Rules vs. Skills vs. Prompts: which do you need?
Every instruction format answers the same two questions differently: when does it reach the model, and who decides? Pick by the job, not the name.
Rules
.mdc · .cursorrules- Reaches the model:
- Automatically, every session
- Who triggers it:
- Nobody — always on
Policy and constraints: “never commit secrets”, “always use our error type”.
You need this when: You keep correcting the agent for the same thing.
Browse rules →Project instructions
AGENTS.md · CLAUDE.md- Reaches the model:
- Automatically, in one repository
- Who triggers it:
- Nobody — ambient context
What this project is, its commands, conventions, and hard limits.
You need this when: Every new session re-explains your project from scratch.
Browse project instructions →Skills
SKILL.md- Reaches the model:
- On demand, when the task matches
- Who triggers it:
- The agent
Reusable procedures: “how we write changelogs”, “how to run visual QA”.
You need this when: You paste the same how-to steps into chats repeatedly.
Browse skills →Prompts
plain text- Reaches the model:
- Once, when you send it
- Who triggers it:
- You
One-shot behaviors: “review this diff like a staff engineer”.
You need this when: You want a behavior right now, in any tool, no setup.
Browse prompts →Rules of thumb
- Correcting the same mistake twice? That correction is a rule.
- Pasting the same steps twice? Those steps are a skill.
- Onboarding the agent to the repo every session? Write AGENTS.md once.
- One-off transformation or persona?That's a prompt — start there, and promote it to a rule or skill when you reach for it again.
- Start small: a focused 20-line rule beats a 300-line manifesto the model skims.
Where do tools, plugins, and MCP fit?
Everything above is knowledge — text the model reads. Tools, plugins, and MCP servers are capabilities: they let an agent do new things (query a database, control a browser) rather than know new things. Waxmark catalogs the knowledge layer; capabilities have their own registries. If a “tool” you found is really a description plus steps, it probably belongs here as a skill.
Whatever you pick, it arrives sealed: how Waxmark scans every file before it reaches your agent.