arc42 + C4 Skill

arc42 is a template for architecture documentation with 12 sections. C4 is a diagram model (Context → Container → Component → Code) that maps directly into arc42's structural sections.

Do not use arc42 for projects under 10 files — use a simple README instead.

When to Activate

• Generating or updating project architecture documentation
• Onboarding a new engineer to the system architecture
• After a major architectural decision (ADR) that changes the structure
• Before a large feature that touches multiple system layers
• When someone asks "how does this system actually work?"

arc42 Sections — Quick Reference