Skill, prompt, or a custom agent?
A simple way to choose, based on how often you repeat the work.
The problem
The three options overlap, so people often build something heavier than they need.
What each one does
A prompt handles one task now. A skill makes a repeated task consistent. A custom agent runs on its own with defined tools and limits.
How to choose
Doing it once: prompt. Doing it repeatedly with you watching: skill. Doing it repeatedly while you are away: custom agent, built on a skill you already trust.
Summary
Move up only when the simpler option has stopped being enough.