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drawio-impl-mermaid-conversion

Design & creation

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Updated 10-16 06:18

About this skill

Draw.io Mermaid Conversion

Purpose

This skill enables correct use of Mermaid diagram conversion in Draw.io. It prevents the most critical AI mistake: assuming Mermaid conversion produces individual mxCells (it does NOT — it produces a single compound shape). It covers supported diagram types, the open_drawio_mermaid MCP tool, limitations, and the decision framework for choosing between Mermaid and native mxGraph XML.

Critical Rules (Memorize These)

  1. NEVER use Mermaid when individual cell editability is required. Mermaid conversion produces a SINGLE compound shape — NOT separate mxCells. You CANNOT select, move, restyle, or connect to individual nodes after conversion.
  2. ALWAYS use native mxGraph XML for production diagrams. Mermaid is ONLY for quick visualization, prototyping, or self-contained read-only diagrams.
  3. NEVER attempt MCP CRUD operations on Mermaid-converted shapes. The drawio-mcp-server (lgazo) edit_cell, delete_cell, move_cell, and restyle tools will NOT work on individual nodes inside a Mermaid compound shape.
  4. ALWAYS validate Mermaid syntax before calling open_drawio_mermaid. Invalid syntax produces a blank diagram with no error feedback.
  5. NEVER mix Mermaid compound shapes with native mxCells on the same canvas when programmatic manipulation is needed — the Mermaid block cannot interact with native cells.
  6. ALWAYS use the content parameter (not data or xml) when calling open_drawio_mermaid.

The Compound Shape Problem

This is the single most important fact about Mermaid in Draw.io:

Mermaid text --> Draw.io renders --> ONE compound shape (grouped SVG)
                                     NOT individual <mxCell> elements
Capability Native mxGraph XML Mermaid Import
Individual cell access YES NO
Style editing per shape YES NO
Programmatic connections YES NO
MCP server CRUD YES NO
Pixel-level positioning YES NO
Layout control Full Mermaid engine decides
Source readability Low (XML) High (text)
Generation speed Slower Faster

Decision Tree: Mermaid vs Native XML

Will individual shapes be edited after creation?
  YES --> Use native mxGraph XML. STOP.
  NO  --> Continue.

Will MCP server tools manipulate the diagram?
  YES --> Use native mxGraph XML. STOP.
  NO  --> Continue.

Does the diagram need precise pixel positioning?
  YES --> Use native mxGraph XML. STOP.
  NO  --> Continue.

Must shapes connect to other elements on the canvas?
  YES --> Use native mxGraph XML. STOP.
  NO  --> Continue.

Is this a quick prototype or read-only visualization?
  YES --> Mermaid is acceptable.
  NO  --> Use native mxGraph XML.

Summary: If ANY of the first four questions is YES, ALWAYS use native mxGraph XML.

Supported Mermaid Diagram Types

Mermaid Type Keyword Best For
Flowchart graph or flowchart Process flows, decision trees
Sequence Diagram sequenceDiagram API calls, message passing
Class Diagram classDiagram OOP structure, UML class diagrams
State Diagram stateDiagram or stateDiagram-v2 State machines, lifecycle flows
ER Diagram erDiagram Database schemas, entity relationships
Gantt Chart gantt Project timelines, scheduling
Pie Chart pie Proportional data visualization
Git Graph gitgraph Branch and merge visualization
Mindmap mindmap Hierarchical brainstorming
Requirement Diagram requirementDiagram Requirements traceability
User Journey journey User experience mapping
C4 Context C4Context Software architecture (C4 model)
Quadrant Chart quadrantChart 2x2 matrix analysis

MCP Tool: open_drawio_mermaid

The official @drawio/mcp server (by jgraph) provides open_drawio_mermaid.

