Mermaid.js Diagram Generator Skill

Mermaid.js Diagram Generator

Generate valid Mermaid.js diagrams that render without syntax errors on the first attempt. When outputting a diagram, always wrap it in a fenced code block with the mermaid language tag.

Syntax Rules

Follow every rule below. Violating any one will produce a render failure.

1. One Node Per Line

%% CORRECT
A[Service A]
B[Service B]

%% WRONG
A[Service A] B[Service B]

2. Alphanumeric Node IDs Only

No spaces, hyphens, or special characters in IDs. Display names go in brackets.

%% CORRECT
AuthService[Auth Service]
UserDB[User Database]

%% WRONG
Auth-Service[Auth Service]
User DB[User Database]

3. Quote Labels with Special Characters

Wrap in double quotes if a label contains parentheses, brackets, colons, math operators, or anything that could be parsed as Mermaid syntax.

%% CORRECT
A["Process (async)"]
B["array[0]"]
C["ratio: 3/4"]
D["calculate(x, y)"]

%% WRONG
A[Process (async)]
B[array[0]]

4. No HTML or Markdown in Labels

Use commas or semicolons instead of <br> or \n.

%% CORRECT
A["Input: raw data, Output: processed"]

%% WRONG
A[Input: raw data<br>Output: processed]

5. No Chained Arrows

Write each connection on its own line.

%% CORRECT
A --> B
B --> C

%% WRONG
A --> B --> C

6. Comments on Their Own Lines

%% comments must not appear at the end of code lines.

%% CORRECT
%% This is a comment
A --> B

%% WRONG
A --> B %% connection

7. Style Only Defined Nodes

Apply style directives only after the node has been defined.

%% CORRECT
A[Server]
style A fill:#f9f,stroke:#333

%% WRONG
style A fill:#f9f,stroke:#333
A[Server]

8. Subgraph Names

Use underscores or wrap in quotes — no bare spaces.

%% CORRECT
subgraph Backend_Services
subgraph "Backend Services"

%% WRONG
subgraph Backend Services

Diagram Type Quick Reference

| Type | Directive | Use For | | --------- | ------------------------------- | ---------------------------------- | | Flowchart | flowchart TD / flowchart LR | Architecture, data flow, decisions | | Sequence | sequenceDiagram | API calls, request/response | | Class | classDiagram | Type relationships, OOP | | State | stateDiagram-v2 | State machines, lifecycles | | ER | erDiagram | Database schemas, entities | | Gantt | gantt | Timelines, schedules | | Git | gitGraph | Branch strategies, merges |

Common Patterns

Architecture Diagram

flowchart TD
    Client[Browser Client]
    API[API Gateway]
    Auth[Auth Service]
    DB[(Database)]

    Client --> API
    API --> Auth
    Auth --> DB

Sequence Flow

sequenceDiagram
    participant C as Client
    participant S as Server
    participant D as Database

    C->>S: POST /api/data
    S->>D: INSERT query
    D-->>S: Result
    S-->>C: 201 Created

Fixing Broken Diagrams

When asked to fix a broken Mermaid diagram:

Identify the error type — parse errors almost always come from one of: chained arrows, unquoted special characters, bare subgraph names, or HTML in labels.
Apply the relevant rules above — check each rule against the broken code line by line.
Rewrite the offending lines — fix only what is broken; preserve the diagram's intent.
Say what you changed and why, naming the rule each fix applies. This skill has no renderer, so state that the corrected diagram is unverified rather than implying it was checked.

If the diagram is structurally ambiguous (missing nodes, unclear relationships), ask one clarifying question before rewriting.

Agent Skills: Mermaid.js Diagram Generator

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