Mermaid Online MCP — Mermaid Share Links & Source

Encode diagram source into share links and recover source from supported Mermaid share tokens, directly from a local MCP client. An independent open-source companion to Mermaid Online, not an official Mermaid.js server.

This MCP serializes text. It does not render diagrams, validate Mermaid syntax, fetch share URLs or read/write files. For a visual editing workflow, use the separate Mermaid Online editor, Mermaid-to-SVG export or Mermaid-to-PNG export.

Install

Requires Node.js 22+ and an MCP client supporting local stdio servers. Common configuration:

{"mcpServers":{"mermaidonline":{"command":"npx","args":["-y","mermaidonline-mcp@0.1.0"]}}}

Each client has its own configuration location. On Windows, if a client cannot spawn npx, install the package and use node with the absolute installed src/cli.js path instead. Running the command without a client starts a protocol server waiting on stdin. Use npx -y mermaidonline-mcp@0.1.0 --help for human-readable help.

Two tools

Tool	Arguments	Result
encode_mermaid_share	code, optional mermaidConfig JSON string and explicit baseUrl	Reversible pako: token and optional URL
decode_mermaid_share	share, optional includeState	Original source and optionally untrusted extension/config data

Ask your agent:

Encode this flowchart into a share link using https://mermaidonline.org/ as the destination. Decode the result and verify the source round-trips. Do not render or open it.

{"code":"flowchart LR\n  A[Write] --> B[Share]\n","baseUrl":"https://mermaidonline.org/"}

Destination is explicit, not silently substituted. No tracking parameters are added. Decoding accepts a token, fragment, or HTTP(S) URL and never requests that address. includeState defaults to false to avoid unnecessarily echoing config and extension fields.

Limits and privacy

• Maximum source: 48,000 UTF-8 bytes; config: 8,000 UTF-8 bytes. Share tokens: 65,536 characters, decoded JSON: 65,536 UTF-8 bytes, container depth: 16. Results are limited to 128 KiB of JSON.
• A link contains the complete diagram state. It is reversible, not encrypted or access controlled. Compression is not redaction.
• Config is stored as data, not applied as renderer options. Treat recovered labels, links and directives as untrusted input to your editor.
• No URL fetching, uploads, file operations, telemetry, account, API key or paid backend calls. Your MCP client/model provider may still transmit or store tool input and output.
• pako:/base64: interoperability does not mean every site accepts every editor field. Unsupported legacy/site-specific fragments are rejected; diagram syntax is not checked.

See share-link workflows and examples. Core implementation: mermaid-share-codec. Commercial website code remains separate.

Verify

npm ci --ignore-scripts
npm run check
npm run audit:public
npm pack --dry-run

Tests negotiate a real MCP stdio session and exercise Unicode/config round trips, explicit destinations, source-only results, malicious schemes, corrupt payloads and bounded decompression. Published in the official MCP Registry as io.github.j-river/mermaidonline, version 0.1.0. Public metadata links directly to Mermaid Online and the npm package. Source metadata is in server.json.

MIT. No customer diagrams or production credentials are included.