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.

When to use

• The user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, or make a codebase more testable.
• Mid-project in tdd-rounds, after the load-bearing seams exist — surface candidates that become dedicated refactor rounds.
• After zoom-out reveals shallow modules in an unfamiliar area.

When to skip

• Designing the shape of a new module from scratch — use design.
• Greenfield system topology — use system-design.
• Single-module local refactor with no cross-module impact — use design or refactor inline.
• Line-level cleanup (renames, dead-code removal) — use the code-hygiene lens + simplify.

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 skills/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 skills/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 (docs/CONTEXT.md) and any ADRs in docs/adr/ for 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?
• 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 skills/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 /grill-plan (see ../formats/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 ../formats/ADR-FORMAT.md.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.

4. Context Splitting

When a single CONTEXT.md becomes a bottleneck (>100 terms), the codebase is likely ready for context splitting.

• Identify Seams: Find logical boundaries where domain terms are largely independent.
• Extract Sub-Contexts: Move terms into <module>/CONTEXT.md files.
• Update CONTEXT-MAP.md: Create or update the root docs/CONTEXT-MAP.md to point to the new sub-contexts.
• AI-Navigability: This reduces context pollution, allowing agents to focus only on the relevant vocabulary for a given module.

Pairing with other skills

• zoom-out — runs before if the area is unfamiliar. Map first, then propose.
• design — shares vocabulary (LANGUAGE.md). design is the greenfield twin of this skill.
• system-design — the system-level twin. Greenfield topology vs brownfield deepening.
• grill-plan — invoked when a candidate hits an ADR that needs revisiting (or when a rejection deserves an ADR).
• tdd / tdd-rounds — runs after the candidate is chosen. Refactor rounds: ACs are "all existing tests still green".
• prod-ready — gate before merge.

Done when

For each candidate the user chose to explore:

• The deepened module's interface has been sketched (small surface, clear seam).
• Implications for tests are named (which existing tests survive; which need updating).
• Any new domain terms used are added to CONTEXT.md.
• Any architectural decision worth preserving is offered as an ADR.

If the user wants to deepen multiple candidates, run the skill again per candidate — don't batch them in one pass.