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Mermaid Diagram Syntax Validator

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Validate a Mermaid diagram definition and flag syntax errors with line numbers before it fails to render

Paste a Mermaid diagram definition and get back a line-numbered list of syntax problems instead of a bare error box on a docs site or GitHub preview. It recognizes flowchart/graph, sequenceDiagram, classDiagram, stateDiagram(-v2), erDiagram, and pie by their header keyword, then runs checks specific to the diagram type it finds: flowcharts are checked for an unterminated subgraph, a stray end, unbalanced node brackets, and an edge arrow missing a node on either side, while sequence diagrams are checked for a participant referenced in a message before its explicit participant or actor declaration later in the file. This is a line-based heuristic over the text, not a full Mermaid grammar or renderer, so it can occasionally misread a bracket-like substring inside a quoted node label, and classDiagram/stateDiagram/erDiagram/pie only get the header-type check today.

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How to use Mermaid Diagram Syntax Validator

  • 1.Paste a Mermaid diagram definition (flowchart, sequenceDiagram, classDiagram, stateDiagram, erDiagram, or pie) into the editor.
  • 2.Read the flagged issues, each with a line number and a plain-English explanation of what looks wrong.
  • 3.Fix the flagged lines and re-check until the diagram reports no issues, before pasting it into a docs site or GitHub README.

Frequently asked questions

Does this render the diagram?
No, it never renders anything. It only reads the text and checks structural rules like matching subgraph/end pairs, balanced node brackets, and valid edge arrows.
Which diagram types get the deepest checks?
Flowchart/graph and sequenceDiagram get real structural checks today. classDiagram, stateDiagram, erDiagram, and pie are recognized by their header keyword but don't get type-specific checks yet.
Why would a valid-looking diagram get flagged?
This is a line-based heuristic, not the real Mermaid parser, so a bracket-like character inside a quoted node label (for example a literal `-->` inside a string) can occasionally produce a false positive.
Is my diagram sent anywhere?
No, in the browser tool everything runs client-side. The REST API and SDKs process whatever text you send them, same as any other API call.

Use via API, SDK, or MCP

cURL# Free: 1,000 req/day · Pro: 10,000 req/day
curl -X POST https://api.utilix.tech/v1/tools/mermaid-syntax-validator \
  -H "Authorization: Bearer utx_live_..." \
  -H "Content-Type: application/json" \
  -d '{"diagram":"flowchart TD\n  A[Start] --> B{Decision}\n  B -->|Yes| C[End]\n  B -->|No| A"}'

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