A writing guide for seqdiag, a notation for sequence diagrams that show messages between participants over time.

Use it to map interactions between clients, servers, browsers, and other components, with output available as PNG, SVG, or PDF through uml-mcp.

It provides the syntax needed to turn a request-and-response flow into a diagram source file.

seqdiag — 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 seqdiag

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

Your own site

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

Per session 54 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 408 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 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	$0.00054	$0.00408
Opus 5.5	$0.00022	$0.00163
Sonnet 5.5	$0.00011	$0.00082
Haiku 4.5	$0.00005	$0.00041

Measured 14d ago against content hash 95290f6dafeb, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

seqdiag 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.

.skill/skills/seqdiag/SKILL.md · 51 lines

What it actually says

seqdiag diagram (uml-mcp)

diagram_type: seqdiag
Backend: seqdiag
Output formats: png, svg, pdf

Sequence diagrams (blockdiag family)

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.

Styling

Participant order left-to-right; keep messages short.
No MCP theme param.

Starter template

seqdiag {
  client -> server [label = "Request"];
  server <-- client [label = "Response"];
}

Example

seqdiag {
  browser  -> webserver [label = "GET /seqdiag/svg/base64"];
  webserver  -> processor [label = "Convert text to image"];
  webserver <-- processor;
  browser <-- webserver;
}

Render with uml-mcp

Set diagram_type to seqdiag exactly.
Call validate_uml when the source is untrusted (prefer strict: true for Mermaid sequence).
Call generate_uml, generate_uml_image (chat PNG), or generate_uml_batch.
Pass theme when this skill's Themes section lists supported values.
Surface the returned url / playground (see the workflow skill).

Confirm formats with uml://formats or list_diagram_types when unsure.

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 · 51 lines · 54 tokens per session scan A 95290f6dafeb

Subscribe to this mod's changes

seqdiag is a skill published in the GitHub repository antoinebou12/uml-mcp (109 stars, last pushed yesterday), licensed MIT. It adds 54 tokens to every session and 408 once invoked, about $0.0002 per session on Opus 5.5. 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 2026-10-24.

mcp uml

seqdiag next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
seqdiagantoinebou12/uml-mcp	Skill	109	yesterday	MIT	A	54	Claude Code, Codex
detailed-designpan94u/forge	Skill	40	5d ago	none	A	27	Claude Code
writing-skillstakaakit/superpowers-uml	Skill	12	1mo ago	none	A	20	Claude Code
brainstormingtakaakit/superpowers-uml	Skill	12	1mo ago	none	A	37	Claude Code
finishing-a-development-branchtakaakit/superpowers-uml	Skill	12	1mo ago	none	A	27	Claude Code
systematic-debuggingtakaakit/superpowers-uml	Skill	12	1mo ago	none	A	21	Claude Code
test-driven-developmenttakaakit/superpowers-uml	Skill	12	1mo ago	none	A	17	Claude Code