d2-diagrams - SKILL.md Agent Skill

name: d2-diagrams description: Create, modify, and render D2 diagrams using the d2 CLI. Use when the user asks for diagrams, architecture visuals, ERDs, sequence diagrams, flowcharts, grid layouts, or any declarative diagramming task. Trigger phrases include "d2 diagram", "create a diagram", "architecture diagram", "sequence diagram", "ERD", "flowchart", "draw", "visualize".

D2 Diagrams

Create, modify, and render diagrams using the D2 declarative diagramming language.

Overview

D2 (Declarative Diagramming) is a text-to-diagram scripting language. You write what you want diagrammed, and the D2 CLI generates SVG or PNG images. This skill covers:

Installing the D2 CLI
Writing D2 source files (.d2)
Rendering diagrams to SVG/PNG
All D2 features: shapes, connections, containers, SQL tables, UML classes, sequence diagrams, grid diagrams, icons, themes, styles, variables, globs, layers, scenarios, and steps

When to Use

User asks to create or modify any kind of diagram
Architecture diagrams, system designs, network topologies
Entity-relationship diagrams (ERDs) with SQL tables
UML class diagrams
Sequence diagrams for API flows or protocols
Flowcharts and process diagrams
Grid layouts for dashboards or comparison tables
Any request mentioning "d2", "diagram", "visualize", "draw", "architecture"

Prerequisites: Installing D2

Before creating diagrams, verify D2 is installed:

d2 version

If not installed, install using one of these methods:

macOS (Recommended: Homebrew)

brew install d2

Any platform (Install script)

curl -fsSL https://d2lang.com/install.sh | sh -s --

From source (requires Go 1.20+)

go install oss.terrastruct.com/d2@latest

Verify installation

d2 version

Workflow

Step 1: Write D2 Source

Create a .d2 file with the diagram definition. Use the language reference in references/D2_LANGUAGE_REFERENCE.md for syntax details.

Step 2: Render the Diagram

# Render to SVG (default, with padding)
d2 --pad=40 input.d2 output.svg

# Verify width is reasonable (<1400px for laptops)
rg -o 'viewBox="0 0 ([0-9]+)' -r '$1' output.svg

# Render to PNG
d2 --pad=40 input.d2 output.png

# With a specific theme (0-based IDs, use `d2 themes` to list)
d2 --theme=200 input.d2 output.svg

# With dark theme support
d2 --dark-theme=200 input.d2 output.svg

# With sketch/hand-drawn style
d2 --sketch input.d2 output.svg

# With a specific layout engine (dagre, elk, tala)
d2 --layout=elk input.d2 output.svg

# With custom padding (default recommendation is 40)
d2 --pad=60 input.d2 output.svg

# Watch mode (opens browser with live reload)
d2 --watch input.d2 output.svg

Step 3: Iterate

Modify the .d2 file and re-render. Use --watch for interactive development.

Quick Reference

Shapes

# Basic shapes (default is rectangle)
my_shape
labeled_shape: My Label

# Specific shape types
db: Database {shape: cylinder}
user: User {shape: person}
decision: Choice {shape: diamond}
q: Queue {shape: queue}
pg: Package {shape: package}
doc: Document {shape: document}
oval_shape: Oval {shape: oval}
circle_shape: Circle {shape: circle}
hexagon_shape: Hex {shape: hexagon}
cloud_shape: Cloud {shape: cloud}

Connections

# Arrow types
a -> b: forward
b <- a: backward
a <-> b: bidirectional
a -- b: undirected

# Chaining
a -> b -> c -> d

# Repeated connections create parallel edges
a -> b: first
a -> b: second

Containers (Nesting)

server: Backend Server {
  api: REST API
  db: Database {shape: cylinder}
  api -> db: queries
}

Styles

x: Shape {
  style: {
    fill: "#f0f0f0"
    stroke: "#333333"
    stroke-width: 2
    stroke-dash: 5
    border-radius: 8
    shadow: true
    opacity: 0.9
    font-size: 16
    font-color: "#000"
    bold: true
    italic: false
    animated: true
    3d: true
    multiple: true
    double-border: true
    fill-pattern: dots
  }
}

Icons

server: Backend {
  icon: https://icons.terrastruct.com/essentials%2F112-server.svg
}

# Standalone icon shape
github: GitHub {
  shape: image
  icon: https://icons.terrastruct.com/dev%2Fgithub.svg
}

SQL Tables

users: {
  shape: sql_table
  id: int {constraint: primary_key}
  name: varchar(255)
  email: varchar(255) {constraint: unique}
  created_at: timestamp
}

Sequence Diagrams

shape: sequence_diagram
alice -> bob: Hello
bob -> alice: Hi back
alice -> bob: How are you?
bob -> alice: Good, thanks!

