This is no different to an MCP, where you rely on the model to use the metadata provided to pick the right tool, and understand how to use it.

Like with MCP, you can provide a deterministic, known-good piece of code to carry out the operation once the LLM decides to use it.

But a skill can evolve from pure Markdown via inlining some shell commands, up to a large application. And if you let it, with Skills the LLM can also inspect the tool, and modify it if it will help you.

All the Skills I use now have evolved bit by bit as I've run into new use-cases and told Claude Code to update the script the skills references or the SKILL.md itself. I can evolve the tooling while I'm using it.