Guidance for using the C4 model, a way to describe software architecture at four levels: the overall system, its major applications and data stores, its modules, and its code. It explains what belongs in each view and how to represent the diagrams with Mermaid.

Use it to create context, container, component, or code views of a system, usually as Mermaid diagrams, when documenting architecture or making design decisions.

It removes uncertainty about how much detail an architecture diagram should contain and keeps diagrams focused on the right audience and level. It also separates the method for planning diagrams from the tool that draws them.

c4-model — Skill for Claude Code and Codex by ulises-jeremias

Install

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.

Any agent

npx skills add ulises-jeremias/agent-toolkit --skill c4-model

Clone the repo

git clone --depth 1 https://github.com/ulises-jeremias/agent-toolkit

Made for: Claude Code, Codex.

Or install agent-toolkit-complete, the plugin that ships this one along with the rest of its 116 skills.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/ulises-jeremias/agent-toolkit/c4-model.svg)](https://agentmods.dev/skills/ulises-jeremias/agent-toolkit/c4-model)

Your own site

<a href="https://agentmods.dev/skills/ulises-jeremias/agent-toolkit/c4-model"><img src="https://agentmods.dev/badge/skills/ulises-jeremias/agent-toolkit/c4-model.svg" alt="Measured on agentmods" height="20"></a>

Per session 56 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 650 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

NVIDIA SkillSpector pass 7 Sept 2026

How audits are shown

Origin original No closer match found in the catalogue.

Token cost

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	$0.00056	$0.00650
Opus 5	$0.00028	$0.00325
Sonnet 5	$0.00011	$0.00130
Haiku 4.5	$0.00006	$0.00065

Measured 8d ago against content hash 65b2d4c2f03c, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-07, from the pricing page.

Security

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 8d 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.

plugins/agent-toolkit-complete/.github/skills/c4-model/SKILL.md · 50 lines

What it actually says

C4 Model — Methodology Overlay (WHAT → HOW via Mermaid)

Sources (2026-08-12): Simon Brown C4 https://c4model.com/ (Context, Container, Component, Code); Structurizr DSL https://github.com/structurizr/dsl (open-source DSL, paid cloud) + structurizr-mcp https://github.com/cubical6/structurizr-mcp + go-structurizr; C4-PlantUML https://github.com/plantuml-stdlib/C4-PlantUML; overarch https://github.com/soulspace-org/overarch (PlantUML/Structurizr generation); rendering via Mermaid mermaid-js/mermaid MIT per §44.

Pack: architecture core (architect + ADR + TRD + threat-model + mermaid (+ C4 via mermaid)) — c4-model teaches what to draw; mermaid renders it. See docs/architecture/research-385-diagram.md for C4 vs Mermaid vs PlantUML/Structurizr decision (Excalidraw/draw.io REJECT).

When to use

Need system Context / Container / Component / Code views.

Instructions

Select level: Context (system + actors) → Container (apps/data stores) → Component (modules) → Code (classes, optional).

Render via Mermaid (C4-inspired): flowchart TD with subgraph for boundaries, e.g.:

flowchart TD
  user([User]) --> web[Web Container]
  web --> api[API Container]
  api --> db[(Database Container)]
  subgraph System Context
    user; web; api; db
  end

Advanced (optional): For consistency checks across views, use Structurizr DSL (structurizr-cli + workspace.dsl → PlantUML/Mermaid) or C4-PlantUML (@startuml + !include C4_Context.puml); note structurizr CLI requires DSL file and is heavier.

Collaboration

architect defines system → c4-model selects C4 level → mermaid renders → ADR records decision.

Anti-patterns

Do not mandate Structurizr DSL for every diagram — Mermaid suffices for most.
Do not dump PlantUML infra as separate skill — keep as optional note.

Changes

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.

8d ago First seen · 50 lines · 56 tokens per session scan A 65b2d4c2f03c

Subscribe to this mod's changes

c4-model is a skill published in the GitHub repository ulises-jeremias/agent-toolkit (16 stars, last pushed today), licensed MIT. It adds 56 tokens to every session and 650 once invoked, about $0.0003 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 2026-08-30.

Building agents Claude Code Planning agent-toolkit ai-agents ai-tools cursor github-copilot

verify

Fact-check claims encountered during reading — dates, names, events, citations. Use when encountering historical facts or disputed claims.

yha9806/academic-writing-toolkit · 26 tokens

iblai-api-agent-skill

Manage an ibl.ai agent's skills via the platform API — browse the org skill catalog, assign/unassign skills to an agent, and create/edit/delete catalog skills. Requires a connected sandbox. Use when giving an agent reusable skill instructions.

iblai/api · 55 tokens

iblai-api-agent-safety

Configure an ibl.ai agent's moderation and safety systems via the platform API — enable flags, system prompts and responses (saved through the agent settings endpoint) — and review or delete flagged-prompt moderation logs. Use when setting guardrails or auditing flagged content.

iblai/api · 59 tokens

pr-alignment-loop

Iterate a PR to its final form by running two opposing reviewer droids (reviewer-robustness and reviewer-minimalist) in a bounded back-and-forth loop. The main orchestrator synthesizes their feedback, makes edits, runs tests, and stops when both approve, on stagnation, or after 3 rounds. Use when the user asks to…

nikships/skills-registry · 122 tokens

vibeflow-quick

Fast-tracks small tasks into a prompt pack in one command. Skips discover, generates an ephemeral spec in memory. Use for well-defined planned tasks that fit in ≤4 files — not for an observed defect with reproducible evidence (that's the vibeflow-hotfix skill).

pe-menezes/vibeflow · 63 tokens

tray-carrinho-compras

API de carrinho de compras da Tray. Permite criar, consultar, atualizar e excluir carrinhos, incluindo suporte a kits de produtos. Contém 6 endpoints.

tray-tecnologia/tray-api-ai-plugin · 41 tokens