MongoDB MCP: MDB_MCP_CONNECTION_STRING is not set (add it to the client env block)

TL;DR: The MongoDB MCP server needs MDB_MCP_CONNECTION_STRING in the client config's env block. Setting it in your terminal does nothing because MCP clients launch servers with a clean environment. Add it to the config, restart the client, done.

Error: MDB_MCP_CONNECTION_STRING environment variable is not set

Fix it

1. Open your MCP client config (claude_desktop_config.json, .mcp.json, etc.) and add the variable to the server's env block:

{
  "mcpServers": {
    "MongoDB": {
      "command": "npx",
      "args": ["-y", "mongodb-mcp-server@latest", "--readOnly"],
      "env": {
        "MDB_MCP_CONNECTION_STRING": "mongodb://cluster-host:27017/myDatabase"
      }
    }
  }
}

For Atlas, use the mongodb+srv:// URL from the Atlas dashboard.

1. Fully quit and restart the client. MCP servers spawn at client launch, not per chat.

Expected: the server starts, tools like find and aggregate appear, no startup error.

When to use this

• The server fails immediately at startup mentioning MDB_MCP_CONNECTION_STRING.
• echo $MDB_MCP_CONNECTION_STRING works in your terminal but the server still complains.

When NOT to use this

• The server starts but queries fail with auth or timeout errors. The variable is set; the value is the problem.
• You use Atlas API credentials (MDB_MCP_API_CLIENT_ID) instead of a connection string. That is a different auth path.

Compatibility

• mongodb-mcp-server (all recent versions).
• Claude Desktop, Claude Code, Cursor, Windsurf, VS Code.

Why it happens

MCP servers are child processes of the client. GUI clients especially do not inherit your interactive shell environment, so export in .bashrc or a terminal session is invisible to the server. The env block in the client config is the only reliable way to pass variables.

Edge cases

• Special characters in the password (@, /, ?, #) must be percent-encoded in the URL, or the parser mangles the connection string.
• JSON-escape backslashes and quotes in passwords.
• On Windows, the config path uses the AppData location. Make sure you are editing the file the client actually reads.