The Mermaid code my AI gave me won't render. Now what?

Short answer

Paste it into the editor. The status line turns into an error bar in plain language with a line number. Three cases cover it.

Case 1: a syntax error

Errors mentioning Parse error or Lexical error are syntax. Generated code has four recurring weak spots — the reserved word end, mixed arrow styles, full-width punctuation, and quotes inside labels. Each one is written up in What does Parse error mean.

If you'd rather not hunt: the error bar has "AI fix". It sends the current code to the engine once, makes the smallest change that parses, verifies the result renders before showing it to you, and presents a line-by-line diff to accept or discard.

Case 2: it isn't Mermaid

Models sometimes hand back PlantUML or Graphviz dot when you asked for Mermaid, and sometimes use syntax from a newer release. Quick check: the first line should be one of flowchart / graph / sequenceDiagram / classDiagram / stateDiagram-v2 / erDiagram / mindmap / gantt / pie. If it's none of those, go back and ask for Mermaid specifically.

Case 3: the diagram is too big

Past 500 edges the engine refuses the flowchart outright — that error is a limit, not a syntax problem. Splitting is the only fix: break it into stages, or collapse the detail layer. Around 300 nodes takes about a second to lay out, which is normal.

What a clean one looks like

stateDiagram-v2
    [*] --> AwaitingPayment
    AwaitingPayment --> Paid: payment captured
    AwaitingPayment --> Cancelled: 30 min timeout
    Paid --> Shipped: left the warehouse
    Shipped --> Complete: delivery confirmed
    Complete --> [*]

Related

• What does Parse error mean, and how do I fix it?
• Editing and fixing errors