---
name: mcp-spy
description: Debug MCP server communication. Use for troubleshooting MCP integrations, viewing traffic, and analyzing latency.
---

# 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

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

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

### Filter by Type

```bash
# 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

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

### Find Slow Calls

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

## Failed Request Analysis

### Find Failures

```bash
# 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

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

### Server Logs

```bash
# 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

```bash
# 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

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

### Validate Requests

```bash
# 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

```bash
# 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

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

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

### Slow Responses

```bash
# 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

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

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

## Monitoring

### Watch Traffic Live

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

### Traffic Stats

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

### Health Dashboard

```bash
#!/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