Mermaid Skill

Use this skill to quickly validate Mermaid diagrams by parsing + rendering them with the official Mermaid CLI. Includes a syntax reference for every Mermaid diagram type.

Arguments

When this skill is invoked, the user may provide a file path and name as an argument:

code

@skills/mermaid/skill.md <filename.mmd>

Examples: vue-architecture.mmd, docs/auth-flow.mmd, db-schema.mmd

•If a filename is provided, use it exactly as given.
•If no filename is provided, derive a descriptive kebab-case name from the diagram's subject (e.g. user-auth-flow.mmd, payment-sequence.mmd, order-er-diagram.mmd). Place it in the project root.
•Never reuse a generic name like diagram.mmd. Every diagram gets its own descriptively named file.

Prerequisites

•Node.js + npm (for npx).
•First run downloads a headless Chromium via Puppeteer. If Chromium is missing, set PUPPETEER_EXECUTABLE_PATH.

Tool

Validate a diagram

The validate script lives inside the skill directory. Use the path relative to the skill, not the working directory:

bash

skills/mermaid/tools/validate.sh <filename.mmd> [output.svg]

•Parses and renders the Mermaid source.
•Non-zero exit = invalid Mermaid syntax.
•Prints an ASCII preview using beautiful-mermaid (best-effort; not all diagram types are supported).
•If output.svg is omitted, the SVG is rendered to a temp file and discarded.

Workflow (short)

•Choose a descriptive filename for the diagram (from the argument or derived from the subject). Use kebab-case with .mmd extension.
•Write the Mermaid source to that file in the project root (or the path given).
•Run skills/mermaid/tools/validate.sh <filename.mmd>.
•Fix any errors shown by the CLI.
•Always show the ASCII preview output to the user. The tool prints an ASCII rendering of the diagram — display it in your response so the user can see the result without opening another tool.
•Once it validates, if the diagram will live in Markdown, copy the Mermaid block into the target Markdown file.

Diagram Type Reference

Pick the right diagram for the job. Each section shows what it's for and the essential syntax.

1. Flowchart

Use for: Process flows, decision trees, workflows, algorithms.

mermaid

flowchart TD
    A[Start] --> B{Decision?}
    B -->|Yes| C[Action 1]
    B -->|No| D[Action 2]
    C --> E[End]
    D --> E

Directions: TB/TD (top-down), BT, LR, RL

Node shapes: A[rect] A(rounded) A([stadium]) A[[subroutine]] A[(cylinder)] A((circle)) A{diamond} A{{hexagon}} A[/parallelogram/] A[/trapezoid\] A(((double circle)))

Links: --> arrow, --- open, -.-> dotted, ==> thick, ~~~ invisible, <--> bidirectional. Add text: -->|text| or -- text -->

Subgraphs: subgraph id[title] ... end

Styling: style A fill:#f9f,stroke:#333 or classDef cls fill:#f9f; A:::cls

2. Sequence Diagram

Use for: API calls, service interactions, request/response flows, protocol sequences.

mermaid

sequenceDiagram
    participant A as Alice
    participant B as Bob
    A->>B: Hello
    B-->>A: Hi back
    A->>B: How are you?
    B-->>A: Great!

Participants: participant, actor. Stereotypes (JSON config): boundary, control, entity, database, collections, queue

Arrows: ->> solid+arrowhead, -->> dotted+arrowhead, -x cross end, -) async open arrow, <<->> bidirectional

Activations: activate A / deactivate A or shorthand A->>+B: msg / B-->>-A: reply

Notes: Note right of A: text or Note over A,B: text

Control flow:

•loop text ... end
•alt text ... else ... end
•opt text ... end
•par [Action 1] ... and [Action 2] ... end
•critical [text] ... option [text] ... end
•break [text] ... end

Grouping: box color Title ... end

Background highlight: rect rgb(0,255,0) ... end

3. Class Diagram

Use for: OOP class structures, inheritance hierarchies, domain models.

mermaid

classDiagram
    Animal <|-- Duck
    Animal <|-- Fish
    Animal : +int age
    Animal : +String gender
    Animal : +isMammal() bool
    Duck : +swim()
    Fish : +canEat() bool

Members: Bracket syntax Class { +method() returnType; -field type }. Visibility: + public, - private, # protected, ~ internal. Classifiers: * abstract, $ static.

