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.
This skill ships reference material, not runnable scripts. Read the relevant reference file with Read, apply the syntax, and write the diagram into the user's artefact. Do not Bash-execute anything from references/ — they are PlantUML/C4 syntax docs, not commands.
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.
