How to get environment variables on the server?

[ilvalerione]

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

[felixweinberger]

This depends on the environment you're running your MCP server in - there's an example for desktop in the README here: https://github.com/modelcontextprotocol/python-sdk?tab=readme-ov-file#claude-desktop-integration

But probably worth adding something to docs on patterns for how to deal with this common use case.

[ameenalkhaldi]

I've opened a PR addressing this: #2296

Added a dedicated documentation page covering environment variable patterns — accessing them in tool functions, passing them via Claude Desktop config / mcp install CLI / programmatic clients, and the built-in MCP_-prefixed settings.

[maxisbey]

The env block in that config becomes the environment of the process the host launches your server in, so inside a tool it's just standard Python: os.environ["API_KEY"] or os.getenv("API_KEY").

import os

from mcp.server.mcpserver import MCPServer

mcp = MCPServer("php-calculator")


@mcp.tool()
def check_key() -> str:
    api_key = os.environ["API_KEY"]  # from the client config's "env" block
    return f"got a {len(api_key)}-character key"

As felixweinberger noted, how the variable gets set is up to the host. For Claude Desktop the config file (including its env field) is covered in Connect to a real host → Claude Desktop, and mcp install -v KEY=VALUE / -f .env will write the variables into that entry for you — see Running your server → The mcp command.

Closing this as part of a wider backlog cleanup around the v2 release. Feel free to reopen if this is still relevant.

_{AI Disclaimer}