Relationships: <|-- inheritance, *-- composition, o-- aggregation, --> association, ..> dependency, ..|> realization

Cardinality: ClassA "1" --> "*" ClassB

Annotations: <<Interface>>, <<Abstract>>, <<Enumeration>>, <<Service>>

Generics: List~int~

Namespaces: namespace Name { class A { } }

Notes: note "text" or note for ClassName "text"

4. State Diagram

Use for: Finite state machines, lifecycle states, object state transitions.

mermaid

stateDiagram-v2
    [*] --> Idle
    Idle --> Processing : submit
    Processing --> Done : complete
    Processing --> Error : fail
    Error --> Idle : retry
    Done --> [*]

States: stateName, stateName : Description, state "Description" as s1

Start/End: [*] --> First, Last --> [*]

Composite: state CompositeState { InnerA --> InnerB }

Choice: state decision <<choice>>

Fork/Join: state fork <<fork>>, state join <<join>>

Notes: note right of State : text ... end note

Concurrency: state Parallel { A --> B -- C --> D } (use -- separator)

Direction: direction LR (or RL, TB, BT)

5. Entity Relationship Diagram

Use for: Database schemas, data models, table relationships.

mermaid

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    PRODUCT ||--o{ LINE-ITEM : "is in"

Cardinality: || exactly one, o| zero or one, }| one or more, o{ zero or more

Line style: -- identifying (solid), .. non-identifying (dashed)

Attributes:

code

ENTITY {
    string name PK
    int age
    string email UK "user email"
}

Key types: PK, FK, UK

6. Gantt Chart

Use for: Project schedules, task timelines, sprint planning.

mermaid

gantt
    title Project Plan
    dateFormat YYYY-MM-DD
    section Phase 1
    Task A           :a1, 2024-01-01, 30d
    Task B           :after a1, 20d
    section Phase 2
    Task C           :2024-02-15, 12d
    Milestone        :milestone, m1, 2024-03-01, 0d

Task format: Task name :tags, id, start, end/duration. Tags: done, active, crit, milestone

Dependencies: after taskId, until taskId

Date config: dateFormat YYYY-MM-DD, axisFormat %Y-%m-%d

Excludes: excludes weekends or specific dates

7. Pie Chart

Use for: Proportions, distributions, market share.

mermaid

pie showData title Browser Market Share
    "Chrome" : 65
    "Safari" : 19
    "Firefox" : 4
    "Other"  : 12

Syntax: pie [showData] [title text] then "label" : value per line. Values must be positive numbers.

8. Git Graph

Use for: Branch strategies, release flows, git workflows.

mermaid

gitGraph
    commit
    branch develop
    checkout develop
    commit
    commit
    checkout main
    merge develop
    commit tag:"v1.0"

Commands: commit, branch name, checkout name, merge name, cherry-pick id:"commitId"

Commit options: id: "abc", tag: "v1.0", type: NORMAL|REVERSE|HIGHLIGHT

9. Mindmap

Use for: Brainstorming, topic hierarchies, concept maps.

mermaid

mindmap
    root((Central Idea))
        Topic A
            Subtopic 1
            Subtopic 2
        Topic B
            Subtopic 3

Hierarchy: Indentation defines parent-child relationships.

Shapes: [square], (rounded), ((circle)), )cloud(, {{hexagon}}, ))bang(( or plain text.

Icons: ::icon(fa fa-book) after node text.

Classes: :::className after node text.

10. Timeline

Use for: Historical events, release histories, roadmaps.

mermaid

timeline
    title Project Milestones
    section 2024 Q1
        January : Kickoff : Team formed
        March   : MVP released
    section 2024 Q2
        June    : Public launch

Syntax: time period : event1 : event2 or stack events on separate lines with : event.

Sections: section Title groups subsequent entries.