Grid Diagrams

grid: {
  grid-rows: 2
  grid-columns: 3
  cell1: A
  cell2: B
  cell3: C
  cell4: D
  cell5: E
  cell6: F
}

Variables

vars: {
  primary-color: "#4A90D9"
  server-icon: https://icons.terrastruct.com/essentials%2F112-server.svg
}
server: Backend {
  icon: ${server-icon}
  style.fill: ${primary-color}
}

Globs (Global Patterns)

# Style all shapes
*.style.fill: "#f0f0f0"
*.style.border-radius: 8

# Style all connections
(* -> *)[*].style.stroke-dash: 3

# Recursive glob
**.style.font-size: 14

Direction

direction: down  # Recommended default — narrower diagrams
# Options: up, down, left, right
# Use 'right' only for explicitly horizontal flows (timelines, pipelines)

a -> b -> c

Themes

Use d2 themes to list available themes. Common theme IDs:

0 - Default
1 - Neutral default
3 - Mixed berry blue
4 - Grape soda
5 - Aubergine
6 - Colorblind clear
8 - Vanilla nitro cola
100 - Origami
200 - Dark Mauve (terminal-style)
300 - Terminal (dark, monospace, caps)
301 - Terminal Grayscale
302 - Retro

Dark themes:

200 - Dark Mauve
201-208 - Various dark themes

Composition (Multi-board)

# Root board
a -> b

# Layers (independent boards)
layers: {
  detail: {
    x -> y -> z
  }
}

# Scenarios (inherit from base)
scenarios: {
  error: {
    a.style.fill: red
    a -> c: error path
  }
}

# Steps (each inherits from previous)
steps: {
  step1: {
    a -> b
  }
  step2: {
    b -> c
  }
}

Classes (Reusable Styles)

classes: {
  server: {
    shape: rectangle
    style: {
      fill: "#dceefb"
      stroke: "#4A90D9"
      border-radius: 8
    }
  }
  database: {
    shape: cylinder
    style: {
      fill: "#e8f5e9"
      stroke: "#66bb6a"
    }
  }
}

api: API Server {class: server}
db: PostgreSQL {class: database}
api -> db

Arrowheads

a -> b: {
  source-arrowhead: {
    shape: diamond
  }
  target-arrowhead: {
    shape: arrow
    label: "1..*"
  }
}

# Arrowhead shapes: triangle, arrow, diamond, circle, cf-one, cf-one-required, cf-many, cf-many-required

Markdown and Code in Labels

explanation: |md
  # Architecture Overview
  - **Frontend**: React SPA
  - **Backend**: Go microservices
  - **Database**: PostgreSQL
|

code_block: |go
  func main() {
    fmt.Println("Hello")
  }
|

Width Control

Diagram width is the most common problem. D2's dagre layout places sibling nodes side-by-side, so diagrams easily exceed screen width. Always target <1400px wide for laptop readability.

Core Principles

Use direction: down (not right) as the default. Vertical stacking is narrower.
Minimize sibling nodes at the same depth. Each peer node adds horizontal width. Chain nodes vertically with connections (a -> b -> c) instead of declaring them as siblings.

After every render, verify the SVG width. D2 doesn't warn about wide output:

# Check viewBox width (first number after "0 0" is width)
rg -o 'viewBox="0 0 ([0-9]+)' -r '$1' output.svg

The Container Width Problem

Nested containers with multiple children are the #1 source of width explosion. Dagre lays out children horizontally within a container, so a container with 4 children produces 4 columns of width.

Bad — 4 children side-by-side, very wide:

services: Backend {
  api: API Gateway
  auth: Auth Service
  core: Core API
  worker: Worker
}

Better — use the "titled container + transparent markdown child" pattern. The container label acts as the heading with proper border-radius, and a single invisible child holds markdown body text:

services: Backend Services {
  style.fill: "#e8f5e9"
  style.stroke: "#4caf50"
  style.border-radius: 10

  details: |md
- API Gateway (:3000)
- Auth Service (JWT)
- Core API (business logic)
- Background Worker (cron)
| {
    style.fill: transparent
    style.stroke: transparent
  }
}

This pattern keeps the rounded-corner container visual while collapsing all content into a single narrow box.

