Deepen

Improve Codebase Architecture

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

Vocabulary

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

The skill is informed by the project's domain model. The domain language names good seams; ADRs record decisions the skill should not re-litigate.

Process

1. Explore

Ground in the project's documentation first — its established domain vocabulary and recorded decisions, as the in-context project CLAUDE.md points to them. With none present, proceed on the code alone.

Then walk the codebase — above ~25 files in scope, fan out Explore subagents (one per area) so the reads stay out of the main window; at or below that, explore inline (see ../DELEGATION.md). 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, 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 — 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 communicates the structure. Mix Mermaid with hand-crafted CSS/SVG visuals — Mermaid when relationships are graph-shaped (call graphs, dependencies, sequences), hand-built divs/SVG for editorial visuals (mass diagrams, cross-sections, collapse animations). Each candidate gets a before/after visualisation. Be visual.

Each candidate renders 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 project's domain vocabulary for the domain, and LANGUAGE.md vocabulary for the architecture. If the project's documentation defines "Order," talk about "the Order intake module" — not "the FooBarHandler," and not "the Order service."

ADR conflicts: if a candidate contradicts an existing ADR, surface it only when the friction warrants revisiting the ADR. Mark it clearly in the card (e.g. a warning callout: "contradicts ADR-0007 — 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. Hand the chosen candidate to design

deepen is a finder: it surfaces candidates and stops. The report is its standalone artifact — steps 1–2 produce it without design in the loop.

Once the user picks a candidate, invoke /design on it for the design conversation. design walks the design tree — constraints, dependencies, the shape of the deepened module, what sits behind the seam, what tests survive — sharpening fuzzy terminology as it goes and offering a load-bearing decision as an ADR inline (see ../contracts/adr.md). Pass the candidate's files, problem, and proposed solution so design opens on the deepening rather than rediscovering it.

Un-picked candidates are rejected, not deferred. Leave them in the report as dead — they are not findings and have no onward path.

Handover

Per ../HANDOVER.md. End an interactive run by rendering this row as one AskUserQuestion.

• artifact: architectural candidates + HTML report
• default: design — design the chosen candidate, sharpening terminology and offering an ADR inline
• alternatives: stop

Interactive-only (per ../HANDOVER.md) — surfacing candidates ends in a pick the user must make; auto never enters it.