C4 Diagramming

C4 Diagramming

C4 Model for structural architecture visualization. Produces diagrams at four zoom levels, mapped into the artifact pyramid.

C4-to-Pyramid Mapping

| C4 Level | Pyramid Layer | Path | |----------|--------------|------| | Level 1: System Context | L1 (Summary) | 01-summary/system-context.md | | Level 2: Container | L2 (Analysis) | 02-analysis/structural-views/container.md | | Level 3: Component | L2 (Analysis) | 02-analysis/structural-views/components.md | | Level 4: Code | L3 (Dossiers) | 03-dossiers/code-level-detail.md |

C4 is the cleanest structural fit — its four-level hierarchy maps to the three pyramid layers with almost no translation, and Levels 2-3 both resolve to separate L2 analysis files.

Authoring Formats

Mermaid (Default for Quick Diagrams)

Use when you need a single diagram embedded in markdown. See the GitHub Rendering Constraint section below for C4-in-Mermaid compatibility notes.

Structurizr DSL (Recommended for Long-Lived Projects)

Structurizr DSL is the C4 model's reference "models as code" implementation, created by Simon Brown. Define the entire architecture model in a single DSL file; all 4 C4 levels are generated from it. This ensures structural consistency across diagrams that hand-written Mermaid cannot guarantee.

workspace {
    model {
        user = person "Customer"
        system = softwareSystem "Your System" {
            webapp = container "Web Application" "TypeScript, React"
            api = container "API" "Go"
            db = container "Database" "PostgreSQL"
            user -> webapp "Uses"
            webapp -> api "Makes API calls"
            api -> db "Reads/writes"
        }
    }
    views {
        systemContext system { include * autolayout lr }
        container system { include * autolayout lr }
        component api { include * autolayout lr }
        theme default
    }
}

Key capabilities:

• !adrs docs/adr — imports Architecture Decision Records (adr-tools, MADR, log4brains) into the workspace, rendered alongside C4 diagrams
• !docs docs/arc42 — imports arc42 documentation as Markdown/AsciiDoc
• Structurizr Lite (Docker) — local preview at http://localhost:8081
• CI commands: validate, inspect, export (PlantUML, Mermaid, static site)

Full reference in references/architecture-as-code-ecosystem.md — tool comparison, DSL cookbook, C4-PlantUML alternative, and the converged repo convention.

Contents

• references/c4-to-pyramid-mapping.md — context→L1, container/component→L2, code→L3 (Mermaid + Structurizr DSL paths)
• references/c4-to-flowchart.md — (in mermaid-diagrams skill) C4 → standard flowchart conversion patterns for GitHub compatibility
• references/architecture-as-code-ecosystem.md — Structurizr DSL, C4-PlantUML, docToolChain, converged repo convention, tool comparison table
• references/ci-pipeline-templates.md — GitHub Actions, GitLab CI, ForgeJo (Gitea Actions, Woodpecker) pipeline templates for Structurizr validation, export, deploy

GitHub Rendering Constraint

GitHub's built-in Mermaid renderer does not bundle the C4 plugin (@mermaid-js/mermaid). Any ```mermaid block using C4Context, C4Container, or C4Component syntax renders as raw code rather than a diagram on GitHub. This affects issues, PR descriptions, discussion comments, and markdown files.

Workaround: Convert C4 diagrams to standard flowchart syntax before embedding in GitHub markdown:

• Person() → [rect] node with label
• System() / System_Ext() → [rect] inside or outside subgraphs
• Container() → [rect with tech stack label]`
• Db() → [(cylinder shape)]
• System_Boundary{} / Container_Boundary{} → subgraph ... end
• Rel() → -- label --> or -.->
• Drop UpdateLayoutConfig() — use flowchart LR or TB directive instead

For the full conversion table with worked examples, load references/c4-to-flowchart.md from the companion mermaid-diagrams skill when it is available.

.mmd files in a DIAGRAMS/ directory must also use standard flowchart syntax if they need to render via mmdc or on GitHub. Files using C4-plugin syntax can only render in tools that bundle the plugin (e.g., Mermaid Live Editor, mmdc with C4 extension config). If you commit .mmd files with C4 syntax to a repo, GitHub's file preview will show them as raw text — convert them to standard syntax or render them to PNG first.

Feature Request Filed

A GitHub Community feature request to bundle the C4 mermaid plugin was filed at https://github.com/orgs/community/discussions/197898 (closed — requires submission through the web UI with the Apps, API and Webhooks discussion template). If this gets implemented, the flowchart conversions below would no longer be necessary for GitHub rendering.

Canonical Reference

• Simon Brown, "The C4 Model" — https://c4model.com/
• GroktoPlan C4 Diagrams (worked examples) — https://github.com/groktopus/groktoplan/blob/main/TECHNICAL_ARCHITECTURE.md

Portability

This skill is intentionally host-neutral. Use your agent's normal mechanisms to load the references, templates, and scripts listed here. Do not assume a particular profile system, task orchestrator, memory service, or response-handoff format.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: magnus919
• Source: magnus919/agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.