c4

name: c4 description: Generate C4 L1 (Context) + L2 (Container) Mermaid diagrams from a project's codebase. argument-hint: "[project-name] [--level=1|2|both] [--force]" allowed-tools: Bash, Read, Grep, Glob, Write

/c4 — Generate C4 Architecture Diagrams

Reads the target project's codebase and produces filled-in Level 1 (System Context) and Level 2 (Container) diagrams as Mermaid markdown. Saves the slog of filling in the templates by hand for a repo you already understand structurally.

This skill complements /handover (which seeds a stub L2 once at onboarding). Use /c4 whenever the architecture changes substantially and the diagrams need a refresh — or for a project that wasn't onboarded via /handover.

Path resolution

Read the registry path via portfolio_registry, the per-project docs dir via portfolio_projects_dir, and the ideas backlog via portfolio_ideas_backlog — all from .claude/hooks/_lib-portfolio-paths.sh. Source the helper at the top of any bash block that touches those paths:

source "$(git rev-parse --show-toplevel)/.claude/hooks/_lib-read-config.sh"
source "$(git rev-parse --show-toplevel)/.claude/hooks/_lib-portfolio-paths.sh"
registry=$(portfolio_registry)

Defaults match today's single-fork layout (./apexyard.projects.yaml, ./projects, ./projects/ideas-backlog.md). Adopters in split-portfolio mode override the portfolio.{registry, projects_dir, ideas_backlog} keys in .claude/project-config.json. Don't hardcode literal apexyard.projects.yaml or projects/ paths in bash blocks — the helper resolves whichever mode the adopter is in. See docs/multi-project.md.

Usage

/c4                                    # current cwd, both levels
/c4 curios-dog                         # registered project, both levels
/c4 curios-dog --level=1               # only the L1 system-context diagram
/c4 . --level=2                        # only the L2 container diagram for cwd
/c4 curios-dog --force                 # overwrite existing diagrams

Output location

Where the files land depends on where the skill is invoked from and what argument is passed:

The split mirrors the existing convention from docs/multi-project.md § "Architecture diagrams".

Process

1. Resolve the target

• If <project-name> is . → use cwd.
• If <project-name> is given and the registry has it → use workspace/<name>/ if it exists, otherwise fall back to ops-view-only mode (no codebase to scan; ask the user to clone or to provide a path).
• If no arg → use cwd; if cwd is the ops fork root, ask whether the diagram is framework-wide or for a registered project.

If the cwd / target doesn't have any of the detection signals listed below (no package.json, no Dockerfile, no template.yaml, etc.), stop and tell the user — there's nothing to scan.

2. Detect

Run these in parallel; collect findings into a structured proposal.

2a. Containers (L2)

A "container" in C4 is a deployable / runnable unit — a frontend, an API, a database, a queue, a worker, a CDN. Not a Docker container (confusing but standard).

Detection sources:

Hard cap: 9 containers max. If detection yields more, collapse the most-similar pair into a single container with a combined label, and surface the collapse to the user during step 3.

2b. External actors and systems (L1)

External actors fall into three buckets — Person (humans), System_Ext (third-party SaaS / APIs), and the System (the box being modelled).

Detection sources:

If a signal could match multiple personas (e.g., the API serves both end users and admins), surface both; the user can collapse during confirm.

Detection should also pull the project's one-sentence description from:

• The README's first non-heading paragraph
• The description field in package.json
• An existing projects/<name>/README.md if the registry has the project

If none of those exist, ask the user for one sentence in step 3.

3. Confirm with the user

Show the detected proposal in a compact table:

For <project>:

External actors (L1):
  [Person] End user — uses the public profile pages
  [Person] Admin — manages reports and users
  [Ext] AWS Cognito — authentication
  [Ext] Amazon Bedrock — text embeddings for similarity
  [Ext] PostHog EU — product analytics
  [Ext] DiceBear — avatar generation

Containers (L2, inside the system boundary):
  Web App         Next.js 16        — public profile pages, sign-in, dashboard
  Backend API     AWS Lambda + SAM  — Q&A endpoints, profiles, likes, search
  Admin App       Next.js + Cognito — moderation console
  DynamoDB        single-table      — questions, answers, profiles, likes
  S3 (uploads)    public-read       — avatar + answer-attachment storage
  CloudFront      asset CDN         — public-asset distribution

One-sentence description:
  "Public Q&A platform — anonymous askers, public answers, share-driven growth."

Edit? (a) accept · (e) edit list · (d) edit description · (q) quit
>

On e: open an interactive add/remove flow — one item per prompt, accept by Enter, or type add: <new item> / remove: <name>. On d: prompt for a one-sentence replacement description. On q: exit without writing. On a: proceed to step 4.

4. Generate the Mermaid

Resolve the C4 templates via the portfolio helper so adopter overrides win when present:

source "$(git rev-parse --show-toplevel)/.claude/hooks/_lib-read-config.sh"
source "$(git rev-parse --show-toplevel)/.claude/hooks/_lib-portfolio-paths.sh"
context_template=$(portfolio_resolve_template architecture/c4-context.md)     # L1 skeleton
container_template=$(portfolio_resolve_template architecture/c4-container.md) # L2 skeleton

Single-fork adopters (no portfolio block) and adopters with no override fall straight through to templates/architecture/c4-{context,container}.md. Adopters who want a customised C4 shape drop their versions at <private_repo>/custom-templates/architecture/c4-{context,container}.md. See templates/README.md for the path-mirroring convention.

