improve-codebase-architecture

name	improve-codebase-architecture
description	Hunt for deepening opportunities — shallow modules whose interface costs as much to learn as their implementation saves — and present them as a visual before/after report, informed by the decisions already recorded in docs/ADR/. Use when the user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, reduce the cost of understanding a subsystem, or make code more testable. Use clean-architecture instead when the question is whether a specific change violates Alt's layer rules.
allowed-tools	Read, Grep, Glob, Bash, Agent
argument-hint	[target path or subsystem]

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, class, package, 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.")
• 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 gives names to good seams; ADRs record decisions the skill should not re-litigate.

Process

1. Explore

Read the accepted ADRs covering the area you're touching first — docs/ADR/ (6-digit files), plus the canonical contracts in docs/plan/ and invariants in docs/wiki/architecture/. Confirm an ADR is still live with docdag resolve <id> before treating it as binding; a superseded decision is not a constraint.

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?
• 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 — use Mermaid when relationships are graph-shaped (call graphs, dependencies, sequences), and hand-built divs/SVG when you want something more editorial (mass diagrams, cross-sections, collapse animations). Each candidate gets a before/after visualisation. Be visual.

For each candidate, the same template as before, but rendered as a card:

• 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 the vault's domain vocabulary for the domain, and LANGUAGE.md vocabulary for the architecture. If the canonical contracts call something a Trail footprint, talk about "the footprint projection module" — not "the FooBarHandler," and not "the footprint service."

ADR conflicts: if a candidate contradicts an accepted ADR, only surface it when the friction is real enough to warrant revisiting the decision. Mark it clearly in the card (e.g. a warning callout: "contradicts [[000722]] — but worth reopening because…"). Don't list every theoretical refactor an ADR forbids.

See HTML-REPORT.md for the full HTML scaffold, diagram patterns, and styling guidance.

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 the vault doesn't define? Pin the term down in the conversation and say which docs/plan/ contract should own it. Ask before editing that contract.
• 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 a future explorer would actually need the reason to avoid re-suggesting the same thing — skip ephemeral reasons ("not worth it right now") and self-evident ones. Hand the writing to the alt-adr-writer skill, which owns this repo's ADR numbering and format.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.