plantuml-skill

SKILL.md

PlantUML Diagram Skill

Overview

Generate .puml PlantUML diagram files and export to PNG/SVG using Kroki — a cloud rendering API that requires no local installation beyond curl.

Format: .puml (PlantUML text)
Renderer: Kroki API (https://kroki.io) — just curl, no Java needed
Output: PNG, SVG
Diagram types: sequence, component, class, ER, activity, use case, state, C4, and more

When to Use

Explicit triggers:

"plantuml diagram", "sequence diagram", "class diagram", "component diagram"

"UML", "activity diagram", "use case diagram", "state machine"

"visualize", "draw", "diagram", "flowchart", "architecture chart"

Proactive triggers:

Explaining a system with 3+ interacting components

Describing API flows, authentication sequences, message passing

Showing class hierarchies, database schemas, or ER models

Illustrating state machines or lifecycle flows

When NOT to use it — route elsewhere:

General, non-UML quick diagrams embedded in Markdown → mermaid.

Freeform, heavily-styled, or branded diagrams needing pixel control → drawio.

A hand-drawn / sketchy look → excalidraw or tldraw.

Modes

Once triggered, route by what the user actually wants — then run the shared render loop (Steps 4–8):

Mode
The user wants…
Entry point

Generate (default)
a diagram from a text description
Steps 1–8 below

From code
a diagram of existing source code
references/from-source-code.md → Steps 4–8

Embed
the PlantUML inside a Markdown doc rendered to images
references/markdown-embed.md

Refine
to change an existing diagram
load its .puml, apply the minimal edit (Step 7), re-render (Steps 4–6)

Review
to know whether an existing diagram is readable / correct
run the Step 6 vision self-check on the image

Prerequisites

Option A: Kroki API (recommended — no install)

# Just needs curl (pre-installed on macOS/Linux/Windows Git Bash)
curl --version

Option B: Local Kroki via Docker (for offline use)

docker run -d -p 8000:8000 yuzutech/kroki
# Then replace https://kroki.io with http://localhost:8000 in commands

Option C: Local PlantUML jar (traditional)

# Requires Java + Graphviz
brew install graphviz   # macOS
sudo apt install graphviz  # Ubuntu
# Download plantuml.jar from https://plantuml.com/download
java -jar plantuml.jar diagram.puml

Workflow

Step 1: Check Dependencies

curl --version

curl is available on all modern systems. If missing, install via package manager.

Step 2: Pick Diagram Type

Choose the most appropriate PlantUML diagram type (see reference below).

Step 3: Generate .puml File

Write the PlantUML source file with @startuml / @enduml markers.

Step 4: Export via Kroki (capture the HTTP status)

Pick the backend first. The default below (public Kroki) uploads the .puml source to kroki.io — for sensitive diagrams use a local backend instead, and never silently fall back. See references/rendering-backends.md. For local Kroki, swap https://kroki.io → http://localhost:8000.

# PNG (recommended) — keep the status code so Step 5 can verify it
http=$(curl -s -w "%{http_code}" -o diagram.png \
  -X POST https://kroki.io/plantuml/png \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.puml")
echo "HTTP $http"

# SVG
http=$(curl -s -w "%{http_code}" -o diagram.svg \
  -X POST https://kroki.io/plantuml/svg \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.puml")
echo "HTTP $http"

Step 5: Validate & self-correct (loop — do NOT skip)

Never report success on a blind curl. Verify the output first; treat the export as failed if any of these hold:

$http is not 200. Kroki returns 400 on a syntax error and writes the error text into the output file, so a .png can exist yet be broken.

The file is empty: [ -s diagram.png ] fails.

The bytes aren't a real image: file diagram.png should report PNG image data; for SVG the file should start with <svg or <?xml.

if [ "$http" != "200" ] || [ ! -s diagram.png ]; then
  echo "Render failed — Kroki said:"
  cat diagram.png    # the 400 body holds the offending line + reason
fi

On failure: cat the output file to read Kroki's error, fix the flagged .puml line (see Common Mistakes), then re-run Step 4. Repeat up to 3 times. If a targeted line fix doesn't clear it, degrade in this order, re-rendering after each step — stop as soon as it renders:

remove exotic shapes → plain rectangle/component/node

strip skinparam / !theme (render plain first)

remove note lines

simplify labels, wrap in "…"

reduce edges

switch to a simpler diagram type rather than forcing the current one

For a per-diagram-type error catalog and the Kroki safe subset, read references/kroki-troubleshooting.md. If it still fails after 3 tries, stop and show the user the raw Kroki error — do not claim the diagram was produced.