Keep the surrounding markdown sections from the resolved templates ("How to use this template" can be trimmed in the generated file since these are real diagrams, not templates).

For L1:

C4Context
    title System Context for {Project Name}

    Person(<id>, "<Display>", "<Description>")
    ...
    System(main, "{Project Name}", "<one-sentence description>")
    System_Ext(<id>, "<Name>", "<Tech / role>")
    ...
    Rel(<from>, <to>, "<Verb>", "<Protocol>")
    ...

For L2:

C4Container
    title Container Diagram for {Project Name}

    Person(<id>, "<Display>", "<Description>")
    ...
    System_Boundary(boundary, "{Project Name}") {
        Container(<id>, "<Name>", "<Tech>", "<Responsibility>")
        ContainerDb(<id>, "<Name>", "<Tech>", "<What it stores>")
    }
    System_Ext(<id>, "<Name>", "<Tech>")
    ...
    Rel(<from>, <to>, "<Verb>", "<Protocol>")
    ...

Relationship inference rules:

• Person → primary Container (Web / API) over HTTPS
• Web → API over HTTPS / JSON
• API → ContainerDb over the DB protocol (SQL / DynamoDB / etc.)
• API → System_Ext over the integration protocol (OAuth / HTTPS / SMTP)
• Worker → Queue if a queue is detected
• All System_Ext arrows point outward from the system boundary

Don't over-annotate. If a relationship is obvious ("Web calls API"), keep the verb to one word.

5. Write the files

Path resolution from step 1's table. Behaviour:

• If the file does not exist → write directly.
• If the file does exist:
  • Without --force: stop, print a diff against the proposed content, ask the user to either re-run with --force or merge by hand.
  • With --force: overwrite. Print a diff so the user sees what changed.

Each generated file ends with a small footer:

---

_Generated by `/c4` on YYYY-MM-DD. Re-run after architecture changes._

This is the skill's signature — readers know it's regenerable, not hand-maintained.

6. Lint the generated Mermaid

Run lint.sh against each file written in step 5. The lint wraps the shared _lib-mermaid-lint.sh — extracts every ```mermaid block and validates each via mmdc (mermaid-cli) so broken syntax is caught at write time, not when a human opens the file on GitHub. Graceful-degrades when Node / npx is unavailable (exit 3, advisory only; doesn't block the skill).

SKILL_DIR="$(dirname "$(realpath "$0")")"
"$SKILL_DIR/lint.sh" "$context_out" || lint_rc=$?
"$SKILL_DIR/lint.sh" "$container_out" || lint_rc=$?

Treat exit 1 (parse error) as a hard fail — print the lint output, ask the operator whether to (a) auto-regenerate the offending block, (b) keep the file as-is and fix by hand, or (c) re-run with --skip-lint if mmdc is misbehaving. Exit 3 (Node missing) prints a one-line warning and proceeds.

7. Confirm to the user

✓ <project>: C4 diagrams written

  L1: <path/to/context.md>
  L2: <path/to/container.md>

  Containers: 6 (max 9)
  External: 6 systems, 2 actors
  Mermaid lint: 2 of 2 files parsed cleanly

Preview: open the file on GitHub — Mermaid renders inline.
Re-run /c4 <project> --force when the architecture changes.

Rules

1. Read-only against the codebase — never modify the project's source. Only writes to the architecture-doc paths in step 5.
2. Never auto-overwrite — existing diagrams require explicit --force. The diagrams may have been hand-edited; clobbering them silently is the worst-case failure.
3. Hard cap at 9 containers — collapse before showing the user, never produce a diagram with 10+ boxes. If a project genuinely needs more than 9, that's an L3 (Component) diagram, which is out of scope for v1.
4. Don't invent integrations — every System_Ext must be backed by a concrete signal in step 2. If you saw nodemailer in package.json, you can list "Email provider"; if you didn't, you can't list "Email provider" because most apps eventually need one.
5. One-sentence description is required — if no source supplies one, ask the user. Never ship a diagram with a placeholder system description like "what the system does".
6. Trim the template's "How to use this template" section in the generated output — that's instructional copy for the templates, not for filled-in diagrams.
7. Footer signature is mandatory — every generated file ends with the Generated by /c4 on YYYY-MM-DD line so future readers know it's regenerable.
8. Refuse if there's nothing to scan — no package.json, no IaC, no Dockerfile, no src/ → stop with an error rather than producing an empty diagram.

When to use this

Out of scope (v1)

• L3 (Component) and L4 (Code) — premature for typical use; teams that need them know they need them
• Auto-diff against existing diagrams — --force is the v1 escape hatch; smarter merging is a separate skill if it proves needed
• Multi-system canvases — single-system per invocation
• Auto-PR creation — the skill writes files; the user commits via the normal PR flow (apexyard hooks ensure that)
• Sequence / deployment / data-flow diagrams — different DSLs, different audiences, different skills if needed

Part of ApexYard — multi-project SDLC framework for Claude Code · MIT.