README - d2mcp by i2y

D2 MCP Server

A Model Context Protocol (MCP) server that provides D2 diagram generation and manipulation capabilities.

D2 is a modern diagram scripting language that turns text to diagrams. This MCP server allows AI assistants like Claude to create, render, export, and save D2 diagrams programmatically.

The server provides 10 tools through the MCP protocol with enhanced descriptions for optimal AI assistant integration, enabling both simple diagram rendering and sophisticated incremental diagram building using the Oracle API.

With the new Oracle API integration, AI assistants can now build and modify diagrams incrementally, making it perfect for:

Converting conversations into architecture diagrams
Building flowcharts step-by-step as requirements are discussed
Creating entity relationship diagrams from database schemas
Generating system diagrams from code analysis
Refining diagrams based on user feedback without starting over

Features

Basic Diagram Operations

d2_create - Create new diagrams with optional initial content (unified approach)
d2_export - Export diagrams to various formats (SVG, PNG, PDF)
d2_save - Save existing diagrams to files

Oracle API for Incremental Editing

d2_oracle_create - Create shapes and connections incrementally
d2_oracle_set - Set attributes on existing elements
d2_oracle_delete - Delete specific elements from diagrams
d2_oracle_move - Move shapes between containers
d2_oracle_rename - Rename diagram elements
d2_oracle_get_info - Get information about shapes, connections, or containers
d2_oracle_serialize - Get the current D2 text representation of the diagram

Additional Features

20 themes - Support for all D2 themes (18 light + 2 dark)

Project Structure

d2mcp/
├── cmd/                  # Application entry point
├── internal/
│   ├── domain/          # Business entities and interfaces
│   │   ├── entity/      # Domain entities
│   │   └── repository/  # Repository interfaces
│   ├── usecase/         # Business logic
│   ├── infrastructure/  # External implementations
│   │   ├── d2/          # D2 library integration
│   │   └── mcp/         # MCP server implementation
│   └── presentation/    # MCP handlers
│       └── handler/     # Tool handlers
└── pkg/                 # Public packages

Prerequisites

Go 1.24.3 or higher
D2 v0.6.7 or higher (included as dependency)
For PNG/PDF export (optional):
- rsvg-convert (from librsvg) or
- ImageMagick (convert command)

Installation

From Source

# Clone the repository
git clone https://github.com/i2y/d2mcp.git
cd d2mcp

# Build the binary
make build

# Or build for all platforms
make build-all

Using Go Install

go install github.com/i2y/d2mcp/cmd@latest

Building

# Simple build
make build

# Run directly
make run

# Cross-platform builds
make build-all

Usage

With Claude Desktop

Add to your Claude Desktop configuration file:

macOS: ~/Library/Application Support/Claude/claude_desktop_config.json Windows: %APPDATA%\Claude\claude_desktop_config.json Linux: ~/.config/Claude/claude_desktop_config.json

For STDIO transport (recommended for Claude Desktop):

{
  "mcpServers": {
    "d2mcp": {
      "command": "/path/to/d2mcp",
      "args": ["-transport=stdio"]
    }
  }
}

For SSE transport:

{
  "mcpServers": {
    "d2mcp": {
      "command": "/path/to/d2mcp",
      "args": ["-transport=sse", "-addr=:3000"]
    }
  }
}

Replace /path/to/d2mcp with the actual path to your built binary.

Standalone

# Run the MCP server (stdio transport)
./d2mcp -transport=stdio

# Run with SSE transport (default)
./d2mcp
# or explicitly
./d2mcp -transport=sse

# Run with Streamable HTTP transport
./d2mcp -transport=streamable

Transport Options

d2mcp now supports multiple transport protocols:

STDIO Transport

The traditional stdio transport for direct process communication:

./d2mcp -transport=stdio

SSE Transport (Server-Sent Events)

HTTP-based transport that allows network connectivity:

# Basic SSE mode (defaults to :3000)
./d2mcp -transport=sse

# Custom configuration
./d2mcp -transport=sse \
  -addr=:8080 \
  -base-url=http://localhost:8080 \
  -base-path=/mcp \
  -keep-alive=30

SSE Configuration Options:

-addr: Address to listen on (default: ":3000")
-base-url: Base URL for SSE endpoints (auto-generated if not specified)
-base-path: Base path for SSE endpoints (default: "/mcp")
-keep-alive: Keep-alive interval in seconds (default: 30)

