C4 Architecture Documentation

C4 is a methodology, not a notation: it documents a system at four zoom levels so each audience sees the right amount of detail. The discipline is choosing the level deliberately and stopping early — most teams over-diagram.

The four levels — pick by audience

Context + Container is enough for most software teams. Reach for Component only when one container is complex enough that its internals are a real question; reach for Deployment when how-it-runs matters. A diagram nobody asked a question about is maintenance debt.

Workflow

1. Scope — decide which level(s) the audience actually needs (default: 1+2).
2. Analyze — read the codebase to identify the real containers (deployable units) and their relationships, not the aspirational architecture.
3. Generate — write the diagrams in Mermaid C4 syntax (see mermaid-diagrams for the rendering mechanics).
4. Document — commit them to markdown with a sentence of context per diagram explaining what decision it informs.

Example — Level 1

C4Context
    title System Context — Orders Service
    Person(customer, "Customer", "Places orders")
    System(orders, "Orders Service", "Accepts and tracks orders")
    System_Ext(payments, "Payments", "Third-party processor")
    Rel(customer, orders, "Places order")
    Rel(orders, payments, "Charges via", "HTTPS")

Keep it true

Architecture docs are only valuable if they match reality — diagram the containers that actually deploy, and update the C4 markdown in the same PR that changes the architecture. A pretty diagram of a system that no longer exists is worse than none.

Adapted from the MIT-licensed softaworks/agent-toolkit c4-architecture skill.