🔧 Fix registerDefaultHandlers to respect existing custom handlers

[aatchison]

🐛 Problem

The registerDefaultHandlers() function was unconditionally overriding custom handlers that servers had already registered. This prevented servers from implementing custom tools/list, prompts/list, and resources/list handlers.

Impact:

• Servers couldn't provide custom tool lists
• Custom MCP protocol implementations were being overwritten
• Framework was not respecting the principle of "custom handlers first"

🔧 Solution

Modified registerDefaultHandlers() to check if handlers already exist before registering defaults:

func (s *Server) registerDefaultHandlers() {
    // Only register default handlers if custom ones don't exist
    if s.toolsListHandler == nil {
        s.toolsListHandler = s.handleToolsList
    }
    if s.promptsListHandler == nil {
        s.promptsListHandler = s.handlePromptsList  
    }
    if s.resourcesListHandler == nil {
        s.resourcesListHandler = s.handleResourcesList
    }
}

✅ Benefits

1. Preserves Custom Handlers - Servers can register custom handlers before calling Start()
2. Backward Compatible - Servers not using custom handlers continue to work unchanged
3. Follows Best Practices - Custom implementations take precedence over defaults
4. Enables Advanced Use Cases - Servers can now provide dynamic tool lists, custom prompts, etc.

🧪 Testing

Before Fix:

// Custom handler gets overwritten
server.RegisterHandler("tools/list", customHandler)
server.Start() // Overwrites with default handler

After Fix:

// Custom handler is preserved
server.RegisterHandler("tools/list", customHandler) 
server.Start() // Keeps custom handler

📁 Files Changed

• pkg/mcp/server.go: Modified registerDefaultHandlers() to check for existing handlers

🔗 Related Issues

This enables the mcp-server-devpod project to provide custom tools/list handlers that include both framework tools (echo) and DevPod-specific tools, fixing Claude Desktop compatibility issues.

✅ Verification

• Backward compatibility maintained
• Custom handlers are preserved
• Default handlers still work when no custom handlers exist
• No breaking changes to existing API