C4 Context

C4 Context Level: System Context

Use this skill when

• Working on c4 context level: system context tasks or workflows
• Needing guidance, best practices, or checklists for c4 context level: system context

Do not use this skill when

• The task is unrelated to c4 context level: system context
• You need a different domain or tool outside this scope

Instructions

• Clarify goals, constraints, and required inputs.
• Apply relevant best practices and validate outcomes.
• Provide actionable steps and verification.
• If detailed examples are required, open resources/implementation-playbook.md.

System Overview

Short Description

[One-sentence description of what the system does]

Long Description

[Detailed description of the system's purpose, capabilities, and the problems it solves]

Personas

[Persona Name]

• Type: [Human User / Programmatic User / External System]
• Description: [Who this persona is and what they need]
• Goals: [What this persona wants to achieve]
• Key Features Used: [List of features this persona uses]

System Features

[Feature Name]

• Description: [What this feature does]
• Users: [Which personas use this feature]
• User Journey: [Link to user journey map]

User Journeys

[Feature Name] - [Persona Name] Journey

3. ...

[External System] Integration Journey

2. ...

External Systems and Dependencies

[External System Name]

• Type: [Database, API, Service, Message Queue, etc.]
• Description: [What this external system provides]
• Integration Type: [API, Events, File Transfer, etc.]
• Purpose: [Why the system depends on this]

System Context Diagram

[Mermaid diagram showing system, users, and external systems]

Related Documentation

• Container Documentation
• Component Documentation

## Context Diagram Template

According to the [C4 model](https://c4model.com/diagrams/system-context), a System Context diagram shows the system as a box in the center, surrounded by its users and the other systems that it interacts with. The focus is on **people (actors, roles, personas) and software systems** rather than technologies, protocols, and other low-level details.

Use proper Mermaid C4 syntax:

```mermaid
C4Context
    title System Context Diagram

    Person(user, "User", "Uses the system to accomplish their goals")
    System(system, "System Name", "Provides features X, Y, and Z")
    System_Ext(external1, "External System 1", "Provides service A")
    System_Ext(external2, "External System 2", "Provides service B")
    SystemDb(externalDb, "External Database", "Stores data")

    Rel(user, system, "Uses")
    Rel(system, external1, "Uses", "API")
    Rel(system, external2, "Sends events to")
    Rel(system, externalDb, "Reads from and writes to")

Key Principles (from c4model.com):

• Focus on people and software systems, not technologies
• Show the system boundary clearly
• Include all users (human and programmatic)
• Include all external systems the system interacts with
• Keep it stakeholder-friendly - understandable by non-technical audiences
• Avoid showing technologies, protocols, or low-level details

Example Interactions

• "Create C4 Context-level documentation for the system"
• "Identify all personas and create user journey maps for key features"
• "Document external systems and create a system context diagram"
• "Analyze system documentation and create comprehensive context documentation"
• "Map user journeys for all key features including programmatic users"

Key Distinctions

• vs C4-Container agent: Provides high-level system view; Container agent focuses on deployment architecture
• vs C4-Component agent: Focuses on system context; Component agent focuses on logical component structure
• vs C4-Code agent: Provides stakeholder-friendly overview; Code agent provides technical code details

Output Examples

When creating context documentation, provide:

• Clear system descriptions (short and long)
• Comprehensive persona documentation (human and programmatic)
• Complete feature lists with descriptions
• Detailed user journey maps for all key features
• Complete external system and dependency documentation
• Mermaid context diagram showing system, users, and external systems
• Links to container and component documentation
• Stakeholder-friendly documentation understandable by non-technical audiences
• Consistent documentation format

🏰 Rei Skills — Curated by Rootcastle Engineering & Innovation | Batuhan Ayrıbaş
Engineering Beyond Boundaries | admin@rootcastle.com