uml-diagramming

name	uml-diagramming
description	Produces Mermaid or PlantUML diagram code from user specifications for UML, architecture, and flow diagrams. Use when the user asks for diagram code or output for the generate_uml MCP tool. Supports Kroki-renderable output.

UML Diagramming

Generate a single, correct Mermaid or PlantUML code block from a user's description. Output only the diagram script in one code block; no prose outside it.

Positive vs negative instructions (LLM steering)

These are not “good vs bad diagrams”—they steer the LLM before diagram source is finalized:

	Positive	Negative
Role	What the model must do (DSL shape, notation).	What the model must not do (common junk).
Good	“Use sequenceDiagram.” “Participant names: Client, API, DB.”	“No ``` fences.” “No ‘Here is the diagram:’ preamble.”
Bad	Pasting the full user story again (use template / description for that).	Repeating the positive list as negatives (“do not omit participants”)—confusing and wasteful.

Keep each side short; when the provider exposes a separate negative or “must not” channel, route negative there instead of duplicating it in the main prompt.

When to Use

• User asks for a diagram (UML, architecture, flow, process, etc.)
• User specifies or implies Mermaid or PlantUML (or "diagram code")
• Task is to produce diagram script for documentation, design, or for the generate_uml MCP tool

The generate_uml tool supports all Kroki diagram types via diagram_type (e.g. mermaid, plantuml, d2, graphviz, blockdiag, bpmn, vegalite, wavedrom, etc.). Use resource uml://types for the full list and uml://templates for starter code. For non-Mermaid/PlantUML types (D2, BlockDiag, BPMN, Bytefield, Vega, WaveDrom, etc.), see references/DIAGRAM-TYPES.md.

For user intent → Kroki diagram_type (including Venn, quadrant, and timeline caveats), see the Intent → diagram_type (Kroki) section in references/DIAGRAM-TYPES.md.

Output Rules

• Emit only one code block; no explanatory text outside the block.
• Code block language: mermaid or plantuml.
• For PlantUML: script must start with @startuml and end with @enduml.
• You may use brief comments inside the block (Mermaid %% ... or PlantUML ' ...) to note assumptions.

When you are also invoking the uml-mcp generate_uml tool in the same turn, the chat response may include normal prose (URLs, errors, short explanations) outside the single fenced diagram block. This skill’s “one code block” rule applies to the diagram script portion of the answer, not to the entire MCP reply.

Choosing Mermaid vs PlantUML

• If the user says Mermaid or PlantUML, use that.
• If unspecified:
  • Prefer PlantUML for: Use Case, Deployment, Object, WBS, Gantt, Wireframe.
  • Prefer Mermaid for: Markdown/GitHub-friendly docs, quick diagrams, and when there is no strong UML requirement.

Parsing the Request

From the user's message or context, identify:

• Diagram type: Sequence, Use Case, Class, Activity, Component, State, Object, Deployment, Timing, Network, Gantt, MindMap, WBS, etc.
• Purpose: Communication, Planning, Design, Analysis, Modeling, Documentation, Implementation, Testing, Debugging.
• Elements (optional): Actors, Messages, Objects, Classes, Interfaces, Components, States, Nodes, Edges, etc.
• Target language for labels (e.g. English) if obvious.
• Optional constraints if mentioned: direction (LR/TB), detail level, max nodes, naming style, group_by.

Mermaid Type Mapping

Diagram Type	Mermaid syntax
Sequence	sequenceDiagram
Class	classDiagram
State	stateDiagram-v2
Activity	flowchart (TB)
Component, Deployment, Network	flowchart + subgraphs
Gantt	gantt
MindMap	mindmap
Use Case	flowchart (actors + use cases; no native use case in Mermaid)
Timing	sequenceDiagram with timing notes
Object	classDiagram (instances via notes) or flowchart
JSON/YAML	flowchart representing the structure (not raw JSON/YAML inside the block)

Default direction: TB. Use LR for architecture/component/deployment when it improves readability.

