MCP Server Debugging Guide

This guide covers debugging techniques specific to MCP (Model Context Protocol) servers when using the Copilot SDK.

Table of Contents

• Quick Diagnostics
• Testing MCP Servers Independently
• Common Issues
• Platform-Specific Issues
• Advanced Debugging

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Quick Diagnostics

Checklist

Before diving deep, verify these basics:

• [ ] MCP server executable exists and is runnable
• [ ] Command path is correct (use absolute paths when in doubt)
• [ ] Tools are enabled (tools: ["*"] or specific tool names)
• [ ] Server implements MCP protocol correctly (responds to initialize)
• [ ] No firewall/antivirus blocking the process (Windows)

Enable MCP Debug Logging

Add environment variables to your MCP server config:

mcpServers: {
  "my-server": {
    type: "local",
    command: "/path/to/server",
    args: [],
    env: {
      MCP_DEBUG: "1",
      DEBUG: "*",
      NODE_DEBUG: "mcp",  // For Node.js MCP servers
    },
  },
}

---

Testing MCP Servers Independently

Always test your MCP server outside the SDK first.

Manual Protocol Test

Send an initialize request via stdin:

# Unix/macOS
echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}' | /path/to/your/mcp-server

# Windows (PowerShell)
'{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}' | C:\path\to\your\mcp-server.exe

Expected response:

{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{}},"serverInfo":{"name":"your-server","version":"1.0"}}}

Test Tool Listing

After initialization, request the tools list:

echo '{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}' | /path/to/your/mcp-server

Expected response:

{"jsonrpc":"2.0","id":2,"result":{"tools":[{"name":"my_tool","description":"Does something","inputSchema":{...}}]}}

Interactive Testing Script

Create a test script to interactively debug your MCP server:

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

SERVER="$1"

# Initialize
echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}'

# Send initialized notification
echo '{"jsonrpc":"2.0","method":"notifications/initialized"}'

# List tools
echo '{"jsonrpc":"2.0","id":2,"method":"tools/list","params":{}}'

# Keep stdin open
cat

Usage:

./test-mcp.sh | /path/to/mcp-server

---

Common Issues

Server Not Starting

Symptoms: No tools appear, no errors in logs.

Causes & Solutions:

Cause	Solution
Wrong command path	Use absolute path: /usr/local/bin/server
Missing executable permission	Run chmod +x /path/to/server
Missing dependencies	Check with ldd (Linux) or run manually
Working directory issues	Set cwd in config

Debug by running manually:

# Run exactly what the SDK would run
cd /expected/working/dir
/path/to/command arg1 arg2

Server Starts But Tools Don't Appear

Symptoms: Server process runs but no tools are available.

Causes & Solutions:

1. Tools not enabled in config:

   mcpServers: {
     "server": {
       // ...
       tools: ["*"],  // Must be "*" or list of tool names
     },
   }
   

2. Server doesn't expose tools:

   • Test with tools/list request manually
   • Check server implements tools/list method

3. Initialization handshake fails:

   • Server must respond to initialize correctly
   • Server must handle notifications/initialized

Tools Listed But Never Called

Symptoms: Tools appear in debug logs but model doesn't use them.

Causes & Solutions:

1. Prompt doesn't clearly need the tool:

   // Too vague
   await session.sendAndWait({ prompt: "What's the weather?" });
   
   // Better - explicitly mentions capability
   await session.sendAndWait({ 
     prompt: "Use the weather tool to get the current temperature in Seattle" 
   });
   

2. Tool description unclear:

   // Bad - model doesn't know when to use it
   { name: "do_thing", description: "Does a thing" }
   
   // Good - clear purpose
   { name: "get_weather", description: "Get current weather conditions for a city. Returns temperature, humidity, and conditions." }
   

3. Tool schema issues:

   • Ensure inputSchema is valid JSON Schema
   • Required fields must be in required array

Timeout Errors

Symptoms: MCP tool call timed out errors.

Solutions:

1. Increase timeout:

   mcpServers: {
     "slow-server": {
       // ...
       timeout: 300000,  // 5 minutes
     },
   }
   

2. Optimize server performance:

   • Add progress logging to identify bottleneck
   • Consider async operations
   • Check for blocking I/O

3. For long-running tools, consider streaming responses if supported.

JSON-RPC Errors

Symptoms: Parse errors, invalid request errors.