Step 6: Self-check (vision)

The Step 5 loop only proves Kroki returned a valid image — not that the diagram is readable. After it renders, use the agent's vision capability to read the PNG and catch what auto-layout (Graphviz) can't prevent. PlantUML positions everything itself, so the failures here are about readability, not your coordinates:

Check
What to look for
Fix

Label truncation / overrun
Text clipped or spilling past a box
Shorten the label, wrap in "…", or break with \n

Component overlap / cramped
Boxes touching or crowded; unreadable
Add together { }, layout hints, or split the diagram

Wrong orientation / aspect
Diagram far too wide or too tall to read
Switch left to right direction ↔ top to bottom direction

Edge spaghetti
Many relations crossing, hard to follow
Reorder declarations, group with package/together, or add hidden edges for layout

Wrong diagram type
Type doesn't suit the content
Switch type (sequence, state, C4, …)

Low contrast
Text blends into the fill / theme
Adjust skinparam / !theme so text contrasts the fill

Max 2 self-check rounds — if issues remain after 2 fixes, show the user anyway.

Re-render (Step 4) and re-validate (Step 5) after every fix.

If vision is unavailable, skip self-check and show the PNG directly.

Step 7: Review loop

After self-check, show the exported image and collect feedback. Apply the minimal .puml edit for each request, then re-render and re-validate:

User request
Edit action

Change a label
Edit the element / message text in the .puml

Add / remove an element or relation
Add or delete the matching line

Change a color
skinparam, !theme, or an inline #color on the element

Change layout direction
Swap left to right direction ↔ top to bottom direction

Restructure / group
Wrap related elements in a package / together { }, or regenerate

Overwrite the same diagram.puml / output file each round — don't create v1, v2, …

Safety valve: after 5 rounds, suggest the user fine-tune the .puml directly or at plantuml.com.

Step 8: Report to User

Only after Steps 5–7 pass. Tell the user:

Path to the .puml source file

Path to the exported PNG/SVG

Brief description of what was generated

Which backend rendered it, and whether the source left the machine — e.g. "via public Kroki (uploaded to kroki.io)" vs "via local Kroki (stayed local)"

Import Workflows

Two non-default modes — load the linked playbook when triggered, then run the same Step 4–8 loop:

Generate a diagram from existing source code — class diagram of a module, sequence from a request handler, component map of a repo, ER from ORM models. Read the code, extract the real entities/relationships, draw only what's there. → references/from-source-code.md

Render PlantUML embedded in Markdown — extract ```plantuml / ```puml blocks (and linked .puml), render each to an image, and rewrite the Markdown with image links (e.g. to publish to Confluence / Notion, which don't render fenced PlantUML). → references/markdown-embed.md

Diagram Types

Type
Keyword
Use for

Sequence
@startuml + sequence syntax
API calls, protocol flows, message passing

Component
@startuml + components
service architecture, module dependencies

Class
@startuml + class syntax
OOP models, data structures

ER / Entity
@startuml + entity syntax
database schemas

Activity
@startuml + activity syntax
workflows, business processes

Use Case
@startuml + actor/usecase
system requirements, user stories

State
@startuml + state syntax
state machines, lifecycle

C4 Context
@startuml + C4 includes
high-level system context maps

Mind Map
@startmindmap
topic breakdowns, concept maps

Gantt
@startgantt
project timelines, schedules

Syntax Reference

Component / Architecture Diagram

@startuml
!theme plain

title Microservices Architecture

actor "Client" as client
rectangle "API Gateway" as gateway #LightBlue

rectangle "Services" {
  component "User Service" as user
  component "Order Service" as order
}

database "User DB" as userdb
database "Order DB" as orderdb
queue "Kafka" as kafka

client --> gateway
gateway --> user
gateway --> order
user --> userdb
order --> orderdb
order --> kafka : events

@enduml

Shape types:

actor "Name" as id — stick figure (user, external actor)

component "Name" as id — component box with [brackets]

rectangle "Name" as id — plain rectangle (for groups/layers)

database "Name" as id — cylinder (database)

queue "Name" as id — queue symbol

cloud "Name" as id — cloud shape (external services)

node "Name" as id — server/node box

frame "Name" as id — frame grouping

package "Name" { } — package grouping

Arrows:

A --> B — solid arrow

A -> B — thin arrow

A ..> B — dashed arrow

A --> B : label — labeled arrow

A <--> B — bidirectional

Colors:

#LightBlue, #LightGreen, #LightYellow, #Pink, #Violet

#AED6F1 (blue), #A9DFBF (green), #FAD7A0 (orange), #F1948A (red)

#D7BDE2 (purple), #F9E79F (yellow), #D3D3D3 (grey)

Sequence Diagram

@startuml
!theme plain
title Login Flow

participant "Client" as C
participant "API Gateway" as G
participant "Auth Service" as A
database "User DB" as D

C -> G : POST /login
G -> A : validateCredentials(user, pass)
A -> D : SELECT * FROM users WHERE email = ?
D --> A : user record
A --> G : 200 OK + JWT token
G --> C : { token: "..." }

@enduml

Arrow types:

