C4

Writing rule

When this skill writes a durable artifact, read .claude/rules/writing-standard.md. Use the controlled technical writing profile.

/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.

For projects that hit Mermaid's ceiling — L3 (component) precision, auto-zoom across levels, tags/perspectives — pass --dsl to emit a Structurizr DSL workspace instead. See "Escape hatch: Structurizr DSL (L3+)" below.

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
/c4 curios-dog --dsl                   # Structurizr DSL escape hatch (L3+), instead of Mermaid
/c4 curios-dog --dsl --force           # overwrite an existing workspace.dsl

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 (or step 4b if --dsl was passed).

4. Generate the Mermaid (skip if --dsl was passed — go to step 4b)

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.

4b. Generate the Structurizr DSL (--dsl only)

Reuse the same detected model from step 2 (containers, external actors, relationships) — the DSL escape hatch does not re-run detection, it re-renders the same proposal the user already confirmed in step 3 into a different output format. See "Escape hatch: Structurizr DSL (L3+)" below for the full mapping and the L3 (component) authoring note.

Resolve the template the same way as step 4:

dsl_template=$(portfolio_resolve_template architecture/c4-structurizr.dsl)

Falls through to templates/architecture/c4-structurizr.dsl for adopters with no override. Adopter overrides live at <private_repo>/custom-templates/architecture/c4-structurizr.dsl (same path-mirroring convention as the Mermaid templates).

5. Write the files

Path resolution from step 1's table (or its --dsl row). 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 Mermaid file ends with a small footer:

---

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

The generated workspace.dsl ends with the DSL-comment equivalent:

// Generated by /c4 --dsl 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 output

Mermaid (skip if --dsl): 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.

Structurizr DSL (--dsl only): run lint-dsl.sh against the written workspace.dsl. The lint wraps the shared _lib-structurizr-lint.sh — a structural check (balanced braces, workspace/model/views blocks present, no duplicate identifiers), not a full DSL parse, because the escape hatch is dependency-free by design (no Java, no Docker required). If structurizr-cli happens to be on the adopter's PATH, the lint also runs a real parse via it — a bonus, never a requirement.

SKILL_DIR="$(dirname "$(realpath "$0")")"
"$SKILL_DIR/lint-dsl.sh" "$dsl_out" || lint_rc=$?

Same exit-code contract as lint.sh: 1 is a hard fail (print, ask), 2 is bad input, --skip-lint bypasses.

7. Confirm to the user

Mermaid mode:

✓ <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.

--dsl mode:

✓ <project>: Structurizr DSL workspace written

  DSL: <path/to/workspace.dsl>

  Containers: 6 (max 9, but --dsl has no cap — L3 detail lives inside them)
  External: 6 systems, 2 actors
  Structural lint: OK (structurizr-cli not found — structural-only; render to verify visually)

Render: paste the file at https://structurizr.com/dsl (free, zero install),
  or install structurizr-cli / run Structurizr Lite if you want a local
  render step. See "Escape hatch: Structurizr DSL (L3+)" below.
Re-run /c4 <project> --dsl --force when the architecture changes.

Escape hatch: Structurizr DSL (L3+)

Mermaid stays the default — nothing above changes for a plain /c4 invocation. --dsl is a side-channel for projects that hit a real wall, not a migration. Decision rationale: AgDR-0085 (a follow-up to the original tool choice in AgDR-0003, which explicitly deferred this).

When to reach for --dsl

If none of those apply, stay on plain /c4 — Mermaid's zero-install, renders-on-GitHub property is still the better default for L1/L2.

What --dsl generates

One workspace.dsl file (Structurizr's canonical extension) capturing:

• model { } — the same person / softwareSystem / container entries the Mermaid path would generate from steps 2–3, PLUS optional component { } blocks nested inside a container for the L3 detail Mermaid can't express
• views { } — a systemContext, a container, and (for any container you added components to) a component view — all autoLayout, all from the one model
• A styles { } block distinguishing Person, External (softwareSystem outside the boundary), and Database (container/component tagged "Database") shapes/colors

L3 components are not auto-detected. Step 2's detection (this skill's core value) stops at L2 — inferring components from source would mean parsing call graphs, which is a materially bigger feature than this escape hatch. The generated workspace.dsl gives you the L1 + L2 model auto-filled exactly like the Mermaid path, with the L3 component { } blocks present as a worked example inside the template (see templates/architecture/c4-structurizr.dsl) for you to extend by hand into whichever container needs the precision.

Rendering the DSL

No render step is required to get value from the file — workspace.dsl is readable as text, and the escape hatch's whole design point (AgDR-0085) is that it introduces no new runtime dependency to the framework. When you do want to see the diagram:

None of these are required by the framework itself — they're the adopter's choice at render time, same as choosing which Markdown viewer to read a .md file in.

Relationship to the Mermaid path

--dsl reuses steps 1–3 (resolve, detect, confirm) unchanged — the detection logic and the user-facing confirmation table are identical whether you're about to render Mermaid or DSL. Only step 4 (generation) and everything downstream branches. This keeps the two output modes from drifting into two different detection engines.

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. Applies to workspace.dsl exactly like the Mermaid files.
3. Hard cap at 9 containers (Mermaid path only) — collapse before showing the user, never produce a Mermaid diagram with 10+ boxes. If a project genuinely needs more container-level detail, that's a signal to reach for --dsl and add L3 component { } blocks instead of further collapsing L2. The --dsl path itself has no hard container cap — L3 detail is where the extra precision belongs.
4. Don't invent integrations — every System_Ext / softwareSystem "..." "External" 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. (The DSL template's leading // header comment is the DSL-equivalent instructional copy — trim the same way.)
7. Footer signature is mandatory — every generated file ends with the Generated by /c4 on YYYY-MM-DD line (or its //-comment DSL equivalent) 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. Applies regardless of --dsl.
9. --dsl never introduces a runtime dependency — the generation and lint (step 6) paths must work with nothing beyond Bash/awk/grep installed. structurizr-cli / Docker / the online editor are adopter-chosen render-time tools, never a framework requirement.

When to use this

Out of scope (v1)

• L4 (Code) — premature for typical use even with the --dsl escape hatch; teams that need it know they need it
• Auto-detected L3 components — --dsl gives you the model and the view, not automated component inference from source; see "What --dsl generates" above
• Auto-diff against existing diagrams — --force is the v1 escape hatch for both Mermaid and DSL; 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
• Bundling a Structurizr renderer or shelling out to one by default — the framework never installs Java/Docker on the adopter's behalf; rendering is always the adopter's explicit, separate choice

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