Why is my MCP server not showing up in Claude Desktop?

The most common causes are JSON syntax errors in claude_desktop_config.json, relative paths, or a missing full restart. Claude Desktop requires absolute paths for commands and arguments. Validate your JSON with a linter, then fully quit and relaunch the app; a window close is not enough. If the server still does not appear, check the logs at ~/Library/Logs/Claude/mcp*.log on macOS or %APPDATA%\Claude\logs on Windows for spawn or connection errors. Ensure Node.js is installed if the server relies on npx or node. Some servers also fail because they depend on environment variables that Claude Desktop does not inherit; try running the command manually in a terminal to confirm it starts.