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mcp-invalid-env-var ​

Environment variables must use proper expansion syntax

Warning Configurable

Rule Details ​

This rule checks that environment variable references in MCP transport fields (command, args, url, headers, and env values) use proper ${VAR} expansion syntax and that variable names match a configurable naming pattern. It warns on empty env values, empty expansion syntax like ${}, variable names that do not match the expected pattern, and bare $VAR references that should use the ${VAR} format. The special variable ${CLAUDE_PLUGIN_ROOT} is always excluded from pattern validation.

Incorrect ​

Bare variable reference without braces

json
{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "env": {
        "API_KEY": "$MY_API_KEY"
      }
    }
  }
}

Empty environment variable value

json
{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "env": {
        "API_KEY": ""
      }
    }
  }
}

Correct ​

Proper variable expansion with braces

json
{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "env": {
        "API_KEY": "${MY_API_KEY}"
      }
    }
  }
}

How To Fix ​

Use the ${VAR_NAME} format for environment variable references. Ensure variable names match the expected pattern (default: uppercase letters, digits, and underscores starting with a letter or underscore). Provide non-empty values for all env entries.

Options ​

Default options:

json
{
  "pattern": "^[A-Z_][A-Z0-9_]*$"
}

Allow lowercase environment variable names:

json
{
  "pattern": "^[a-zA-Z_][a-zA-Z0-9_]*$"
}

Resources ​

Version ​

Available since: v0.2.0