I usually instruct the agent to use the skills explicitly, e.g. "/writing-tests write the tests for @some-class.cpp"

So the skills are mostly a sort of on-demand AGENTS.md specific to the task.

Another example is I have a `plan-review` skill, so when planning something I add at the end of the prompt something like: "plan the task, .... then launch claude and codex /plan-review agents in parallel and take their findings into account before producing the final plan".

The 4% usage was about our internal team, and we have skills setup. So it is not necessary that they are not built, but rather that they were not used, when we expected them to be used. So we adapted our CLAUDE.md to make claude more eager to use them. Also the 4% usage was on the 4.5 models, 4.6 got much better with invoking skills.