# C4 Diagram

> Generates C4 model architecture diagrams using Structurizr DSL (primary) or Mermaid C4 (fallback). Use when: user asks to "draw a diagram", "create a C4 diagram", "show architecture as a diagram", "generate architecture diagram", "document the system", or when explain-code reaches its diagram step. DO NOT USE for code explanation or walkthroughs (use `explain-code`), class/sequence/ER diagrams (C4 is system-architecture only), or when a diagram is incidental to a code change — only when a C4 diagram is the primary deliverable.

- Skill: `bsene/c4-diagram` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add bsene/c4-diagram`
- Raw SKILL.md: https://api.skillmd.com/api/skills/bsene/c4-diagram/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: bsene (https://skillmd.com/u/bsene)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/bsene/c4-diagram

---


# C4 Diagram

## Workflow

1. **Pick the C4 level** — match diagram depth to the question (see table below)
2. **Choose the tool** — Structurizr DSL by default; Mermaid for quick inline previews (see table below)
3. **Generate the diagram** — follow Diagram Rules, then the DSL reference for the chosen tool
4. **Render** — follow the Rendering fallback chain

## C4 Level Selector

| Question type                                | C4 level                          | Structurizr view type                             |
| -------------------------------------------- | --------------------------------- | ------------------------------------------------- |
| "Show the whole organisation / all systems"  | **Supporting — System Landscape** | `systemLandscape` (no Mermaid equivalent)         |
| "What does this system do?" / "Who uses it?" | **Level 1 — Context**             | `systemContext`                                   |
| "What are the main services/apps/databases?" | **Level 2 — Container**           | `container`                                       |
| "How is this service structured internally?" | **Level 3 — Component**           | `component`                                       |
| "How does this specific class/module work?"  | **Level 4 — Code**                | No diagram — Level 4 has no C4 diagram equivalent |

**Default to Level 2 (Container)** — it covers most "show me the architecture" requests. Level 1 for stakeholder overviews, Level 3 for deep dives into a single service.

## Tool Selector

| Need                                | Use                                                |
| ----------------------------------- | -------------------------------------------------- |
| Quick inline preview in chat        | Mermaid → `references/c4-mermaid.md`               |
| Export PNG/SVG for docs/wiki        | Structurizr → `references/c4-structurizr.md`       |
| Multiple views from one model       | Structurizr                                        |
| Organisation-wide system landscape  | Structurizr (no Mermaid C4 equivalent)             |
| Dynamic / numbered interaction flow | Either (`C4Dynamic` or Structurizr `dynamic`)      |
| Deployment / infrastructure view    | Structurizr (Mermaid `C4Deployment` is limited)    |
| Class/method level detail (Level 4) | Mermaid `classDiagram` — no Structurizr equivalent |
| No tooling available                | Mermaid                                            |

**Default to Structurizr DSL** unless the user explicitly wants a quick Mermaid preview.

## Diagram Rules

- Every element must have: **name**, **type** (Person/System/Container/Component), and **brief description**
- Containers and components must state their **technology** (e.g. "Node.js Express", "PostgreSQL")
- Every relationship must be labeled with **action + protocol** (e.g. "Reads events via gRPC", not just "Uses")
- Mark external systems/people with `tags "External"` in the model
- Boundaries are implicit: `systemContext` and `container` views auto-scope. Use `group` for logical sub-groupings within a boundary
- Keep diagrams under ~12 elements — split into multiple diagrams if larger

## Rendering

Run in order, stop at first success. Native `svg`/`png` export does not exist in the Structurizr CLI — always export to PlantUML/Mermaid/D2 first, then render with that tool.

1. **CLI available** (binary is `structurizr.sh` from local install / Docker, or `structurizr-cli` from Homebrew/Scoop) — save DSL to `/tmp/<name>.dsl`, then:

   ```bash
   structurizr.sh export -workspace /tmp/<name>.dsl -format plantuml -output /tmp/structurizr-out/
   # render the .puml to SVG/PNG with `plantuml /tmp/structurizr-out/*.puml`
   ```

   Share output path with user.

2. **Docker available** — save DSL to `/tmp/<name>.dsl`, give user:

   ```bash
   # Active replacement (recommended):
   docker run -it --rm -p 8080:8080 -v /tmp:/usr/local/structurizr structurizr/structurizr local
   # Legacy (archived but functional):
   docker run -it --rm -p 8080:8080 -v /tmp:/usr/local/structurizr structurizr/lite
   ```

   Point to `http://localhost:8080`.

3. **Neither** — output DSL as a ` ```dsl ` code block. Note: paste at `https://playground.structurizr.com/` to render online.

---

→ Structurizr DSL syntax: `references/c4-structurizr.md`
→ Mermaid C4 syntax: `references/c4-mermaid.md`

---

## Benchmark

Scenario: `.benchmarks/scenarios/c4-diagram-001-container-view.md` · Run: 2026-08-31 · Log: `.benchmarks/runs/2026-08-31/c4-diagram-001-container-view.json`

| Model             | Without | With | Delta |
| ----------------- | ------- | ---- | ----- |
| claude-opus-4-8   | 67%     | 100% | +33%  |
| claude-sonnet-4-6 | 100%    | 100% | +0%   |
| claude-haiku-4-5  | 100%    | 100% | +0%   |

> **PASS (run 2026-08-31)**. Opus +33 (67→100); sonnet/haiku already at ceiling. No regressions. Gate per `.agents/skills/skill-optimizer/rules/release-gates.md`.