Common causes:

1. Server writes to stdout incorrectly:

   • Debug output going to stdout instead of stderr
   • Extra newlines or whitespace

   // Wrong - pollutes stdout
   console.log("Debug info");
   
   // Correct - use stderr for debug
   console.error("Debug info");
   

2. Encoding issues:

   • Ensure UTF-8 encoding
   • No BOM (Byte Order Mark)

3. Message framing:

   • Each message must be a complete JSON object
   • Newline-delimited (one message per line)

---

Platform-Specific Issues

Windows

.NET Console Apps / Tools

// Correct configuration for .NET exe
["my-dotnet-server"] = new McpLocalServerConfig
{
    Type = "local",
    Command = @"C:\Tools\MyServer\MyServer.exe",  // Full path with .exe
    Args = new List<string>(),
    Cwd = @"C:\Tools\MyServer",  // Set working directory
    Tools = new List<string> { "*" },
}

// For dotnet tool (DLL)
["my-dotnet-tool"] = new McpLocalServerConfig
{
    Type = "local", 
    Command = "dotnet",
    Args = new List<string> { @"C:\Tools\MyTool\MyTool.dll" },
    Cwd = @"C:\Tools\MyTool",
    Tools = new List<string> { "*" },
}

NPX Commands

// Windows needs cmd /c for npx
["filesystem"] = new McpLocalServerConfig
{
    Type = "local",
    Command = "cmd",
    Args = new List<string> { "/c", "npx", "-y", "@modelcontextprotocol/server-filesystem", "C:\\allowed\\path" },
    Tools = new List<string> { "*" },
}

Path Issues

• Use raw strings (@"C:\path") or forward slashes ("C:/path")
• Avoid spaces in paths when possible
• If spaces required, ensure proper quoting

Antivirus/Firewall

Windows Defender or other AV may block:

• New executables
• Processes communicating via stdin/stdout

Solution: Add exclusions for your MCP server executable.

macOS

Gatekeeper Blocking

# If server is blocked
xattr -d com.apple.quarantine /path/to/mcp-server

Homebrew Paths

// GUI apps may not have /opt/homebrew in PATH
mcpServers: {
  "my-server": {
    command: "/opt/homebrew/bin/node",  // Full path
    args: ["/path/to/server.js"],
  },
}

Linux

Permission Issues

chmod +x /path/to/mcp-server

Missing Shared Libraries

# Check dependencies
ldd /path/to/mcp-server

# Install missing libraries
apt install libfoo  # Debian/Ubuntu
yum install libfoo  # RHEL/CentOS

---

Advanced Debugging

Capture All MCP Traffic

Create a wrapper script to log all communication:

#!/bin/bash
# mcp-debug-wrapper.sh

LOG="/tmp/mcp-debug-$(date +%s).log"
ACTUAL_SERVER="$1"
shift

echo "=== MCP Debug Session ===" >> "$LOG"
echo "Server: $ACTUAL_SERVER" >> "$LOG"
echo "Args: $@" >> "$LOG"
echo "=========================" >> "$LOG"

# Tee stdin/stdout to log file
tee -a "$LOG" | "$ACTUAL_SERVER" "$@" 2>> "$LOG" | tee -a "$LOG"

Use it:

mcpServers: {
  "debug-server": {
    command: "/path/to/mcp-debug-wrapper.sh",
    args: ["/actual/server/path", "arg1", "arg2"],
  },
}

Inspect with MCP Inspector

Use the official MCP Inspector tool:

npx @modelcontextprotocol/inspector /path/to/your/mcp-server

This provides a web UI to:

• Send test requests
• View responses
• Inspect tool schemas

Protocol Version Mismatches

Check your server supports the protocol version the SDK uses:

// In initialize response, check protocolVersion
{"result":{"protocolVersion":"2024-11-05",...}}

If versions don't match, update your MCP server library.

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Debugging Checklist

When opening an issue or asking for help, collect:

• [ ] SDK language and version
• [ ] CLI version (copilot --version)
• [ ] MCP server type (Node.js, Python, .NET, Go, etc.)
• [ ] Full MCP server configuration (redact secrets)
• [ ] Result of manual initialize test
• [ ] Result of manual tools/list test
• [ ] Debug logs from SDK
• [ ] Any error messages

See Also

• MCP Overview - Configuration and setup
• General Debugging Guide - SDK-wide debugging
• MCP Specification - Official protocol docs