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 — CONTEXT.md and any docs/adr/. The domain language gives names to good seams; ADRs record decisions the skill should not re-litigate. See references/CONTEXT-FORMAT.md and references/ADR-FORMAT.md.

When to load references/domain-model.md: before this skill's first pass on a repo with no CONTEXT.md yet, or one too thin to name the seams below — run its grilling interview to establish the domain vocabulary first. Skip it once CONTEXT.md already reflects the domain.

Process

1. Explore

Read existing documentation first:

• CONTEXT.md (or CONTEXT-MAP.md + each CONTEXT.md in a multi-context repo)
• Relevant ADRs in docs/adr/ (and any context-scoped docs/adr/ directories)

If any of these files don't exist, proceed silently — don't flag their absence or suggest creating them upfront.

Optional pre-pass — export inventory injection (mapper-supported projects only):

Before dispatching the Explore subagent, read each known entry-point file (e.g. index.ts, src/index.ts, the main export barrel) and pass its contents to extractSemanticSlices(filePath, content) from scripts/lib/language-mappers/index.mjs. The signature is (filePath, content, options?) — the second argument is the raw file text, not an options object, and there is no slice-kind filter parameter. It resolves to an array of SemanticSlice records ({ kind, name, exported, … }); keep the ones with exported === true, format them as structured context, and inject that into the Explore subagent prompt. This gives the subagent an immediate map of the codebase's public surface without requiring it to grep manually.

## Export inventory (auto-generated)
- src/lib/foo.ts: exports `createFoo`, `FooConfig`, `FooError`
- src/lib/bar.ts: exports `processBar`

This pre-pass is OPTIONAL — only activate when the entry-point file is mapper-supported (.ts, .tsx, .js, .jsx, .mjs, .cjs, .md, .mdx, .swift, .py — the EXT_TO_LANG map in index.mjs is the source of truth). For any other extension extractSemanticSlices throws rather than returning an empty array, so wrap the call in a try/catch and skip the pre-pass silently on failure. Never block Explore dispatch on a mapper error.

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

Present a numbered list of deepening opportunities. For each candidate:

• 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 also in how tests would improve

Use CONTEXT.md vocabulary for the domain, and LANGUAGE.md vocabulary for the architecture. If CONTEXT.md 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, only surface it when the friction is real enough to warrant revisiting the ADR. Mark it clearly (e.g. "contradicts ADR-0007 — but worth reopening because…"). Don't list every theoretical refactor an ADR forbids.

Do NOT propose interfaces yet. 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 the domain-model grilling reference (see references/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 to avoid re-suggesting the same thing — skip ephemeral reasons ("not worth it right now") and self-evident ones. See references/ADR-FORMAT.md.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.