Arc42 Framework

Origin: plugin-original

arc42-framework

This skill is the knowledge base behind the arc42 plugin. It turns a target codebase into arc42 architecture documentation by (1) reading the repo into a fact base and (2) authoring the twelve arc42 sections from that evidence. It does not copy arc42's prose; it routes each generated claim to recorded evidence, an explicit inference, or a typed GAP flag.

The bundled references under references/ are the plugin's working knowledge; the generated output format is fixed by references/output-conventions.md.

Recipe overview

1. Build the evidence base. Parse manifests, import/dependency graphs, configs, IaC, route tables, and naming conventions into fact records (F-NNN) with a precise source and an extraction_method. This is the only place facts enter the system.
2. Author sections in dependency order (see Waves). Each sub-section pulls from the fact base, cites fact ids per claim, and chooses a diagram per references/diagram-conventions.md.
3. Flag, never fabricate. Where the code cannot answer, emit GAP:human-input; where evidence should exist but was not found, emit GAP:no-evidence. All gaps aggregate into _gaps.md.
4. Lint and check consistency. Apply references/lint-checklist.md (per-section rules) and references/consistency-rules.md (cross-section invariants) before delivery.

The exact output layout, YAML front-matter, fact-record schema, per-sub-section arc42-meta blocks, per-claim anchors, and typed GAP flags are specified in references/output-conventions.md.

Evidence-tier table

Each section is classed by how far the code alone can carry it. The tier sets the default provenance and confidence, and decides whether a missing answer becomes a Mermaid diagram, a caveated diagram, or a GAP:human-input.

These tiers are recorded per section in references/sections/NN-*.md under ## Evidence tier.

Generation waves (code-derivable first)

Sections are generated in three waves by the generate command. The wave order is code-derivable structural sections first → dependent sections → synthesis, not a strict topological sort of the Depends on edges declared in references/sections/NN-*.md.

Cross-section consistency is enforced post-hoc by the consistency-checker (Phase C) — particularly the §3↔§5 interface symmetry (interfaces-match). Other Depends on edges are best-effort cross-references: a section may be authored before all of its declared upstream sections exist, and that is expected. Do not assume that all inputs pre-exist when a section is dispatched.

• Wave 0 — Evidence base. Extract fact records from the repo. Precedes all sections.
• Wave 1 — §3 Context and Scope, §5 Building Block View, §7 Deployment View, §12 Glossary. These four sections are code-derivable (structure readable directly from manifests, import graphs, IaC, and route tables). They run first with no prior wave output, in parallel.
• Wave 2 — §6 Runtime View, §8 Crosscutting Concepts, §4 Solution Strategy, §9 Architecture Decisions, §2 Constraints. These five sections receive Wave 1 output as upstream context where available. Because §1 is not yet generated, upstream deps that reference §1 are absent and treated as best-effort. All five run in parallel.
• Wave 3 — §1 Introduction and Goals, §10 Quality, §11 Risks and Technical Debt. These synthesis sections are authored last, after the structural and dependent sections are complete. §10 and §11 receive Wave 2 output. All three run in parallel.

Why the knowledge base is derived, not copied

The references/ corpus is rewritten from the arc42 material rather than bundled verbatim, for four reasons:

• Licensing. arc42 material is CC BY-SA 4.0. Derived, reworded, attributed references keep the plugin's obligations clean and auditable instead of shipping large verbatim excerpts.
• Portability. The plugin ships self-contained reference files with no runtime dependency on the upstream arc42 site or its asset tree; generation works fully offline.
• Fitness for agents. The corpus is restructured into machine-actionable artifacts — a lint checklist, cross-section consistency rules, a keyword taxonomy, a diagram-convention map, and an expected-topics catalog — which an agent can apply directly, unlike narrative documentation.
• Augmentation. Each section spec adds plugin-original evidence-routing (what to look for in a repo, which extraction method, which Mermaid type, which GAP to emit) that arc42 itself does not provide, because arc42 documents the template, not how to derive it from code.

Corpus Derivation Contract (the bar for future editors)

Every arc42-derived reference file MUST keep sourcing from the crawled corpus snapshot (.../crawl-arc42-doc/data/output), MUST carry a Source: line citing the corpus-relative paths it derives from, MUST be reworded (no verbatim runs from the source), and MUST be accounted for in references/COVERAGE.md (every corpus file maps to a destination or a justified dropped: reason). Plugin-original files (this SKILL.md, output-conventions.md, COVERAGE.md) are marked Origin: plugin-original and need no Source:. The gate is plugins/arc42/tests/check_corpus_contract.py; keep it green and keep its --corpus pointed at the corpus root.

arc42 is the work of Dr. Gernot Starke and Dr. Peter Hruschka; this knowledge base is derived from the arc42 framework material, used under CC BY-SA 4.0.