How to get environment variables on the server?

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

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.

docs: add environment variables guide

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.

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}

Navigation Menu

Repository navigation

How to get environment variables on the server? #1277

Question

Additional Context

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

How to get environment variables on the server? #1277

Description

Question

Additional Context

Activity

felixweinberger commented on Oct 7, 2025

ameenalkhaldi commented on Mar 15, 2026

maxisbey commented on Jul 24, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions