c4-diagrams

C4 Diagrams

Overview

Use C4 diagrams to clarify software architecture before detailed design. Focus on boundaries, responsibilities, actors, dependencies, and data flow; draw only the levels that add value.

First Gates

Before diagramming, determine these choices. If the user already specified one, honor it. If multiple choices are ambiguous, ask one concise combined question.

Choice	Options	Default behavior
Purpose	Existing code, new system, design review	Existing code requires codebase exploration first; new systems require assumptions called out.
Format	ASCII, Mermaid	Ask the user. Mermaid means plain Mermaid syntax, not C4-specific Mermaid.
Rigor	Strict C4, lightweight C4-inspired, hybrid	Ask the user. Do not decide silently.

If the user asks to skip questions for speed, still preserve unresolved choices as explicit assumptions and keep the first diagram lightweight.

Workflow

1. Establish purpose, format, and rigor.
2. For existing code, inspect entry points, runtime boundaries, integrations, and persistence before drawing. For a quick pass, inspect enough to identify language/framework, launch entry points, major directories, external integrations, and data stores; label anything else as unknown.
3. Pick the smallest useful diagram set. Start with system context or container; add component, dynamic, or deployment only when it answers a real question.
4. Draw the diagram using ASCII or plain Mermaid.
5. Explain the diagram in 3-6 bullets: boundaries, responsibilities, key relationships, assumptions, and open questions.
6. Stop before detailed design unless the user approves the diagram and asks to continue.

C4 Level Selection

Level	Use when	Avoid when
System context	Identifying users, external systems, and scope	The scope is already obvious and local.
Container	Showing deployable/runnable units, data stores, APIs, CLIs, queues	You only need code-level call flow.
Component	Explaining internals of one container	The container has few meaningful internal parts.
Code	Rarely, for critical classes/modules	A normal component diagram would be enough.
Dynamic	Showing request, event, or workflow sequence	Static structure is the actual question.
Deployment	Showing infrastructure, nodes, networks, runtime placement	Deployment is unknown or irrelevant.

For small systems, challenge requests for all four C4 levels and offer a smaller set. If the user still wants all four, produce them but mark low-value levels as lightweight sketches and explain why they may not be worth maintaining.

Output Rules

• Mermaid diagrams must use plain flowchart or sequenceDiagram syntax for portability.
• ASCII diagrams should favor readable boxes and arrows over decorative complexity.
• Keep labels concrete: actor, system, container, component, database, queue, external service.
• Do not mix levels accidentally: containers are runnable/deployable units; components live inside one container.
• Always include assumptions when diagramming a future system or incomplete codebase.
• Always include open questions when boundaries, ownership, data flow, or deployment are uncertain.

Templates

Use templates.md in this skill directory for compact ASCII and Mermaid starting points.

Common Mistakes

Mistake	Fix
Drawing all four C4 levels by default	Draw only levels that answer the current question.
Jumping into detailed design immediately	Validate context/container boundaries first.
Using C4-specific Mermaid syntax	Use plain Mermaid flowchart or sequenceDiagram.
Treating modules as containers	Containers are runnable/deployable units; modules are usually components.
Hiding uncertainty	State assumptions and open questions after the diagram.