Parameters

Parameter Type Required Description
content string YES The Mermaid diagram definition
dark string NO Dark mode: "auto", "true", "false". Default: "auto"
lightbox boolean NO Read-only view mode. Default: false

Usage Pattern

open_drawio_mermaid(
  content: "graph TD\n  A[Start] --> B{Decision}\n  B -->|Yes| C[Action]\n  B -->|No| D[End]"
)

What Happens After the Call

  1. Draw.io editor opens in the browser
  2. The Mermaid text is rendered as a single compound shape
  3. The user sees the diagram but CANNOT select individual nodes
  4. To edit: select the shape, press Enter, modify the Mermaid source text

Mermaid Syntax Quick Reference

Flowchart

graph TD
  A[Rectangle] --> B{Diamond}
  B -->|Yes| C[Process]
  B -->|No| D[End]
  C --> D

Direction keywords: TD (top-down), LR (left-right), BT (bottom-top), RL (right-left).

Node shapes: [text] rectangle, {text} diamond, (text) rounded, ([text]) stadium, [[text]] subroutine, [(text)] cylinder, ((text)) circle, >text] asymmetric.

Sequence Diagram

sequenceDiagram
  participant A as Client
  participant B as Server
  A->>B: Request
  B-->>A: Response

Arrow types: ->> solid with arrowhead, -->> dashed with arrowhead, -x solid with cross, --x dashed with cross.

Class Diagram

classDiagram
  class Animal {
    +String name
    +int age
    +makeSound() void
  }
  Animal <|-- Dog
  Animal <|-- Cat

State Diagram

stateDiagram-v2
  [*] --> Idle
  Idle --> Processing : start
  Processing --> Done : complete
  Processing --> Error : fail
  Done --> [*]

ER Diagram

erDiagram
  CUSTOMER ||--o{ ORDER : places
  ORDER ||--|{ LINE-ITEM : contains
  PRODUCT ||--o{ LINE-ITEM : "is in"

Cardinality: || exactly one, o| zero or one, }| one or more, }o zero or more.

Gantt Chart

gantt
  title Project Plan
  dateFormat YYYY-MM-DD
  section Phase 1
  Task A :a1, 2024-01-01, 30d
  Task B :after a1, 20d

Mindmap

mindmap
  root((Central Topic))
    Branch A
      Leaf 1
      Leaf 2
    Branch B
      Leaf 3

Conversion Methods (Beyond MCP)

Method 1: URL Parameter

{"type": "mermaid", "compressed": true, "data": "BASE64_DEFLATED_MERMAID_TEXT"}

Method 2: Draw.io UI

Arrange > Insert > Mermaid or the + toolbar icon > Mermaid.

Method 3: Embed Mode postMessage

{"action": "load", "descriptor": {"format": "mermaid", "data": "graph TD; A-->B;"}}

Method 4: ELK Layout Option

Draw.io supports the Mermaid ELK layout engine for more compact flowcharts. This is activated within the Mermaid source by using %%{init: {'flowchart': {'defaultRenderer': 'elk'}}}%% at the top.

Workarounds for the Compound Shape Limitation

When you need BOTH Mermaid readability AND individual cell control:

Workaround 1: Mermaid as Specification, XML as Output

  1. Write the diagram logic in Mermaid syntax (for human review)
  2. Translate the Mermaid structure into native mxGraph XML
  3. Generate the XML with individual mxCells for each node and edge
  4. This gives you readable specification AND full editability

Workaround 2: Two-Phase Approach

  1. Use open_drawio_mermaid for initial visualization and stakeholder review
  2. Once approved, recreate the diagram in native mxGraph XML for production use
  3. Discard the Mermaid version

Workaround 3: Side-by-Side on Separate Pages

  1. Put the Mermaid compound shape on Page 1 (reference view)
  2. Put the native mxGraph XML version on Page 2 (editable version)
  3. Maintain both as needed

NEVER attempt to "ungroup" or "explode" a Mermaid compound shape into individual cells. This is NOT supported by Draw.io.

Reference Links