Cowork desktop: Custom MCP connector URL field doesn't expand environment variables

Summary

The "Agregar conector personalizado" (Add custom connector) UI in Cowork desktop does not expand environment variable references like ${VAR} or ${VAR:-default} in the URL field. The string is sent literally to the MCP server, causing DNS/connection failures.

Reproduction

1. Open Cowork desktop → Customize → Conectores → Agregar conector personalizado
2. In the URL field, paste: ${POWI_MCP_URL:-https://us-central1-powi-staging-488705.cloudfunctions.net/powiMcpServer}
3. (Optional) Configuración avanzada → set Authorization header: Bearer ${POWI_MCP_TOKEN}
4. Save → click "Conectar"

Expected

Cowork resolves ${POWI_MCP_URL} from the environment (or applies the :- default value) and connects to the MCP server.

Actual

Error: "Couldn't reach the MCP server. You can check the server URL and verify the server is running."
Reference ID: ofid_667d0f69e01971cc

The literal string ${POWI_MCP_URL:-...} is sent as the URL, which is not a valid HTTP target.

Why this matters

Plugins distributed via marketplace declare MCP servers in .mcp.json using the documented ${VAR} syntax (supported by Claude Code CLI). When the same plugin is consumed via Cowork desktop, the connector pre-fills with the literal placeholder and breaks. This forces users to manually edit and paste expanded URLs/tokens at install time, which defeats the portability of distributing plugins with env-var defaults.

In our case the plugin lives at marketplace admin431/PoWi-Mobile, version powi-claude-plugin@1.4.0, and declares an HTTP MCP server with Bearer auth — both fields use \\${VAR}\ placeholders that work fine via CLI but fail in Cowork.

Workaround

Replace the placeholder with the literal expanded URL/token in the connector form. Loses portability across staging/prod environments.

Environment

• Cowork desktop (Windows 11)
• Plugin: \powi-claude-plugin@1.4.0\ from marketplace \admin431/PoWi-Mobile\
• MCP server type: \http\, Bearer auth header