Mermaid MCP Server

Model Context Protocol server that bundles the official Mermaid syntax docs and a syntax validator so agents can search, quote, and check diagrams before sending them to users.

What It Does

• Serves Mermaid syntax Markdown as MCP resources (flowchart, sequence, class, C4, etc.) plus a guide prompt for quick routing.
• search_mermaid_docs finds sections in the bundled docs (mermaid/docs/syntax) with optional diagram filtering and snippet/full modes. Results come from a startup cache for speed.
• validate_mermaid runs mermaid.parse to flag syntax issues; DOM-related errors are treated as pass-through so headless environments still work.

Architecture

flowchart TD
    Client[MCP client] -->|stdio| StdioTransport
    Client -->|HTTP /mcp| HttpTransport
    StdioTransport --> Server
    HttpTransport --> Server
    Server[Mermaid MCP server] -->|registers| Resources[Docs resources + guide prompt]
    Server -->|tools| Tools
    Tools --> Search[search_mermaid_docs -> cached syntax md]
    Tools --> Validate[validate_mermaid -> mermaid.parse]
    Search --> DocsCache[Docs cache: mermaid/docs/syntax]
    Validate --> MermaidLib[Mermaid parser]

Install & Run

• Clone (includes Mermaid submodule):
  • git clone --recurse-submodules <repo-url>
  • or, if already cloned: git submodule update --init --recursive
• Prereq: Node 18+
• Install: npm install
• Build: npm run build
• Start (STDIO for MCP clients): npm run start:stdio
• Start (HTTP for testing): npm run start:http (health at /health, MCP endpoint at /mcp)

MCP Client Wiring (STDIO)

{
  "mcpServers": {
    "mermaid": {
      "command": "node",
      "args": ["/absolute/path/to/mcp-mermaid/dist/server.js"]
    }
  }
}

Agent Usage Tips

• Always search first: search_mermaid_docs with a single-term query (e.g., flowchart, subgraph, arrow); use diagram_type to narrow by file; mode: "full" returns entire docs.
• Always validate after editing: validate_mermaid on the final diagram string.
• Use the guide prompt for quick diagram-type routing and common pitfalls.

Tests

• Run unit tests: npm test

Project Layout

• src/server.ts – MCP server, resources, tools, docs cache
• src/httpServer.ts – Express HTTP wrapper for /mcp
• guides/ – guide.md prompt and config
• mermaid/docs/syntax/ – Bundled official Mermaid syntax Markdown
• dist/ – Built JS output

Request Flow (User → Answer)

sequenceDiagram
    participant U as User
    participant A as MCP Client/Agent
    participant S as Mermaid MCP Server
    participant Docs as Docs Cache (mermaid/docs/syntax)
    participant V as validate_mermaid (mermaid.parse)

    U->>A: Ask for Mermaid help
    A->>S: search_mermaid_docs {query}
    S-->>Docs: Read cached syntax
    Docs-->>S: Return snippets/full sections
    S-->>A: Matched reference sections
    A->>S: validate_mermaid {code}
    S->>V: Parse diagram
    V-->>S: Valid / warnings / error
    S-->>A: Validation result
    A-->>U: Deliver answer + validated diagram