If the same guidance — safety rules, a style guide, a report format — appears in more than one prompt, extract it and reference it.
Instruction files are markdown fragments, authored in the web interface. Attach one to an instruction, decision or review step via instructionFileIds, and its content is appended to the prompt. Or embed it anywhere in instruction content with {{include:<name-or-id>}}.
Skills are standalone reusable procedures, discoverable by agents through ccw_skill_list and fetched with ccw_skill_call. A skill can nest {{skill:<name-or-id>}} and {{include:<name-or-id>}} references; the engine resolves them recursively, to a depth of ten, reporting cycles inline.
Both travel with a personal profile's library across projects, and both are bound to the instance they were authored on.