Engineer Design Diagram

Engineer Design Diagram

Generates production-grade engineering design diagrams as single-file HTML with inline SVG, grounded in real repository topology. Credit: design palette + arrow-masking pattern inspired by Cocoon AI's architecture-diagram-generator (MIT). See THIRD_PARTY_LICENSES.md.

Overview

Most diagramming tools produce pretty pictures disconnected from reality. This skill does the opposite: it reads the actual repo (package manifests, docker-compose, k8s, terraform, import graph) and emits a diagram that reflects the real system. It also knows how the system changed — PR-diff mode highlights structural deltas, trace mode turns a stack/log into a sequence diagram, and drift mode detects when the architecture has wandered from a stored fingerprint.

Four modes share a common pipeline (DCI grounding → node/edge graph → template fill → fingerprint write). Output is a single self-contained HTML file that opens in any browser, plus a Mermaid text block for copy-paste into docs. Dark theme with semantic OKLCH color coding by component role. Accessible by default (ARIA, <title>/<desc>, reduced-motion, keyboard navigation).

Layout Philosophy — pick the shape before you draw

Dense technical systems want to render wide. This is how Anthropic docs, Linear docs, and Vercel architecture pages present multi-component systems: a sticky left rail for context (nav, invariants, legend) and a generous main column for the diagram itself. Mimic that pattern when your content is dense, and use the simpler single-SVG hero when it isn't.

Two supported output shapes, one decision up front:

Widescreen-first for docs-layout. Target min-width: 1024px; do not add mobile breakpoints for docs-layout output — it's architecture documentation, not a landing page. The diagram needs horizontal breathing room. On narrow viewports the diagram stage scrolls horizontally inside its card while the sidebar stays visible.

Hybrid rendering in docs-layout mode: HTML/CSS for all node cards (easier to maintain, free hover states, content edits don't trigger collision math), SVG overlay positioned absolutely over the node grid for arrows only. Arrows are the one place SVG still wins — fixed pixel coords, clean arrowheads, no brittle CSS-line math. Pure-SVG stays the default for the single-hero shape because the payoff of HTML flex doesn't materialize at small node counts.

Layout-decision signal: if during Step 2 the graph has any of — (a) more than one semantic lane/plane, (b) subcomponent lists of 4+ items per node, (c) the user asks for "docs page" / "architecture page" / references Anthropic/Linear/Vercel docs as exemplars — pick docs-layout. Otherwise stay with single-SVG.

See docs-layout.md for the full widescreen spec (grid geometry, design tokens, sidebar anatomy, arrow overlay placement, hover states).

Prerequisites

• Git repository (for generate/diff/watch modes)
• At least one of: package.json, pyproject.toml, Cargo.toml, go.mod, docker-compose.yml, k8s/*.yaml, terraform/*.tf
• Python 3.9+ (for scripts/fingerprint.py)
• A browser to view output
• Optional: kubectl (cluster introspection), terraform (state parsing), jq (manifest parsing)

Environment Detection (DCI)

!git rev-parse --show-toplevel 2>/dev/null || echo "not a git repo" !git rev-parse --short HEAD 2>/dev/null || echo "no-head"; git branch --show-current 2>/dev/null || echo "no-branch" !ls package.json pyproject.toml Cargo.toml go.mod requirements.txt docker-compose.yml Dockerfile 2>/dev/null | head -10 || echo "no manifests" !command -v jq >/dev/null 2>&1 && jq -r '.name,(.dependencies // {} | keys[]?)' package.json 2>/dev/null | head -20 || echo "no package.json / no jq" !docker compose config --services 2>/dev/null | head -30 || echo "no docker-compose" !find . -maxdepth 3 -type d \( -name k8s -o -name kubernetes -o -name manifests \) 2>/dev/null | head -5 || echo "no k8s dir" !find . -maxdepth 3 -name '*.tf' 2>/dev/null | head -10 || echo "no terraform"

Instructions

Step 1: Mode Detection

Parse the first token of $ARGUMENTS:

All modes share Steps 2-5. Mode-specific variations documented in the per-mode playbook.

Step 2: Build the Node/Edge Graph

Use the DCI block output + targeted reads to populate a working graph:

• Nodes from: docker-compose service names, k8s kind: Deployment/Service/StatefulSet, terraform resource blocks, package manifest names (for standalone apps), detected binaries in bin/ or cmd/.
• Edges from: docker-compose depends_on + exposed ports, k8s Service selectors + NetworkPolicy, terraform resource references, import-graph via Grep (import.*from, require(, use crate::), exposed HTTP/gRPC routes.
• Role classification per node using drawing-rules.md § Role inference. Defaults to slate (external/unknown) when heuristics don't match.
• Groups from: k8s namespaces, terraform modules, docker-compose networks, directory boundaries for monorepos.

For graphs with >50 nodes, fall back to Mermaid (see Step 3 template selection).

Step 3: Choose Template and Render

Apply the layout-selection rule from Layout Philosophy first, then pick the template for the chosen mode:

For single-SVG hero: fill placeholders per drawing-rules.md — color palette, SVG arrow-masking, z-order, 40px spacing, dashed boundaries, legend placement. Keep the grid <pattern>, #020617 canvas, pulsing header dot (with reduced-motion guard).

For docs-layout page: fill placeholders per docs-layout.md — GitHub-inspired palette (#0f1117 / #161b22 / #1c2128), Inter + JetBrains Mono, 260px sticky sidebar, fixed-pixel node stage with SVG arrow overlay, hover states on node cards. No mobile breakpoints. Populate the sidebar with: (a) version badge, (b) "On this page" nav, (c) "Architectural invariants" list (surface up to 5 load-bearing constraints you inferred from the code/config — e.g. "Kernel owns durable state", "All ops return Result<T,E>"), (d) node-type legend.

If node count >50 OR the model signals layout failure (overlapping boxes, arrows crossing through nodes), switch to templates/mermaid-fallback.html regardless of layout shape.

Step 4: Update Fingerprint State

For generate/diff/watch modes, write structural state to ${CLAUDE_PLUGIN_DATA}/arch-state.json (or ~/.claude-state/arch-state.json fallback if the env var is unset):

python3 ${CLAUDE_SKILL_DIR}/scripts/fingerprint.py write --input /tmp/graph.json

Schema documented in fingerprint-spec.md. Fingerprint persists across sessions so watch mode can detect drift.

Step 5: Validate and Open

Run the HTML validator before presenting output:

python3 ${CLAUDE_SKILL_DIR}/scripts/validate_html.py $CWD/.arch/<mode>-<timestamp>.html

Validator confirms: ARIA labels present, reduced-motion rule exists, no unexpected external script sources beyond Google Fonts. If validation fails, iterate on the template fill — don't ship an inaccessible diagram.

Open the result via ${CLAUDE_SKILL_DIR}/scripts/open_in_browser.sh (OS-aware: xdg-open on Linux, open on macOS, wslview on WSL). Echo the Mermaid equivalent to the chat as a copy-pasteable text block.