ~/.claude/debug/mcp-*.log

# Specific tool
grep "beads_add" ~/.claude/debug/mcp-*.log

Latency Analysis

Measure Response Times

# Time a specific tool
time claude --print "Run beads_ready" --dangerously-skip-permissions 2>&1 | head -1

Find Slow Calls

# Look for latency warnings in logs
grep -i "timeout\|slow\|latency" ~/.claude/debug/mcp-*.log

Failed Request Analysis

Find Failures

# All errors
grep -i "error" ~/.claude/debug/mcp-*.log

# Parse error responses
grep "\"error\":" ~/.claude/debug/mcp-*.log | jq '.'

Common Issues

1. Connection refused - Server not running
2. Timeout - Server too slow
3. Invalid JSON - Malformed request/response
4. Unknown tool - Tool not registered

MCP Server Debug

Check Server Status

# Test server connection
curl -X POST http://localhost:3000/mcp/list_tools \
  -H "Content-Type: application/json" \
  -d '{}'

Server Logs

# View server logs (if running as process)
tail -f logs/mcp-server.log

# Or in terminal running server
# Logs appear in stdout

Test Tool Directly

# Call tool directly
curl -X POST http://localhost:3000/mcp/call_tool \
  -H "Content-Type: application/json" \
  -d '{
    "name": "beads_ready",
    "args": {}
  }'

Protocol Debugging

Inspect Messages

# Pretty print JSON messages
grep "message" ~/.claude/debug/mcp-*.log | jq '.'

Validate Requests

# Check request format
gemini -m pro -o text -e "" "Validate this MCP request format:

$(grep "request" ~/.claude/debug/mcp-*.log | tail -1)

Check:
1. Required fields present
2. Types correct
3. Schema compliance"

Troubleshooting Guide

Server Won't Start

# Check if port in use
lsof -i :3000

# Check server process
ps aux | grep mcp

# Start with verbose
node server/index.ts --verbose

Tool Not Found

# List available tools
curl http://localhost:3000/mcp/list_tools | jq '.tools[].name'

# Check tool registration
grep "registerTool\|toolRegistry" server/*.ts

Slow Responses

# Profile tool execution
time curl -X POST http://localhost:3000/mcp/call_tool \
  -H "Content-Type: application/json" \
  -d '{"name": "slow_tool", "args": {}}'

# Check for blocking operations
grep -i "await\|sync" tools/slow_tool/index.ts

JSON Parse Errors

# Find malformed JSON
grep -B 5 "JSON\|parse" ~/.claude/debug/mcp-*.log | grep -i error

# Validate JSON
echo '{"test": ...}' | jq '.'

Monitoring

Watch Traffic Live

# Real-time log monitoring
tail -f ~/.claude/debug/mcp-*.log | grep --line-buffered "tool_use\|result"

Traffic Stats

# Count calls per tool
grep "tool_use" ~/.claude/debug/mcp-*.log | \
  grep -oP '"name":"[^"]*"' | \
  sort | uniq -c | sort -rn

Health Dashboard

#!/bin/bash
# mcp-health.sh

echo "=== MCP Server Health ==="

# Check server
if curl -s http://localhost:3000/health > /dev/null 2>&1; then
  echo "Server: UP"
else
  echo "Server: DOWN"
fi

# Recent errors
ERRORS=$(grep -c "error" ~/.claude/debug/mcp-*.log 2>/dev/null || echo 0)
echo "Recent errors: $ERRORS"

# Tool count
TOOLS=$(curl -s http://localhost:3000/mcp/list_tools 2>/dev/null | jq '.tools | length' || echo 0)
echo "Registered tools: $TOOLS"

Best Practices

1. Enable logging - Keep debug logs on during development
2. Check health first - Verify server running before debugging
3. Test isolation - Test tools directly before through Claude
4. Monitor latency - Watch for degradation
5. Log rotation - Don't let logs grow unbounded
6. Error alerts - Set up monitoring for failures