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Browser-side validation

Free Mermaid Syntax Checker

Validate Mermaid diagrams in your browser before adding them to GitHub, Obsidian, docs, or slide decks. Paste code, see parser feedback, then export valid diagrams.

No signup · No upload for the core check · Works with AI-generated Mermaid code · Independent tool

Paste Mermaid code to validate

Replace the sample, load an example, or paste AI-generated Mermaid code.

Auto-checks after you stop typing. No upload, no account, no diagram-content tracking.

Common Mermaid syntax problems

Most diagrams that fail to render were generated by a model and pasted straight in. These six account for almost all of it. Each broken example below really does fail on Mermaid 11.15.0, and each fix really does parse.

The Markdown fence came along with the code

Copying a diagram out of a chat answer usually brings the ```mermaid wrapper with it. Mermaid reads that first line as the diagram type and gives up.

Fails to render

```mermaid
flowchart LR
  A --> B
```

Renders

flowchart LR
  A --> B

A label contains brackets or quotes

Parentheses, quotes and angle brackets inside a label end the label early unless the whole label is wrapped in double quotes.

Fails to render

flowchart LR
  A[Start (init)] --> B[End]

Renders

flowchart LR
  A["Start (init)"] --> B[End]

The arrow belongs to a different diagram type

Models mix syntaxes between diagram families. The -->> arrow is sequence-diagram syntax and is not valid inside a flowchart.

Fails to render

flowchart LR
  A ->> B

Renders

flowchart LR
  A --> B

A node is named end

end closes blocks such as subgraph, so it cannot be used as a node id. This one is easy to miss because the error points at the following line.

Fails to render

flowchart TD
  start --> end
  end --> done

Renders

flowchart TD
  start --> finish
  finish --> done

A brace, bracket or subgraph was never closed

Long generated diagrams get truncated mid-line. The parser reports the line where it noticed, which is usually after the line that is actually wrong.

Fails to render

flowchart TD
  subgraph S1
    A --> B

Renders

flowchart TD
  subgraph S1
    A --> B
  end

The very first word is mangled

We have seen models emit sequencequenceDiagram and similar doubled openings. Because the first token decides the diagram type, the whole thing fails with "No diagram type detected" rather than a line error.

Fails to render

sequencequenceDiagram
  A->>B: hi

Renders

sequenceDiagram
  A->>B: hi

Once the diagram is valid, use the embedded exporter or the Mermaid Online editor.

FAQ

Is the Mermaid syntax checker free?

Yes. Validating a diagram runs entirely in your browser, with no signup and no limit on how many times you check. Exporting a valid diagram in a standard style is free too.

What does the checker catch that my editor does not?

It runs the same Mermaid parser the renderer uses, so it reports the exact error and line Mermaid itself would raise — including diagrams that look fine but fail because a label, arrow, or block is not valid for that diagram type.

Why does the Mermaid code from ChatGPT or Claude keep failing?

Usually one of five things: the ```mermaid fence was copied along with the code, a label contains unquoted brackets, an arrow from a different diagram type was used, a node was named end, or a brace was never closed. Paste the code above and the parser will point at the line; the table on this page shows a working version of each case.

The error says a line number that looks fine. Why?

Mermaid reports the line where parsing broke, not the line that caused it. An unclosed brace or subgraph on line 3 is usually reported on line 4 or later, so check the lines above the one named in the error.

My diagram renders on mermaid.live but not on GitHub.

Different tools bundle different Mermaid versions, and GitHub's is often older than the one an online editor runs. If the syntax is new, GitHub can reject code that is otherwise valid. Exporting the diagram to SVG or PNG and committing the image avoids the version question entirely.

Can this fix the code for me, not just tell me what is wrong?

Yes. When the check fails you can hand the code to AI repair, which rewrites it and re-checks the result. Signing in gives you 10 free credits; validating and exporting the standard styles stays free either way.

Does my Mermaid code get uploaded?

For the core editor and export workflow, Mermaid code is processed locally in your browser and is not uploaded to our server.

Is Mermaid Online affiliated with Mermaid.js or Mermaid Chart?

No. This is an independent tool and is not affiliated with Mermaid.js, Mermaid Chart, or their maintainers.