I don't understand why tool calling isn't the primitive. A "skill" could have easily just been a tool that an agent can build an execute in its own compute space.

I really don't see why we need two forms of RCP...

If you look through the example skills most of them are about calling existing tools, often Python via the terminal.

Take a look at this one for working with PDFs for example: https://github.com/anthropics/skills/blob/main/document-skil... - it includes a quickstart guide to using the Python pypdf module, then expands on that with some useful scripts for common patterns.

> A "skill" could have easily just been a tool

The problem skills solve is initial mapping of available information. A tool might hide what information it contains until used, this approach puts a table of contents for docs in the context, so the model is aware and can navigate to desired information as needed.