Distinguish reusable expertise from tools and model training.
1. Instructions as a package
A skill packages task guidance and optional supporting files for an agent. It does not train the underlying model or guarantee that instructions will always be followed.
2. Skill versus tool
A tool performs an operation. A skill explains when and how to use available capabilities. An MCP server supplies a tool interface; it is not interchangeable with a skill.
3. Activation scope
A clear description helps an agent select the skill for relevant tasks. Broad descriptions cause irrelevant activation and unnecessary context.
Worked scenario
A Compose review skill should activate for UI state review, not every repository question merely because Kotlin files exist.
Apply it
Write intended and excluded task examples for one narrowly scoped Android skill.
Check your understanding
You can explain why the skill exists separately from the tools it uses. Explain the decision and show evidence from your implementation or design. If you cannot demonstrate it yet, revisit the relevant section before continuing.