A writing guide for C4 PlantUML diagrams. C4 diagrams show software systems at different levels, such as the people and systems around an application or the containers inside it.

It is for creating context and container views in C4 notation, with output that can be rendered as PNG, SVG, PDF, text, or base64.

It helps the agent produce diagram text in the expected notation instead of using an incompatible format. The text can then be validated or rendered through uml-mcp.

c4plantuml — Skill for Claude Code and Codex by antoinebou12/uml-mcp

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 antoinebou12/uml-mcp --skill c4plantuml

Clone the repo

git clone --depth 1 https://github.com/antoinebou12/uml-mcp

Made for: Claude Code, Codex.

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/antoinebou12/uml-mcp/c4plantuml/github.svg)](https://agentmods.dev/skills/antoinebou12/uml-mcp/c4plantuml)

Your own site

<a href="https://agentmods.dev/skills/antoinebou12/uml-mcp/c4plantuml"><img src="https://agentmods.dev/badge/skills/antoinebou12/uml-mcp/c4plantuml/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/antoinebou12/uml-mcp/c4plantuml"><img src="https://agentmods.dev/badge/skills/antoinebou12/uml-mcp/c4plantuml.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 62 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 775 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.

Origin 84% copy Near-identical to another mod 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	$0.00062	$0.00775
Opus 5.5	$0.00025	$0.00310
Sonnet 5.5	$0.00012	$0.00155
Haiku 4.5	$0.00006	$0.00077

Measured 14d ago against content hash 9686f1a2f0ae, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

c4plantuml 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 14d 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.

Origin

This is a copy

84% identical to activity — 65 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

.skill/skills/c4plantuml/SKILL.md · 77 lines

How it starts

The opening of the file, as written. The whole thing — 77 lines — stays where its author put it; the contents beside it link to each section on GitHub.

c4plantuml diagram (uml-mcp)

diagram_type: c4plantuml
Backend: c4plantuml
Output formats: png, svg, pdf, txt, base64

C4 model diagrams using PlantUML

Load the workflow skill uml-mcp-diagrams for URL-first replies, progressive discovery, and which MCP tool to call. This file is notation only; it does not add an MCP tool.

Notation tips

Use PlantUML C4 includes; pick context or container view.
Keep relationships labeled; stay under ~30 elements or split views.

Themes

Pass theme to generate_uml / generate_uml_image (PlantUML built-in name). The server injects !theme <name> after @startuml when the source has no !theme yet.

Recommended: cerulean, materia, sketchy, cyborg, plain, aws-orange, blueprint, sandstone.

Match the page background (also see MCP prompt diagram_theme):

light / white: cerulean, materia, plain, sketchy, sandstone
dark / black: blueprint, cyborg, black-knight, carbon-gray, crt-green
B&W / print: plain, sketchy, sketchy-outline, mimeograph, lightgray

Or embed in source:

@startuml
!theme cerulean
' … diagram …
@enduml

Starter template

!include <C4/C4_Context>
title System Context
Person(user, "User")
System(system, "My System")
Rel(user, system, "Uses")

Example

!include <C4/C4_Context>
title System Context diagram for Internet Banking System
Person(customer, "Banking Customer", "A customer of the bank, with personal bank accounts.")
System(banking_system, "Internet Banking System", "Allows customers to check their accounts.")
System_Ext(mail_system, "E-mail system", "The internal Microsoft Exchange e-mail system.")
System_Ext(mainframe, "Mainframe Banking System", "Stores all of the core banking information.")
Rel(customer, banking_system, "Uses")
Rel_Back(customer, mail_system, "Sends e-mails to")
Rel_Neighbor(banking_system, mail_system, "Sends e-mails", "SMTP")
Rel(banking_system, mainframe, "Uses")

Read the full file on GitHub · 77 lines

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.

14d ago First seen · 77 lines · 62 tokens per session scan A 9686f1a2f0ae

Subscribe to this mod's changes

c4plantuml is a skill published in the GitHub repository antoinebou12/uml-mcp (109 stars, last pushed yesterday), licensed MIT. It adds 62 tokens to every session and 775 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. It is 84% identical to activity, differing in 65 lines, and is treated as a copy.

Design and UX mcp uml

c4plantuml next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
c4plantumlantoinebou12/uml-mcp	Skill	109	yesterday	MIT	A	62	Claude Code, Codex
detailed-designpan94u/forge	Skill	40	5d ago	none	A	27	Claude Code
architecture-diagram-creatorbookforge-ai/bookforge-skills	Skill	42	6mo ago	CC-BY-SA-4.0	A	128	Claude Code
webgl-holographic-foilnexu-io/open-design	Skill	106k	yesterday	Apache-2.0	A	41	Claude Code, Codex
html-ppt-hermes-cyber-terminalnexu-io/open-design	Skill	106k	yesterday	Apache-2.0	A	53	Claude Code, Codex
html-ppt-taste-brutalistnexu-io/open-design	Skill	106k	yesterday	Apache-2.0	A	78	Claude Code, Codex
visual-ralphYeachan-Heo/oh-my-codex	Skill	34k	3d ago	MIT	A	52	Codex