4.4 Skills: Packaging Reusable Workflows

Picture the difference between a note that says "format currency as $X.XX" and handing someone your actual finance team's full style guide — when to use parentheses for negatives, which currencies get symbols versus codes, how zero-decimal currencies like JPY get handled. The note is a fact. A Skill is the style guide: a whole reasoning process, packaged as a reusable MCP tool, so Codex can pick it up and apply it correctly without you re-explaining it every time.

That's the distinction that matters and is easy to blur with AGENTS.md: AGENTS.md is always loaded, every session, describing the whole repo — commands, conventions, guardrails. A Skill loads only when it's relevant to the specific task at hand, which keeps your context window from filling up with instructions for work you're not currently doing. Put it this way — AGENTS.md is "who we are and how this repo works." A Skill is "here's exactly how we do this one specific thing, done right, every time." Skills bundle into profiles too, so a "careful" profile might load a different set than a "fast" one, and dependency discovery happens automatically rather than you manually wiring each one in.

The candidates for a Skill are the same tasks that made you write a custom slash command in other tools: something you do the same specific way every time, where the value isn't in the individual instruction but in never having to re-explain the whole process again. How you structure a database migration in this codebase. How you write a PR description that matches your team's actual template. How you scaffold a new API endpoint end to end, including the test file and the docs update everyone forgets.