How to get environment variables on the server?

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Question

I haven't been able to figure out how to get environment variables in tool functions on an MCP server, or if they're actually supported.

This should be a valid client configuration:

{
    "mcpServers": {
        "php-calculator": {
            "command": "python",
            "args": ["/absolute/path/to/your/mcp-server.py"], 
            "env": {"API_KEY": "xxxx"}
        }
    }
}

How can I get the API_KEY environment variable on the server?

Additional Context

No response