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

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 0. Update check (notify, don't pull) — first use per conversation

Throttle to one check per 24 hours per installation; never mutate the skill directory without explicit user consent.

1. If <this-skill-dir>/.last_update exists and is less than 24 hours old, skip this step entirely.

2. Otherwise, fetch the latest tag from upstream:

   git -C <this-skill-dir> ls-remote --tags origin 'v*' 2>/dev/null \
     | awk '{print $2}' | sed 's|refs/tags/||' \
     | sort -V | tail -1
   

3. Compare with this skill's metadata.version from the frontmatter. If the upstream tag is strictly newer (semver), tell the user one line and ask:

   "A newer version of this skill is available: vX.Y.Z → vA.B.C. Want me to git pull?"

   If they say yes, run git -C <this-skill-dir> pull --ff-only. Refresh .last_update either way so the prompt doesn't repeat for 24 hours.

4. If upstream is the same or older, refresh .last_update silently and continue.

5. On any failure (offline, not a git checkout — e.g. ClawHub-installed copy, read-only path, no permission), swallow the error silently and continue with the user's task. Do not mention the failure.

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

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

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

Step 5: Report to User

Tell the user:

• Path to the .puml source file
• Path to the exported PNG/SVG
• Brief description of what was generated

---

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

---

Export Commands

# 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

Mistake	Fix
curl POST returns HTML error page	Check network; try curl -v to see error details
Kroki returns 400 Bad Request	Validate PlantUML 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 via Kroki	Use Kroki's c4plantuml diagram type instead of plantuml for C4 diagrams
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