I've not explicitly used skills or MCP, but have had zero issues with Claude calling apis via curl as an example. I'm not sure what the MCP server or skill is actually enabling at this point. If I wanted CC to talk to SQL Server, I'd have it open a nix-env with the tools needed to talk to the database. One of my primary initial claude.md entries has to do with us running on NixOS and that temporarily installing tools is trivial and it should do things in the NixOS way whenever possible. Since then it has just worked with practically everything I've thrown at it. Very rarely do I see it trying to use a tool that isn't installed anymore. CC even uses my local vaultwarden where I have a collection of credentials shared with it. All driven through claude.md.