MCP Config Debugger
Paste your MCP config file and get instant diagnostics. Catches the most common reasons MCP servers fail to connect.
Last updated Mar 2026
Common MCP Connection Problems
1. spawn npx ENOENT (Windows)
The most common MCP error on Windows. When Claude Desktop or Cursor tries to start an MCP server using npx, it fails with "spawn npx ENOENT" because npx is a .cmd script on Windows, not a binary executable. MCP clients need cmd /c npx to run it correctly. This affects npx, npm, pnpm, yarn, and bunx.
2. npx Missing -y Flag
When using npx to run an MCP server package, the -y flag is required to skip the interactive confirmation prompt. Without it, npx asks "Need to install the following packages: ... Ok to proceed? (y)" and waits for input — which never comes because the MCP client doesn't have a terminal. The server silently hangs. Always add -y as the first argument.
3. Windows MSIX Config Path
Claude Desktop on Windows is distributed as an MSIX package with filesystem virtualization. When you click "Edit Config" in Claude Desktop, it may open a wrong config file. Your edits go to the wrong file and nothing changes. The actual config path is under the MSIX virtualized LocalCache directory.
4. PATH / nvm Issues
If you installed Node.js via nvm, mise, or Homebrew, the MCP client (Claude Desktop, Cursor) may not inherit your terminal's PATH. Your terminal finds node and npx fine, but the MCP client gets "command not found." Fix: use the full absolute path to the binary (find it with which npx or where npx).
5. Wrong Config Format Per Client
Each MCP client uses a slightly different JSON structure. Claude Desktop and Cursor use {"mcpServers": {}}. VS Code uses {"mcp": {"servers": {}}}. Using the wrong format results in the client silently ignoring your servers. Use our Config Generator to create the correct format for your client.