Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add zekiriabd/SDD-Pro --skill c4-modelgit clone --depth 1 https://github.com/zekiriabd/SDD-ProWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/zekiriabd/sdd-pro/c4-model)<a href="https://agentmods.dev/skills/zekiriabd/sdd-pro/c4-model"><img src="https://agentmods.dev/badge/skills/zekiriabd/sdd-pro/c4-model.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00198 | $0.03362 |
| Opus 5 | $0.00099 | $0.01681 |
| Sonnet 5 | $0.00040 | $0.00672 |
| Haiku 4.5 | $0.00020 | $0.00336 |
Grade A, and why
c4-model scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 3d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 249 lines — stays where its author put it; the contents beside it link to each section on GitHub.
C4 model — diagramming a software architecture
Simon Brown's C4 model (c4model.com) describes a software architecture through four hierarchical levels of abstraction. This skill covers the full cycle around a C4 diagram: design, retro-documentation, review, update.
The 4 levels
| Level | Diagram | Mermaid | Zoom | Audience |
|---|---|---|---|---|
| 1 | Context | C4Context |
System + actors / external systems | Everyone (technical and non-technical) |
| 2 | Container | C4Container |
Applications, services, internal data stores | Technical team (dev, ops, architecture) |
| 3 | Component | C4Component |
Internal components of a container | Developers of that container |
| 4 | Code | classDiagram (UML) |
Classes/functions of a component | Developers (often generated by the IDE) |
Simon Brown's golden rule: "Context + Container diagrams are sufficient for most software development teams." Only generate levels 3 and 4 when the user explicitly asks for them or when they bring genuine value (complex container, onboarding, high-risk zone). When in doubt, produce levels 1 and 2 and propose to go deeper.
C4 also supports supporting diagrams: System Landscape, Deployment, Dynamic. See supporting-diagrams.md.
Mode detection (router)
Always start by identifying the mode. If it's not obvious from the user's message, ask one question:
"Do you want to: (a) design a new architecture, (b) retro-document an existing system (code or prose), (c) review or explain an existing diagram, or (d) update a C4 that's already in place?"
Signals to infer without asking
| User signal | Mode |
|---|---|
| Path to a repo, code snippet pasted, "here's my codebase" | Document-code |
| README, ADR, spec, meeting transcript pasted | Document-prose |
| Diagram (Mermaid/PlantUML/Structurizr) pasted + "what is this?" / "is this good?" | Review |
| Existing diagram + "add/remove/change X" | Update |
| Vague idea, no artifact, "I want to design…" | Design |
What ships with it
12 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
- examples/01-context.example.md 4.7 KB
- examples/02-container.example.md 7.2 KB
- level-template.md 2.5 KB
- LICENSE.txt 1.0 KB
- mermaid-c4-syntax.md 10 KB
- mode-design.md 4.5 KB
- mode-document-code.md 4.3 KB
- mode-document-prose.md 2.5 KB
- mode-review.md 3.5 KB
- mode-update.md 2.9 KB
- review-checklist.md 3.6 KB
- supporting-diagrams.md 4.1 KB
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 3d ago First seen · 249 lines · 0 tokens per session scan A e7a43d6f25f6
c4-model is a skill published in the GitHub repository zekiriabd/SDD-Pro (191 stars, last pushed 3d ago), licensed Apache-2.0. It adds 198 tokens to every session and 3,362 once invoked, about $0.0010 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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