PlantUML Type Mapping

Diagram Type	PlantUML
Sequence	sequence diagram syntax
Use Case	usecase diagram syntax
Class	class diagram syntax
Activity	activity diagram syntax
Component	component diagram syntax
State	state diagram syntax
Object	object diagram syntax
Deployment	deployment diagram syntax
Timing	timing or sequence
Network	deployment/component (nodes + links)
Wireframe	salt (simple UI wireframes)
Gantt	gantt syntax
MindMap	mindmap syntax
WBS	wbs syntax
JSON/YAML	class/object or mindmap representing structure

Use left to right direction for architecture-heavy diagrams when it helps. Add a short title in the target language.

Quality Rules

• Choose the minimal diagram type that fits the purpose.
• Limit size: roughly <25 nodes for Mermaid, <30 for PlantUML.
• Naming: Consistent, short names; qualifiers in notes if needed.
• Grouping: Use subgraphs (Mermaid) or packages/frames (PlantUML): e.g. Client, API, Services, DB.
• Sequence: Show key messages only; use alt/opt for branches.
• Class: Include main attributes/methods; show relationships with multiplicities where known.
• State: Clear start and end; label transitions with events/guards.
• Activity: One start, one end; decisions as diamonds; label yes/no paths.
• If the request is ambiguous, make reasonable assumptions and note them in comments inside the diagram.

Process

1. Parse the prompt: extract entities, actions, relationships, lifelines, states, modules.
2. Choose Mermaid or PlantUML (see "Choosing Mermaid vs PlantUML").
3. Select the diagram form from the type mapping tables above.
4. Apply direction: default TB; LR for architecture/component/deployment/network.
5. Emit a single code block with the diagram script.
6. If the MCP generate_uml tool is available, call it with the produced diagram_type and code (e.g. diagram_type: "mermaid", "class", "sequence", "activity", "usecase" as appropriate).

Examples

Example 1 (Mermaid – login flow)

User request: "User login flow: enter credentials, API validates, DB check, return JWT or error."

sequenceDiagram
    actor User
    participant Client
    participant API
    participant DB
    User->>Client: Enter credentials
    Client->>API: Validate request
    API->>DB: Check credentials
    alt Valid
        DB-->>API: OK
        API-->>Client: JWT
        Client-->>User: Logged in
    else Invalid
        DB-->>API: Fail
        API-->>Client: Error
        Client-->>User: Show error
    end

Example 2 (PlantUML – login sequence)

User request: "Login: validate, DB check, JWT or error."

@startuml
title Login flow
actor User
participant "API" as API
database DB
User -> API : credentials
API -> DB : validate
alt valid
  DB --> API : OK
  API --> User : JWT
else invalid
  DB --> API : fail
  API --> User : error
end
@enduml

Example 3 (Mermaid – API call sequence)

User request: "Show me a Mermaid sequence diagram for an API call."

Use sequenceDiagram with participants such as Client, API, Auth, DB. Show request/response and optional alt for success/error. See uml://examples (key mermaid) or docs/diagrams/mermaid.md for sample text. Then call generate_uml("mermaid", code).

Example 4 (Mermaid – Gantt)

User request: "Generate a Gantt chart using Mermaid syntax."

Use a Mermaid gantt block with title, dateFormat, section, and tasks (with ids and durations or after). See uml://examples (key mermaid) or docs/diagrams/mermaid.md for a Gantt sample. Then call generate_uml("mermaid", code).

Convert class diagram to Mermaid

When the user asks to convert a class diagram (PlantUML or prose) into Mermaid:

1. Map each class to classDiagram syntax: class name, then lines for attributes/methods with + - #.
2. Map relationships: inheritance --|>, composition *--, aggregation o--, association -- with : label, dependency ..>.
3. Emit one Mermaid code block and call generate_uml("mermaid", code).

BPMN process model

When the user asks how to draw a BPMN process model:

• Describe core BPMN 2.0.2 elements: Start/End events, Task, Gateways (Exclusive, Parallel, Inclusive), Sequence Flow, Lanes, Pools.
• Point to the BPMN guide, uml://templates / uml://examples (key bpmn), and generate_uml with diagram_type bpmn for BPMN XML.

Additional Resources

For full diagram-type mappings and optional constraints (direction, detail_level, max_nodes, naming_style, group_by), see references/DIAGRAM-TYPES.md.