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Correct answer: B. The skill's body loads only when used, so an idle reference costs almost nothing
Explanation
The principle — where a long document lives decides when you pay for it.
Why the key is correct — Anthropic states it directly: unlike CLAUDE.md content, a skill's body loads only when it is used, so long reference material costs almost nothing until you need it. That is why a rarely-needed reference belongs in a skill rather than in the file read every time.
Why the others are wrong — the cost is not the same either way; that contrast is the whole point. Always in force is not the same as free. And a skill does not run in isolated context — it is the option for a workflow that runs in the main conversation.
Remember this — a reference you need occasionally goes in a skill, which charges you only on the days you use it.
Sources — Anthropic's Claude Code skills and subagents documentation.
Sources
“Unlike CLAUDE.md content, a skill's body loads only when it's used, so long reference material costs almost nothing until you need it.”
“Consider Skills instead when you want reusable prompts or workflows that run in the main conversation context rather than isolated subagent context.”
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