Mermaid Validator

MCP Badge

A Model Context Protocol (MCP) server built with Bun and TypeScript that validates Mermaid diagram syntax.

Quick Start

This package is published on npm. You can run it directly with npx:

npx -y mermaid-validator-mcp

MCP Client Configuration

Add the server to your MCP client (e.g. Claude Desktop) configuration:

Using npx (recommended)

{
  "mcpServers": {
    "mermaid-validator": {
      "command": "npx",
      "args": ["-y", "mermaid-validator-mcp"]
    }
  }
}

Using local source

{
  "mcpServers": {
    "mermaid-validator": {
      "command": "bun",
      "args": [
        "/path/to/mermaid-validator-mcp/index.ts"
      ]
    }
  }
}

Or if you have it installed globally:

{
  "mcpServers": {
    "mermaid-validator": {
      "command": "mermaid-validator-mcp"
    }
  }
}

Features

• Single tool – validate_mermaid: Pass any Mermaid diagram text and receive either a success response with the detected diagram type, or a detailed parse-error message.
• Supports all major Mermaid diagram types: flowchart, sequence, class, pie, gantt, state, ER, gitGraph, mindmap, timeline, and more.
• No dependency on mermaid-cli or any external rendering process.
• Uses JSDOM to provide the minimal DOM environment required by Mermaid.

Requirements

• Bun ≥ 1.0

Getting Started

# Install dependencies
bun install

# Start the MCP server (stdio mode)
bun start

Tool: validate_mermaid

Input

Field	Type	Description
diagram	string	The Mermaid diagram text to validate

Output (JSON text)

Valid diagram

{ "valid": true, "diagramType": "flowchart-v2" }

Invalid diagram

{
  "valid": false,
  "error": "Parse error on line 2:\n...TD\n  --> B\n-------^\nExpecting 'SEMI', ..."
}

Development

# Run with auto-reload
bun dev

# Build
bun run build