---

name: explain-code description: Explain source code using structured text explanations and Excalidraw diagrams. Use this skill when users ask to visualize code, explain code architecture, create class diagrams, flow diagrams, ER diagrams, feature diagrams, component diagrams, sequence diagrams, or any software engineering visualization from source code. Also use when documenting codebases, understanding complex logic, debugging workflows, or creating technical documentation. Outputs .excalidraw JSON files editable at excalidraw.com or embeddable via @excalidraw/excalidraw npm package.

Code Visualizer with Excalidraw

Explain source code through structured text explanations and hand-drawn style Excalidraw diagrams.

About Excalidraw

• Open source at excalidraw.com and GitHub
• Hand-drawn/sketchy aesthetic
• Real-time collaboration support
• Browser-based, no sign-up required
• .excalidraw JSON format
• Embeddable via @excalidraw/excalidraw npm package

Workflow

1. Receive code input with optional context
2. Analyze code structure, patterns, and flow
3. Select appropriate diagram type(s)
4. Generate text explanation with key insights
5. Create .excalidraw JSON diagram
6. Output to user's working directory or specified location

Input Handling

Accept code in these formats:

<code language="python">
def example():
    pass
</code>

Or with context:

<code language="typescript" context="Authentication module for REST API">
...
</code>

Or request type:

<request type="class_diagram">
Visualize the User authentication system
</request>

Code Analysis

Parse and Identify

Code Element	What to Extract
Functions	Name, params, return type, calls
Classes	Name, methods, properties, inheritance
Modules	Imports, exports, dependencies
APIs	Endpoints, methods, request/response
Data	Models, schemas, transformations

Identify Patterns

• Control flow (conditionals, loops, error handling)
• Data flow (input → processing → output)
• Design patterns (singleton, factory, observer, etc.)
• Architecture patterns (MVC, layered, microservices)

Diagram Selection

Code Characteristic	Recommended Diagram
OOP with inheritance	Class Diagram
Process/algorithm logic	Flow Diagram
Database models	ER Diagram
System modules	Component Diagram
API interactions	Sequence Diagram
Feature breakdown	Feature Diagram
State management	State Diagram
High-level overview	Architecture Diagram

Multiple diagrams may be appropriate for complex code.

Explanation Structure

Every code explanation includes:

1. High-Level Overview

Brief summary of what the code does and its purpose.

2. Key Components

List main classes, functions, or modules with roles.

3. Step-by-Step Logic

Walk through the execution flow with numbered steps.

4. Data Flow

Describe how data moves through the system.

5. Design Decisions

Note patterns used and architectural choices.

6. Performance Notes (if relevant)

Complexity, bottlenecks, optimization opportunities.

7. Potential Improvements (if relevant)

Suggestions for refactoring or enhancement.

Excalidraw JSON Structure

{
  "type": "excalidraw",
  "version": 2,
  "source": "code-visualizer",
  "elements": [...],
  "appState": {
    "gridSize": 20,
    "viewBackgroundColor": "#ffffff"
  },
  "files": {}
}

Element Reference

CRITICAL: Every element MUST include these base properties for proper rendering:

{
  "version": 1, "versionNonce": 1, "isDeleted": false,
  "seed": <unique_number>, "angle": 0, "opacity": 100,
  "groupIds": [], "frameId": null, "boundElements": [],
  "updated": 1, "link": null, "locked": false
}

Text elements also need: containerId, originalText, lineHeight, verticalAlign Lines/arrows also need: lastCommittedPoint, startBinding, endBinding, startArrowhead, endArrowhead

See references/diagram-templates.md for complete examples.

Shapes

Rectangle (classes, entities, processes) - key properties:

{
    "type": "rectangle",
    "id": "rect_1",
    "x": 100,
    "y": 100,
    "width": 200,
    "height": 120,
    "strokeColor": "#1e1e1e",
    "backgroundColor": "#a5d8ff",
    "fillStyle": "solid",
    "strokeWidth": 2,
    "roughness": 1,
    "roundness": { "type": 3 }
}

Diamond (decisions) - key properties:

{
    "type": "diamond",
    "id": "diamond_1",
    "x": 150,
    "y": 200,
    "width": 100,
    "height": 80,
    "backgroundColor": "#ffec99"
}

Ellipse (start/end, actors) - key properties:

{
    "type": "ellipse",
    "id": "ellipse_1",
    "x": 100,
    "y": 100,
    "width": 80,
    "height": 80,
    "backgroundColor": "#b2f2bb"
}

Connectors

Arrow (relationships, flow) - key properties:

{
    "type": "arrow",
    "id": "arrow_1",
    "x": 300,
    "y": 160,
    "points": [
        [0, 0],
        [100, 0]
    ],
    "strokeWidth": 2,
    "endArrowhead": "arrow"
}

Line (connections, dividers) - key properties:

{
    "type": "line",
    "id": "line_1",
    "x": 100,
    "y": 145,
    "points": [
        [0, 0],
        [200, 0]
    ],
    "strokeWidth": 1
}

Text

Key properties (must also include text-specific required props):

{
    "type": "text",
    "id": "text_1",
    "x": 110,
    "y": 110,
    "text": "ClassName",
    "fontSize": 16,
    "fontFamily": 1,
    "textAlign": "center",
    "originalText": "ClassName",
    "lineHeight": 1.25,
    "verticalAlign": "top",
    "containerId": null
}

Color Palette

Element Type	Background	Meaning
Classes	#a5d8ff	Primary code units
Interfaces	#d0bfff	Abstract contracts
Entities/Data	#b2f2bb	Data structures
Decisions	#ffec99	Branch points
Start/End	#ffc9c9	Boundaries
Processes	#e9ecef	Actions
External	#ffd8a8	APIs, services
Errors	#ff8787	Exception paths

Relationship Arrows

Relationship	Arrowhead	Style
Association	arrow	solid
Inheritance	triangle	solid
Implementation	triangle	dashed
Dependency	arrow	dashed
Composition	diamond (start)	solid
Aggregation	diamond_outline	solid
Data flow	arrow	solid
Return	arrow	dashed, gray

Layout Guidelines

1. Grid: Align to 100px grid
2. Spacing: Minimum 50px between elements
3. Flow: Left-to-right or top-to-bottom
4. Grouping: Cluster related components
5. Hierarchy: Important elements at top/left
6. Labels: Center text, 10px padding from edges

ID Generation

Format: {type}_{index}_{random5}

import random, string
def gen_id(t, i):
    return f"{t}_{i}_{''.join(random.choices(string.ascii_lowercase+string.digits,k=5))}"

Output Format

For each visualization:

1. Text explanation in conversation
2. Diagram file saved to ./{name}.excalidraw or user-specified path
3. Usage instructions for opening/editing

Example naming: auth-system-class-diagram.excalidraw

Detailed Patterns

For specific diagram patterns, templates, and complete examples, see:

• references/diagram-templates.md - Element templates
• references/code-patterns.md - Code-to-diagram mapping