11. User Journey

Use for: UX flows, customer experience mapping, satisfaction scoring.

mermaid

journey
    title User Purchase Flow
    section Browse
        Visit site: 5: User
        Search product: 3: User
    section Checkout
        Add to cart: 5: User
        Enter payment: 2: User, System
        Confirm order: 4: User, System

Syntax: Task name: score: actor1, actor2. Score is 1-5 (1=bad, 5=great).

Sections: Group tasks into phases.

12. Quadrant Chart

Use for: Priority matrices, competitive analysis, effort-vs-impact plots.

mermaid

quadrantChart
    title Feature Priority
    x-axis Low Effort --> High Effort
    y-axis Low Impact --> High Impact
    quadrant-1 Do First
    quadrant-2 Plan
    quadrant-3 Eliminate
    quadrant-4 Delegate
    Feature A: [0.8, 0.9]
    Feature B: [0.2, 0.7]
    Feature C: [0.6, 0.3]

Points: Label: [x, y] where x,y are 0-1. Optional: radius: 12, color: #ff3300

13. Sankey Diagram

Use for: Flow quantities, energy/resource distribution, budget allocation.

mermaid

sankey
    Source A,Target X,100
    Source A,Target Y,50
    Source B,Target Y,80
    Source B,Target Z,30

Syntax: CSV format source,target,value. Wrap commas in quotes: "A, Inc",B,100

14. XY Chart (Bar/Line)

Use for: Data trends, comparisons, statistics.

mermaid

xychart
    title "Monthly Sales"
    x-axis "Month" [Jan, Feb, Mar, Apr, May]
    y-axis "Revenue" 0 --> 500
    bar [120, 200, 150, 300, 450]
    line [100, 180, 160, 280, 420]

Orientation: xychart horizontal for horizontal layout.

X-axis: categorical ["a", "b"] or numeric min --> max. Y-axis: numeric only.

Data: bar [values] and/or line [values].

15. Block Diagram

Use for: System architecture layouts, infrastructure diagrams, component arrangements.

mermaid

block-beta
    columns 3
    A["Frontend"] B["API"] C["Database"]
    D["Cache"]:2 E["Queue"]
    A --> B --> C
    B --> D

Columns: columns N sets grid width. Blocks span columns with :N.

Shapes: Same as flowchart: [rect], (rounded), ((circle)), {diamond}, [(cylinder)], etc.

Links: -->, --- with optional text.

Nesting: Blocks can contain other blocks for composite layouts.

Space: Use space or space:N to insert empty cells.

16. Packet Diagram

Use for: Network protocol headers, binary data formats, memory layouts.

mermaid

packet-beta
    0-15: "Source Port"
    16-31: "Destination Port"
    32-63: "Sequence Number"
    64-95: "Acknowledgment Number"

Syntax: start-end: "Field name" for ranges, or +N: "Field name" for N-bit auto-positioned fields.

17. Architecture Diagram

Use for: Cloud infrastructure, CI/CD pipelines, service topology.

mermaid

architecture-beta
    group api(cloud)[API Layer]

    service db(database)[PostgreSQL] in api
    service server(server)[App Server] in api
    service client(internet)[Client]

    client:R --> L:server
    server:R --> L:db

Groups: group id(icon)[Title] — containers for services. Nest with in parentId.

Services: service id(icon)[Title] — individual nodes. Nest with in groupId.

Edges: serviceA:direction --> direction:serviceB. Directions: T, B, L, R.

Junctions: junction id — four-way connection points.

Icons: Built-in: cloud, database, disk, internet, server. Custom via iconify: "logos:aws"

18. Kanban Board

Use for: Task boards, workflow stages, agile boards.

mermaid

kanban
    todo[To Do]
        task1[Design mockups]
        task2[Write specs]
    inprog[In Progress]
        task3[Build API]
    done[Done]
        task4[Setup repo]

Columns: id[Title] at root level.

Tasks: id[Description] indented under a column.

Metadata: @{ assigned: "Alice", ticket: "PROJ-123", priority: "High" } after task.

Config: ticketBaseUrl in YAML frontmatter creates clickable ticket links.