MCP server can't see your API keys or env vars: declare them under "env"

Symptom

The server reports that DEMO_API_KEY is not set, even though it's exported in the shell that launched the client.

Environment and tested versions

Debian GNU/Linux 13 (trixie) x86_64, node 20.19.2, npm 11.21.1; stdio transport

• @modelcontextprotocol/sdk (TypeScript) 1.32.2
• node 20.19.2

Client used in the test: Reproduced with the official MCP TypeScript SDK stdio client (StdioClientTransport) 1.32.2 as a stand-in for a desktop app (Claude Desktop, Cursor, etc.), which can't run on the Linux test box. Not tested in the desktop app itself.

Root cause

stdio clients don't pass their whole environment to servers. The SDK's getDefaultEnvironment() passes only a safe list on POSIX (HOME, LOGNAME, PATH, SHELL, TERM, USER; DEFAULT_INHERITED_ENV_VARS in client/stdio.js). Desktop apps launched from the Dock or Start menu never had your shell variables in the first place.

Fix steps

1. Declare each variable the server needs under "env" in that server's config entry.
2. Restart the client.

Verification

Tested 31 Oct 2026 in a throwaway directory on a Linux box (fake credentials only). Before (A) and after the fix:

A key only in parent env: CALL: "ERROR: DEMO_API_KEY is not set"
B key under "env":        CALL: "DEMO_API_KEY is set (len 19)"

Sources

• https://modelcontextprotocol.io/legacy/tools/debugging