For Mermaid flowcharts, keep node IDs separate from visible labels, avoid the lowercase word end, and quote label text when punctuation could be mistaken for syntax. Watch for lowercase o or x immediately after a connector, too: each can be interpreted as an edge marker. These checks apply to flowcharts, not automatically to every Mermaid diagram type.
Start with the distinction between node IDs and labels
A flowchart node ID is the identifier you use to refer to a node in a connection. Its label is the text displayed in the shape. They do not have to match: in A[Start] --> B[Review], the IDs are A and B, while the visible labels are “Start” and “Review.” You can connect to an existing node by its ID without writing its label again.
Using descriptive IDs and treating labels as display text makes diagrams easier to edit. For example, reviewStep{"Ready?"} can keep the same ID while you revise the wording shown to readers.
Check for flowchart words and connector ambiguities
Avoid lowercase end
Mermaid’s Flowchart Syntax documentation warns that lowercase end in a flowchart node can break the diagram. Use End, END, or a different word. This documented warning concerns flowcharts; it is not evidence that every Mermaid diagram grammar treats the same word identically.
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Keep lowercase o and x clear of connectors
A lowercase o or x immediately after a connector may be read as an edge marker—a circle or cross—instead of the start of a node ID. For example, A---oB can be parsed as a circle edge, and A---xB as a cross edge. Put a space after the connector, as in dev--- ops, or capitalize the first letter of the ID, as in dev---Ops. Mermaid Chart’s flowchart guide applies the same caution to connection labels beginning with o or x: add a space or capitalize the initial letter.
Make punctuation in visible labels unambiguous
When punctuation overlaps with flowchart shape syntax, put the visible text in quotes. Mermaid’s flowchart documentation uses id1["This is the (text) in the box"] to show parentheses treated as label text rather than as shape delimiters. Quoting is also a useful approach for label text containing commas or semicolons.
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If a quote or other character still needs explicit representation, use Mermaid’s documented entity codes. The flowchart reference shows #quot; for a double quote and #9829; for a decimal character code. Numeric codes are base 10, so #35; represents #; HTML character names are supported as well. Prefer these documented forms over assuming a general backslash-escape rule.
Choose traditional or Markdown strings for the label you need
For ordinary text, a quoted label is often sufficient. For formatted or multiline text, Mermaid flowcharts also support Markdown strings. The syntax uses outer double quotes and inner backticks:
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A["`A **bold** label`"]Markdown strings support bold and italics, automatic text wrapping, and physical line breaks. The feature applies to node, edge, and subgraph labels. Traditional strings can use <br> for line breaks.
| Label approach | Use it when | Example |
|---|---|---|
| Plain label | The text is simple and does not risk being read as flowchart syntax. | A[Review] |
| Quoted label | Punctuation in the visible text could be confused with shape syntax. | id1["This is the (text) in the box"] |
| Traditional string with line break | You want a line break without Markdown formatting. | A[First line<br>Second line] |
| Markdown string | You want formatting, automatic wrapping, or physical line breaks. | A["`A **bold** label`"] |
Use a safe pattern in a complete flowchart
This example uses descriptive IDs, avoids lowercase end, and keeps the edge labels from starting with lowercase o:
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flowchart LR
startNode["Begin (draft)"] --> reviewNode{"Ready?"}
reviewNode -->| Yes| finishNode[End]
reviewNode -->| No| startNodeThe declaration starts with flowchart and a direction. Common directions are LR (left to right), TD or TB (top to bottom), BT (bottom to top), and RL (right to left). Common node forms include A[Label] for a rectangle, A(Label) for a rounded rectangle, A{Label} for a diamond, and A((Label)) for a circle. Parallelogram, cylinder, and subroutine forms are also available. Since delimiters define shapes, quoted label text helps when the visible words include punctuation that resembles those delimiters.
Do not treat this as an exhaustive reserved-word list
The cited flowchart reference documents the lowercase end warning and the o/x edge-marker ambiguity; it does not establish a complete list of reserved words. Mermaid grammars differ by diagram type. For example, the official use-case diagram syntax documentation describes its own reserved delimiters and sequences. Apply the rules for the diagram grammar you are writing rather than assuming flowchart rules cover all Mermaid diagrams.
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Mermaid syntax also evolves. The flowchart reference marks collapsible subgraphs as available in Mermaid v11.17.0 and later. Check the Mermaid version in the editor, documentation system, or publishing host where the diagram will render before relying on newer syntax.
Quick checklist before rendering
- Give each flowchart node an ID you can use consistently in connections; do not confuse it with the visible label.
- Replace lowercase
endwithEnd,END, or another label. - After connectors, check for lowercase
oorxat the start of an ID or connection label; add a space or capitalize the initial letter if needed. - Quote visible label text when punctuation could overlap with shape syntax.
- Use Mermaid entity codes for characters that need explicit representation.
- Choose traditional strings or Markdown strings according to whether you need line breaks and formatting.
- Confirm that the host rendering the diagram supports any newer syntax you use.
Optional reference
Mermaid’s free syntax documentation is the practical reference for authoring. For a broader printed reference, Mermaid Chart’s page for The Official Guide to Mermaid.js identifies the book by Knut Sveidqvist and Ashish Jain. Google Books records Packt Publishing as the publisher and 2021 as the publication year; the book is optional, not a prerequisite for using Mermaid’s documentation.
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