It's a balance and we use both.

Skills doesn't totally deprecate documenting things in CLAUDE.md but agree that a lot of these can be defined as skills instead.

Skill frontmatter also still sits in the global context so it's not really a token optimization either.

The skill lets you compress the amount loaded to just the briefest description, with the “where do I go to get more info” being implicit. You should use a SKILL.md for evry added tool. At which point, putting instructions in CLAIDE.md becomes redundant and confusing to the LLM.