Fix MCP Server Invalid JSON Config — Mac Syntax Error Guide

What causes this error?

Before applying fixes, it is helpful to understand the root cause. On macOS, this error typically stems from one of the following: incorrect Node.js or NPX installation paths, missing environment variables in your shell profile, permission restrictions on the MCP server binary or config file, or an incompatible transport protocol configuration in your Claude Desktop or Cursor settings. Identifying the specific cause narrows down the fix and prevents wasted effort.

Step 1: Verify your Node.js and NPX installation

Open a terminal and confirm that Node.js and NPX are correctly installed and accessible from your shell. Run the following commands:

node --version   # should show v18.x or higher
npx --version    # should show a version number
which npx        # should return a valid path

Step 2: Check your configuration file

Locate your MCP client configuration file (e.g., claude_desktop_config.json for Claude Desktop). Verify the JSON syntax is valid — a single missing comma or unquoted string can silently break server startup. Validate with a JSON linter or by pasting your config into an online validator.

Step 3: Apply the fix

Based on your diagnosis, apply the appropriate fix. For path issues, add the Node.js binary directory to your shell profile (~/.bashrc or ~/.zshrc). For permission errors, adjust file ownership with chmod or chown. For port conflicts, free the port using lsof -i :PORT.

Step 4: Verify the server is running

Restart your MCP client, configure the server connection, and verify that tools appear in the available tools list. If they do not, check the server process log output for additional error messages.

Preventative measures

To avoid this error recurring: pin your Node.js version with a version manager (nvm for Unix), add a JSON schema validator to your editor for MCP config files, and set up a health-check script that verifies server connectivity on startup.

Summary

After following these steps, your MCP server should be running correctly. If the issue persists, check the server-specific documentation on MCPHub for additional troubleshooting, or search for similar issues in the server's GitHub repository.

Related articles