Markdown Labels and border-radius

Gotcha: When a node's label is markdown (|md ... |), D2 renders the content inside a foreignObject and the SVG <rect> gets rx="0" — meaning border-radius is ignored on the outer shape even if set via class or inline style. Plain-text labels render border-radius correctly.

Workaround: Use the titled container pattern above. The parent container uses a plain-text label (which gets proper border-radius), and the markdown goes in a transparent child node inside it.

Sequence Diagram Width

Sequence diagram width is proportional to actor count and cannot be controlled by direction or layout. The only way to reduce width is to merge related actors:

# Wide — 5 actors
shape: sequence_diagram
user: User
ios: iOS App
llm: On-Device LLM
server: Server
db: Database

# Narrower — 3 actors (merge ios+llm, server+db)
shape: sequence_diagram
user: User
ios: iOS (App + LLM)
server: Server (+ DB)

Other Width Tactics

Shorten labels. Every character counts. Use abbreviations.
Use --pad=40 to add breathing room without changing layout width.
Chain nodes linearly (a -> b -> c) to force dagre to stack them vertically rather than placing them side-by-side.
If a diagram is still too wide after all optimizations, split it into two diagrams rather than fighting the layout engine.

Known Gotchas

@ prefix is D2 import syntax. Labels like @github/copilot-sdk or @MainActor will be interpreted as imports. Wrap in quotes or rephrase: "@github/copilot-sdk" or just Copilot SDK.
Sequence diagrams don't support style blocks on connections. Unlike regular diagrams, you cannot style individual arrows in a sequence diagram — the render will fail. Remove any style.stroke, style.stroke-width, etc. from sequence diagram connections.
Markdown labels lose border-radius on the outer SVG rect (see Width Control above).
Dagre layout is non-deterministic for peer nodes. Re-rendering the same .d2 file can produce slightly different widths. Always verify after re-rendering.

Best Practices

Use meaningful keys: api_server not box1. Keys become default labels.
Default to direction: down: Vertical layouts fit screens better. Only use right for explicitly horizontal flows (timelines, pipelines).
Use the titled container + transparent md child pattern: For nodes that need both rounded corners and rich content (see Width Control section).
Use classes for consistency: Define reusable styles with classes:.
Use globs for defaults: Set *.style.border-radius: 8 instead of styling each shape.
Use variables for colors: Define a palette in vars: for easy theme changes.
Use icons: Add icons from https://icons.terrastruct.com for professional diagrams.
Choose the right layout: dagre (default) for hierarchical, elk for complex graphs.
Keep it readable: Use nested syntax and indentation for complex diagrams.
Use composition: Split complex diagrams into layers, scenarios, or steps.
Always render with --pad=40: Gives diagrams breathing room. Default padding is too tight.
Verify width after every render: Check the SVG viewBox. Target <1400px for laptop screens.

Rendering Tips

Always use --pad=40 for comfortable whitespace around diagrams. The default is too tight.

Always verify width after rendering:

d2 --pad=40 input.d2 output.svg
rg -o 'viewBox="0 0 ([0-9]+)' -r '$1' output.svg  # should be <1400

SVG is the default and best format for web embedding and scalability.
PNG is good for embedding in documents, Slack, etc.
Use --theme for professional presentation-ready output.
Use --sketch for informal, hand-drawn appearance.
Animated SVGs: Use --animate-interval=1200 with composition for animated diagrams.

Troubleshooting

"d2: command not found": D2 is not installed. Follow install instructions above.
Layout issues: Try a different layout engine (--layout=elk or --layout=tala).
Overlapping labels: Shorten labels, adjust font-size, or use a different layout.
Missing icons: Ensure icon URLs are accessible. Use https://icons.terrastruct.com for reliable icons.
Diagram too wide: See the Width Control section. Most likely cause is nested containers with many sibling children.
Square corners on markdown boxes: border-radius doesn't apply to |md ... | labels. Use the titled container + transparent md child pattern instead.
Render fails on sequence diagram styles: Sequence diagrams don't support style blocks on connections. Remove them.
@ interpreted as import: Quote or rephrase labels containing @ (e.g., "@MainActor").

Additional References

references/D2_LANGUAGE_REFERENCE.md - Complete language syntax reference
references/D2_PATTERNS.md - Common diagram patterns and templates
assets/ - Example .d2 template files ready to use
Official docs: https://d2lang.com/tour/intro/
Playground: https://play.d2lang.com
Icons: https://icons.terrastruct.com