plantuml-syntax

PlantUML Syntax Reference

A comprehensive reference for PlantUML diagram types with a focus on C4-PlantUML for architecture diagrams. This skill provides syntax documentation adapted from the SpillwaveSolutions/plantuml project, supplemented with ArcKit-specific C4 layout conflict rules and best practices.

Supported Diagram Types

Select the appropriate diagram type and read the corresponding reference file:

Type	Reference	ArcKit Commands Using It
C4-PlantUML	c4-plantuml.md	/arckit.diagram (C4 Context, Container, Component)
Sequence Diagram	sequence-diagrams.md	/arckit.diagram (Sequence mode)
Class Diagram	class-diagrams.md	—
Activity Diagram	activity-diagrams.md	—
State Diagram	state-diagrams.md	—
ER Diagram	er-diagrams.md	—
Component Diagram	component-diagrams.md	—
Use Case Diagram	use-case-diagrams.md	—
Deployment Diagram	deployment-diagrams.md	—

Styling & Errors

Topic	Reference
Common Syntax Errors	common-syntax-errors.md
Styling Guide	styling-guide.md

C4-PlantUML (Primary Use Case)

The C4-PlantUML reference is the most important file for ArcKit users. It covers:

• Include URLs for C4 libraries
• Element syntax (Person, System, Container, Component, and their variants)
• Boundary syntax (System_Boundary, Container_Boundary)
• Directional relationships (Rel_Down, Rel_Right, Rel_Up, Rel_Left, Rel_Neighbor)
• Layout constraints (Lay_Right, Lay_Down, Lay_Distance)
• LAYOUT CONFLICT RULES — critical rules to prevent rendering failures when layout hints contradict relationship directions
• Tier-based layout patterns
• Worked examples for Context, Container, and Component diagrams

For C4 layout science (Sugiyama algorithm, edge crossing targets, declaration ordering), also see the Mermaid skill's c4-layout-science.md — Section 7 covers PlantUML directional hints.

Common Syntax Gotchas

These are the most common PlantUML syntax errors encountered when generating diagrams:

Gotcha	Problem	Fix
Rel_Down contradicts Lay_Right	Layout engine receives conflicting direction hints for the same element pair	Ensure every Rel_* direction is consistent with any Lay_* constraint on the same pair
Missing @startuml/@enduml	Diagram fails to render entirely	Always wrap PlantUML code in @startuml and @enduml
Wrong !include URL	C4 macros not found, syntax errors on Person/System/Container	Use exact URL from plantuml-stdlib: https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Context.puml
Generic Rel instead of directional	Layout engine places elements randomly without direction hints	Always use Rel_Down, Rel_Right, etc. instead of plain Rel
Missing element declaration	Relationship references an undeclared element ID	Declare ALL elements before ANY relationships
Spaces in element IDs	Parser fails on IDs with spaces or special characters	Use camelCase or underscores: paymentApi, payment_api
Nested boundaries without content	Empty boundaries may cause rendering errors	Ensure every boundary contains at least one element
\n in descriptions	Expects literal \n text but PlantUML renders it as a line break	This is expected behavior — PlantUML interprets \n as line breaks natively. Use \\n if literal text is needed

ArcKit Integration

This skill handles conversational PlantUML syntax questions — quick lookups, syntax examples, troubleshooting rendering issues, and learning about diagram types.

For formal architecture diagram generation with document control, project integration, layout science, and governance compliance, use the /arckit:diagram command instead. It generates versioned diagram artifacts saved to your project directory with full traceability to requirements and architecture principles.