A -> B — synchronous call

A --> B — return / dashed

A ->> B — async message

A -[#red]-> B — colored arrow

activate A / deactivate A — show activation box

Class Diagram

@startuml
!theme plain

class User {
  +int id
  +String name
  +String email
  +login() : bool
  +logout()
}

class Order {
  +int id
  +Date createdAt
  +float total
  +place()
  +cancel()
}

class Product {
  +int id
  +String name
  +float price
}

User "1" --> "*" Order : places
Order "*" --> "*" Product : contains

@enduml

Relationships:

A --> B — association

A --|> B — inheritance

A ..|> B — implements interface

A *-- B — composition

A o-- B — aggregation

A "1" --> "*" B : label — with multiplicities

ER Diagram

@startuml
!theme plain

entity "USER" as user {
  * id : int <<PK>>
  --
  name : varchar
  email : varchar
  created_at : datetime
}

entity "ORDER" as ord {
  * id : int <<PK>>
  --
  * user_id : int <<FK>>
  total : decimal
  status : varchar
}

entity "PRODUCT" as prod {
  * id : int <<PK>>
  --
  name : varchar
  price : decimal
}

user ||--o{ ord : places
ord }o--|{ prod : contains

@enduml

Activity / Flowchart

@startuml
!theme plain

start

:Receive Order;

if (Payment valid?) then (yes)
  :Process Payment;
  :Send Confirmation Email;
  :Update Inventory;
  :Ship Order;
  :Mark as Delivered;
else (no)
  :Send Payment Failed Email;
  :Cancel Order;
endif

stop

@enduml

State Diagram

@startuml
!theme plain

[*] --> Pending

Pending --> Processing : payment_received
Processing --> Shipped : packed
Shipped --> Delivered : confirmed
Processing --> Cancelled : cancel
Pending --> Cancelled : cancel

Delivered --> [*]
Cancelled --> [*]

@enduml

C4 Context Diagram

C4 uses the bundled C4-PlantUML standard library via !include <C4/...>, which Kroki and recent local jars resolve with no network fetch. Export with the standard plantuml endpoint (the c4plantuml Kroki type also works).

@startuml
!include <C4/C4_Context>

title System Context — Internet Banking

Person(customer, "Banking Customer", "A customer of the bank")
System(banking, "Internet Banking System", "Lets customers view their accounts")
System_Ext(mail, "E-mail System", "The internal Microsoft Exchange system")

Rel(customer, banking, "Uses", "HTTPS")
Rel(banking, mail, "Sends e-mail via", "SMTP")
@enduml

Other levels: <C4/C4_Container> (Container, ContainerDb), <C4/C4_Component> (Component). Common macros: Person, System, System_Ext, Container, Rel, Boundary. Do not use a remote !includeurl https://… — Kroki cannot fetch external URLs; always use the bundled <C4/…> form.

Export Commands

Quick reference for the renderer variants. The Kroki ones drop the status capture for brevity — when actually exporting, use the Step 4 form and run the Step 5 validation loop.

# PNG via Kroki API (recommended)
curl -s -X POST https://kroki.io/plantuml/png \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.puml" \
  -o diagram.png

# SVG via Kroki API
curl -s -X POST https://kroki.io/plantuml/svg \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.puml" \
  -o diagram.svg

# Via local Kroki Docker (offline)
curl -s -X POST http://localhost:8000/plantuml/png \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.puml" \
  -o diagram.png

# Via local PlantUML jar (if installed)
java -jar plantuml.jar diagram.puml
# Output: diagram.png in same directory

Themes

!theme plain       ← clean, minimal (recommended)
!theme cerulean    ← blue-tinted
!theme blueprint   ← dark blue background
!theme aws-orange  ← AWS style
!theme vibrant     ← vivid colors

Or use skinparam for custom styling:

skinparam backgroundColor #FAFAFA
skinparam componentBorderColor #555555
skinparam ArrowColor #333333
skinparam FontName Arial

Common Mistakes

Quick table below; for a per-diagram-type error catalog, the Kroki safe subset, and the failure-degradation ladder, see references/kroki-troubleshooting.md.

Mistake
Fix

curl POST returns HTML error page
Check network; try curl -v to see error details

Kroki returns 400 Bad Request
cat the output file — Kroki wrote the offending line + reason there; fix it and re-render via the Step 5 loop. Validate syntax at https://www.plantuml.com/plantuml/uml/

Arrow direction unexpected
Use --> for downward/right; explicitly use -up->, -down->, -left->, -right->

Diagram too large/crowded
Split into multiple diagrams or use package/rectangle grouping

Missing @startuml / @enduml
Always wrap diagram in these markers

Special chars in labels
Wrap in quotes: "Label: value"

C4 includes not found
Use the bundled !include <C4/C4_Context> (resolved on the standard plantuml endpoint and c4plantuml); never a remote !includeurl https://… — Kroki cannot fetch external URLs

Component overlap
Use together { } or explicit layout hints (top to bottom direction)

Sequence participants out of order
Declare participant explicitly at top in desired left-to-right order