Diagramming Skill | Agent Skills

Diagramming

Overview

Create maintainable technical diagrams in Mermaid (renders in GitHub, GitLab, and most doc tools). Diagrams live in version control next to code — they must stay in sync or they mislead. This skill covers type selection, syntax, and the parsing footguns that waste the most time.

Choose the diagram type

| Type | Use when | Not when | | --------------- | ----------------------------------------------- | ---------------------------------------- | | Sequence | Interactions over time across participants | Showing static structure | | Flowchart | Decision logic, process/pipeline steps | Timing between services (use sequence) | | State | An entity's lifecycle + transitions/guards | Data flow or call order | | ERD | Data model, tables, cardinality | Runtime behavior | | Class | OO structure, interfaces, inheritance | Deployment or infra | | C4 (Context) | System boundary + external actors/systems | Internal code detail | | C4 (Container) | Deployable units + their tech + data stores | Class-level detail |

Rule of thumb: structure → flowchart/C4/class/ERD; behavior over time → sequence; lifecycle → state. For architecture, prefer C4's layered zoom (Context → Container → Component) over one sprawling diagram.

Gotchas (Mermaid parsing footguns)

These cause silent render failures or garbled output far more often than logic errors.

end is reserved in flowcharts. A lowercase node id end breaks the parser. Use End, END, or quote it: id["end"]. Same care with subgraph/click/class as bare ids.
Quote labels with special characters. Parentheses (), {}, [], #, :, ;, and quotes inside a label break parsing — wrap the text: A["fetch(url)"], B["step: parse"]. For literal special chars inside a quoted label, use HTML entities: #quot; won't work — use ", &, # (for #).
Line breaks: <br/> inside a label, not \n: A["Line one<br/>Line two"].
Edge labels with special chars must be quoted: A -->|"retry (max 3)"| B.
Leading o/x on an edge become arrowheads. A---oB renders a circle end, A---xB a cross. Add a space (A --- oB is still risky) or rename the node so an edge doesn't touch a bare o/x.
Subgraph direction: set direction LR inside the subgraph; note that edges crossing subgraph boundaries can override a subgraph's internal direction.
Comments are %% on their own line. Semicolons are optional line terminators.
C4 diagrams (C4Context/C4Container) have limited, sometimes experimental layout control and lag other Mermaid features — verify they render in your target tool before committing to them; a plain flowchart with subgraphs is a robust fallback.
Don't build the giant diagram. Past ~15-20 nodes it's unreadable and un-reviewable. Split by concern or zoom level. One diagram, one idea.

Keep diagrams in sync

Diagrams are code artifacts: update the diagram in the same PR as the code it depicts, review it in the diff, and prefer a diagram that's easy to regenerate over a pixel-perfect one that rots. A wrong diagram is worse than none.

Syntax quick reference

Direction: flowchart TB (top-bottom), LR (left-right). Sequence diagrams auto-layout.
Node shapes: [Rect] process · (Rounded) start/end · {Diamond} decision · [(DB)] store · ((Circle)) connector.
Edges: --> solid · -.-> dotted/optional · ==> emphasis · ->> (sequence) sync message · -->> async/return.
Sequence keywords: participant, autonumber, alt/else/end, loop/end, par/and/end, Note over A,B.

Examples

C4 Context (Level 1)

C4Context
    title System Context — E-Commerce Platform
    Person(customer, "Customer", "Browses and purchases")
    System(ecommerce, "E-Commerce Platform", "Core system")
    System_Ext(payment, "Payment Gateway", "Processes payments")
    Rel(customer, ecommerce, "Uses")
    Rel(ecommerce, payment, "Charges", "HTTPS")

C4 Container (Level 2)

C4Container
    title Containers — E-Commerce Platform
    Person(customer, "Customer")
    Container_Boundary(ec, "E-Commerce Platform") {
        Container(web, "Web App", "React", "Customer UI")
        Container(api, "API Gateway", "Node.js", "REST API")
        ContainerDb(db, "Database", "PostgreSQL", "Stores data")
        ContainerQueue(queue, "Queue", "RabbitMQ", "Async events")
    }
    Rel(customer, web, "Uses", "HTTPS")
    Rel(web, api, "Calls", "JSON/HTTPS")
    Rel(api, db, "Reads/writes")
    Rel(api, queue, "Publishes")

Sequence — request flow with conditionals

sequenceDiagram
    autonumber
    participant C as Client
    participant S as Service
    participant D as Database
    C->>S: POST /api/users
    S->>S: Validate input
    alt Validation fails
        S-->>C: 400 Bad Request
    else Valid
        S->>D: INSERT user
        alt Constraint violation
            D-->>S: Duplicate key
            S-->>C: 409 Conflict
        else Success
            D-->>S: Created
            S-->>C: 201 Created
        end
    end

Sequence — parallel work

sequenceDiagram
    participant O as Order Service
    participant I as Inventory
    participant P as Payment
    par Check inventory and authorize payment
        O->>I: Check stock
        I-->>O: Available
    and
        O->>P: Authorize
        P-->>O: Authorized
    end
    O->>P: Capture payment

Flowchart — decision logic

flowchart TD
    A[User Login] --> B{Valid credentials?}
    B -->|No| D[Show error]
    D --> A
    B -->|Yes| C{2FA enabled?}
    C -->|No| G[Create session]
    C -->|Yes| E[Send 2FA code]
    E --> F{Code valid?}
    F -->|Yes| G
    F -->|No| H{Attempts < 3?}
    H -->|Yes| E
    H -->|No| I[Lock account]
    G --> J[Dashboard]

ERD

Cardinality: || exactly one · o| zero-or-one · }| one-or-more · }o zero-or-more (left char = min, right char = max, read toward the entity).

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ ORDER_ITEM : contains
    ORDER_ITEM }|--|| PRODUCT : references
    USER {
        uuid id PK
        string email UK
        timestamp created_at
    }
    ORDER {
        uuid id PK
        uuid user_id FK
        string status
    }

State machine

stateDiagram-v2
    [*] --> Draft: create
    Draft --> Pending: submit
    Pending --> Confirmed: payment received
    Pending --> Cancelled: payment failed / timeout
    Confirmed --> Shipped: fulfill
    Shipped --> Delivered: confirmed
    Delivered --> [*]
    note right of Pending
        Auto-cancels after 24h
    end note

Class diagram

classDiagram
    class Repository~T~ {
        <<interface>>
        +findById(id) T
        +save(entity: T) T
    }
    class UserRepository {
        -db: Database
        +findByEmail(email) User
    }
    Repository~T~ <|.. UserRepository
    UserRepository --> User

Verify before done

[ ] Diagram renders in the target tool (GitHub/GitLab/docs), not just a local previewer — especially for C4.
[ ] Labels with (), #, :, ,, or quotes are wrapped in "…"; no bare end node id in a flowchart.
[ ] Diagram type matches intent (behavior→sequence, structure→flowchart/C4, lifecycle→state).
[ ] ≤~20 nodes; split otherwise. One diagram, one concept.
[ ] Matches current code; updated in the same PR as the change it depicts.

Agent Skills: Diagramming

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