SSE Endpoints: When running in SSE mode, the following endpoints are available:

SSE stream: http://localhost:3000/mcp/sse
Message endpoint: http://localhost:3000/mcp/message

Streamable HTTP Transport

The modern HTTP-based transport that simplifies bidirectional communication:

# Basic Streamable HTTP mode
./d2mcp -transport=streamable

# Custom configuration
./d2mcp -transport=streamable \
  -addr=:8080 \
  -endpoint-path=/mcp \
  -heartbeat-interval=30 \
  -stateless

Streamable HTTP Configuration Options:

-addr: Address to listen on (default: ":3000")
-endpoint-path: Endpoint path for Streamable HTTP (default: "/mcp")
-heartbeat-interval: Heartbeat interval in seconds (default: 30)
-stateless: Enable stateless mode (default: false)

Streamable HTTP Endpoint: When running in Streamable HTTP mode, a single endpoint handles all communication:

Endpoint: http://localhost:3000/mcp

Tools

d2_create

Create a new diagram with optional initial content (unified approach):

Empty diagram (for Oracle API workflow):

{
  "id": "my-diagram"
}

With initial D2 content:

{
  "id": "my-diagram",
  "content": "a -> b: Hello\nserver: {shape: cylinder}"
}

d2_export

Export a diagram to a specific format:

{
  "diagramId": "my-diagram",
  "format": "png"  // Options: "svg", "png", "pdf"
}

d2_save

Save a diagram to a file:

{
  "diagramId": "my-diagram",
  "format": "pdf",
  "path": "/path/to/output.pdf"  // Optional, defaults to temp directory
}

Oracle API Tools

The Oracle API tools enable incremental diagram manipulation without regenerating the entire diagram. These tools are ideal for building diagrams step-by-step or making surgical edits.

d2_oracle_create

Create a new shape or connection:

{
  "diagram_id": "my-diagram",
  "key": "server"  // Creates a shape
}

{
  "diagram_id": "my-diagram", 
  "key": "server -> database"  // Creates a connection
}

d2_oracle_set

Set attributes on existing elements:

{
  "diagram_id": "my-diagram",
  "key": "server.shape",
  "value": "cylinder"
}

{
  "diagram_id": "my-diagram",
  "key": "server.style.fill",
  "value": "#f0f0f0"
}

d2_oracle_delete

Delete elements from the diagram:

{
  "diagram_id": "my-diagram",
  "key": "server"  // Deletes the server and its children
}

d2_oracle_move

Move elements between containers:

{
  "diagram_id": "my-diagram",
  "key": "server",
  "new_parent": "network.internal",  // Moves server into network.internal
  "include_descendants": "true"       // Also moves child elements
}

d2_oracle_rename

Rename diagram elements:

{
  "diagram_id": "my-diagram",
  "key": "server",
  "new_name": "web_server"
}

d2_oracle_get_info

Get information about diagram elements:

{
  "diagram_id": "my-diagram",
  "key": "server",
  "info_type": "object"  // Options: "object", "edge", "children"
}

d2_oracle_serialize

Get the current D2 text representation of the diagram:

{
  "diagram_id": "my-diagram"
}

Returns the complete D2 text of the diagram including all modifications made through Oracle API.

Creating Sequence Diagrams

D2 has built-in support for sequence diagrams. Use d2_create with proper D2 sequence diagram syntax:

{
  "id": "api-flow",
  "content": "shape: sequence_diagram\n\nClient -> Server: HTTP Request\nServer -> Database: Query\nDatabase -> Server: Results\nServer -> Client: HTTP Response\n\n# Add styling\nClient -> Server.\"HTTP Request\": {style.stroke-dash: 3}\nDatabase -> Server.\"Results\": {style.stroke-dash: 3}"
}

Example with actors and grouping:

{
  "id": "auth-flow",
  "content": "shape: sequence_diagram\n\ntitle: Authentication Flow {near: top-center}\n\n# Define actors\nClient: {shape: person}\nAuth Server: {shape: cloud}\nDatabase: {shape: cylinder}\n\n# Interactions\nClient -> Auth Server: Login Request\nAuth Server -> Database: Validate Credentials\nDatabase -> Auth Server: User Data\n\ngroup: Success Case {\n  Auth Server -> Client: Access Token\n  Client -> Auth Server: API Request + Token\n  Auth Server -> Client: API Response\n}\n\ngroup: Failure Case {\n  Auth Server -> Client: 401 Unauthorized\n}"
}

