improve-codebase-architecture

name	improve-codebase-architecture
description	Find deepening opportunities in the codebase, informed by the domain language in CONTEXT.md, the project's design tenets, and the decisions in docs/ADR/. Use when the user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, or make the codebase more testable and AI-navigable — or says 「アーキテクチャ改善」「リファクタの種を探して」「設計を深く」.

Improve Codebase Architecture

Surface architectural friction and propose deepening opportunities — refactors that turn shallow modules into deep ones. The aim is testability and AI-navigability.

Glossary

Use these terms exactly in every suggestion. Consistent language is the point — don't drift into "component," "service," "API," or "boundary." Full definitions in LANGUAGE.md.

• Module — anything with an interface and an implementation (function, trait + impl, crate module, tier-spanning slice).
• Interface — everything a caller must know to use the module: types, invariants, error modes, ordering, config. Not just the type signature.
• Implementation — the code inside.
• Depth — leverage at the interface: a lot of behaviour behind a small interface. Deep = high leverage. Shallow = interface nearly as complex as the implementation.
• Seam — where an interface lives; a place behaviour can be altered without editing in place. (Use this, not "boundary" — "boundary" is reserved for Plecto's capability/trust boundary.)
• Adapter — a concrete thing satisfying an interface at a seam.
• Leverage — what callers get from depth.
• Locality — what maintainers get from depth: change, bugs, knowledge concentrated in one place.

Key principles (see LANGUAGE.md for the full list):

• Deletion test: imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep.
• The interface is the test surface.
• One adapter = hypothetical seam. Two adapters = real seam.

This skill is informed by the project's domain model. The domain language (CONTEXT.md) gives names to good seams; the project's design tenets (summarised in CLAUDE.md) and ADRs (docs/ADR/) record decisions the skill should not re-litigate (e.g. Fork 3 instance lifecycle, Fork 4 stateless filters).

Process

1. Explore

Read the project's domain glossary, the relevant Forks/Tenets (summarised in CLAUDE.md), and any ADRs in the area you're touching first.

Then use the Agent tool with subagent_type=Explore to walk the codebase. Don't follow rigid heuristics — explore organically and note where you experience friction:

• Where does understanding one concept require bouncing between many small modules?
• Where are modules shallow — interface nearly as complex as the implementation?
• Where have pure functions been extracted just for testability, but the real bugs hide in how they're called (no locality)?
• Where do tightly-coupled modules leak across their seams? (Watch the fast-path ↔ extension-plane seam and the host-API capability surface especially — those are the seams that must stay clean.)
• Which parts of the codebase are untested, or hard to test through their current interface?

Apply the deletion test to anything you suspect is shallow: would deleting it concentrate complexity, or just move it? A "yes, concentrates" is the signal you want.

2. Present candidates as an HTML report

Write a self-contained HTML file to the OS temp directory so nothing lands in the repo. Resolve the temp dir from $TMPDIR, falling back to /tmp (or %TEMP% on Windows), and write to <tmpdir>/architecture-review-<timestamp>.html so each run gets a fresh file. Open it for the user — xdg-open <path> on Linux, open <path> on macOS, start <path> on Windows — and tell them the absolute path.

The report uses Tailwind via CDN for layout and styling, and Mermaid via CDN for diagrams where a graph/flow/sequence reliably communicates the structure. Mix Mermaid with hand-crafted CSS/SVG visuals. Each candidate gets a before/after visualisation. Be visual. See HTML-REPORT.md for the full scaffold and diagram patterns.

For each candidate, render a card with:

• Files — which files/modules are involved
• Problem — why the current architecture is causing friction
• Solution — plain English description of what would change
• Benefits — explained in terms of locality and leverage, and how tests would improve
• Before / After diagram — side-by-side, custom-drawn, illustrating the shallowness and the deepening
• Recommendation strength — one of Strong, Worth exploring, Speculative, rendered as a badge

End the report with a Top recommendation section: which candidate you'd tackle first and why.

Use CONTEXT.md vocabulary for the domain, and LANGUAGE.md vocabulary for the architecture. If CONTEXT.md defines "Filter," talk about "the filter-dispatch module" — not "the FooHandler," and not "the filter service."

ADR / design-draft conflicts: if a candidate contradicts a recorded decision (an ADR, or a Fork stance in the design tenets), only surface it when the friction is real enough to warrant revisiting that decision. Mark it clearly in the card (e.g. "contradicts Fork 4 (stateless filters) — but worth reopening because…"). Don't list every theoretical refactor a decision forbids.

Do NOT propose interfaces yet. After the file is written, ask the user: "Which of these would you like to explore?"

3. Grilling loop

Once the user picks a candidate, drop into a grilling conversation. Walk the design tree with them — constraints, dependencies, the shape of the deepened module, what sits behind the seam, what tests survive.

Side effects happen inline as decisions crystallize:

• Naming a deepened module after a concept not in CONTEXT.md? Add the term to CONTEXT.md — same discipline as grill-with-docs (see CONTEXT-FORMAT.md). Create the file lazily if it doesn't exist.
• Sharpening a fuzzy term during the conversation? Update CONTEXT.md right there.
• User rejects the candidate with a load-bearing reason? Offer an ADR, framed as: "Want me to record this as an ADR so future architecture reviews don't re-suggest it?" Only offer when the reason would actually be needed by a future explorer. See ADR-FORMAT.md, or hand off to plecto-adr-writer.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md. For the deepening categories and testing strategy, see DEEPENING.md.