Architecture Diagrams (PlantUML)

Generates physical diagram artifacts via the public PlantUML server — no Java required. Renders C4 models, dense class diagrams, and network topologies that Mermaid cannot express well.

Tool boundary — Mermaid vs PlantUML

Mermaid for diagrams embedded in Markdown — sequence, flow, state, gitgraph, mindmap, ER — quick logic inside docs, wikis (obsidian-workflow), and reports. Validated inline by the validate-mermaid hook.

PlantUML exclusively for physical artifacts (.svg/.png files):

• C4 models (Context / Container / Component / Deployment)
• Dense class diagrams (many entities, relations, generics)
• Network / deployment topology

If the user wants a diagram inside a Markdown doc → Mermaid. If they want a file they can open, print, or attach → PlantUML via this skill.

Where it is

• Renderer: extensions/diagram-tools/plantuml_renderer.py (installed at %APPDATA%\devin\extensions\diagram-tools\) — stdlib-only; Deflate + PlantUML modified-Base64 + GET to plantuml.com/plantuml/{svg,png}/.

Flow

1. Write the PlantUML source to a file (e.g. .devin/scratch/diagram.puml or a path the user chooses). Never render from a heredoc — keep the source file as the editable artifact.

2. Syntax sanity before the network (mandatory):

   python extensions/diagram-tools/plantuml_renderer.py --check diagram.puml

   Fix all reported errors before proceeding.

3. Render:

   python extensions/diagram-tools/plantuml_renderer.py diagram.puml            # .svg
   python extensions/diagram-tools/plantuml_renderer.py diagram.puml --format png

   Verify ok: true in the JSON output, then open the artifact for the user.

4. If the server returns a syntax-error image (non-SVG body), read the error it reports, fix the source, re-check, re-render.

C4 model rules

Use the C4-PlantUML stdlib — always remote-include, never vendor:

@startuml
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml

Person(user, "User", "End user")
System_Boundary(sys, "System") {
  Container(app, "App", "Python", "Does X")
  ContainerDb(db, "DB", "SQLite", "Stores Y")
}
Rel(user, app, "Uses")
Rel(app, db, "Reads/Writes")
@enduml

• Layer !include by depth: C4_Context.puml ⊂ C4_Container.puml ⊂ C4_Component.puml; C4_Deployment.puml for topology.
• One diagram per abstraction level — do not mix Context and Component.
• Every element gets alias, label, technology, description (4 args).
• Rel(a, b, "verb") for every arrow; direction = call order.
• Keep aliases stable across levels so diagrams stay cross-referenceable.

Gotchas

• The public server requires network — if offline, say so and keep the .puml source; it renders later unchanged.
• Server-side syntax errors return an image containing the error — the script detects this and exits non-zero. Treat it as a fix-and-retry.
• PlantUML is whitespace/keyword-sensitive: @enduml (not @end uml), arrows are --> / ..> not ->.
• Large C4 models hit URL limits (~8KB encoded); split into per-level files.