improve-codebase-architecture

# 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](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](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 project's domain glossary and any ADRs in the area you're touching first. Then use an available exploration subagent/tool to walk the codebase, or do the exploration directly if the runtime has no dedicated explorer. 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](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` with a short definition, the canonical wording to use, and any aliases to avoid. 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. Keep the ADR concise: context, decision, consequences. - **Want to explore alternative interfaces for the deepened module?** See [INTERFACE-DESIGN.md](INTERFACE-DESIGN.md).