Example Oracle API Workflow

Starting from scratch:

// 1. Create an empty diagram
d2_create({ id: "architecture" })

// 2. Add shapes incrementally
d2_oracle_create({ diagram_id: "architecture", key: "web" })
d2_oracle_create({ diagram_id: "architecture", key: "api" })
d2_oracle_create({ diagram_id: "architecture", key: "db" })

// 3. Set properties
d2_oracle_set({ diagram_id: "architecture", key: "db.shape", value: "cylinder" })
d2_oracle_set({ diagram_id: "architecture", key: "web.label", value: "Web Server" })

// 4. Create connections
d2_oracle_create({ diagram_id: "architecture", key: "web -> api" })
d2_oracle_create({ diagram_id: "architecture", key: "api -> db" })

// 5. Export final result
d2_export({ diagramId: "architecture", format: "svg" })

Starting with existing content (unified approach):

// 1. Create diagram with initial content
d2_create({ 
  id: "architecture",
  content: "web -> api -> db\ndb: {shape: cylinder}"
})

// 2. Enhance incrementally using Oracle API
d2_oracle_set({ diagram_id: "architecture", key: "web.label", value: "Web Server" })
d2_oracle_create({ diagram_id: "architecture", key: "cache" })
d2_oracle_create({ diagram_id: "architecture", key: "api -> cache" })

// 3. Export final result
d2_export({ diagramId: "architecture", format: "svg" })

When to Use Each Tool

d2_create: Always use for new diagrams - both empty (for incremental building) and with initial D2 content
d2_oracle_*: Use for incremental modifications to any diagram created with d2_create
d2_export: Use to render the final diagram in your desired format

Development

Running tests

# Run all tests
make test

# Run with coverage
go test -cover ./...

# Run specific test
go test -v ./internal/presentation/handler

Code Quality

# Format code
make fmt

# Run linter
make lint

# Clean build artifacts
make clean

Adding new features

Define entities in internal/domain/entity
Add repository interfaces in internal/domain/repository
Implement business logic in internal/usecase
Add infrastructure implementations
Create MCP handlers in internal/presentation/handler
Wire dependencies in cmd/main.go

Project Structure

cmd/: Application entry point
internal/domain/: Core business logic and entities
internal/infrastructure/: External service integrations
internal/presentation/: MCP protocol handlers
internal/usecase/: Application business logic

Troubleshooting

PNG/PDF Export Not Working

If you get errors when exporting to PNG or PDF formats, install one of these tools:

macOS:

# Using Homebrew
brew install librsvg
# or
brew install imagemagick

Ubuntu/Debian:

sudo apt-get install librsvg2-bin
# or
sudo apt-get install imagemagick

Windows: Download and install ImageMagick from the official website.

MCP Connection Issues

Ensure the binary has execute permissions: chmod +x d2mcp
Check Claude Desktop logs for error messages
Verify the path in your configuration is absolute

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Changelog

v0.5.0 (Latest)

Added SSE (Server-Sent Events) transport support for network connectivity
Added Streamable HTTP transport support for modern bidirectional communication
New command-line flags for transport configuration
Support for stateful and stateless modes in Streamable HTTP
Maintained backward compatibility with stdio transport
Improved error handling and logging for different transport modes

v0.4.0

Simplified API to unified d2_create for all diagram creation needs
Enhanced tool descriptions for better AI assistant integration
Improved Oracle API error handling and validation
Reduced API surface from 14 to 10 tools
Breaking Change: Removed d2_render, d2_render_to_file, d2_import, d2_from_text - use d2_create instead

v0.3.0

Added d2_oracle_serialize tool to get current D2 text representation

v0.2.0

Added D2 Oracle API integration for incremental diagram manipulation
6 new MCP tools for creating, modifying, and querying diagram elements
Support for stateful diagram editing sessions

v0.1.0

Initial release with basic D2 diagram operations
Support for rendering, creating, exporting, and saving diagrams
20 built-in themes
MCP protocol integration

License

This project is licensed under the MIT License - see the file for details.