# Improve Codebase Architecture

> Find deepening opportunities in a codebase, informed by the domain language in CONTEXT.md and the decisions in docs/adr/. USE FOR: improving architecture, finding refactoring opportunities, consolidating tightly-coupled modules, making code more testable and AI-navigable. DO NOT USE FOR: writing new features, debugging, quick fixes.

- Skill: `habonn/improve-codebase-architecture` (Agent Skill)
- Install (CLI): `npx skillmds@latest add habonn/improve-codebase-architecture`
- Raw SKILL.md: https://api.skillmd.com/api/skills/habonn/improve-codebase-architecture/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: habonn (https://skillmd.com/u/habonn)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/habonn/improve-codebase-architecture

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# 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."

- **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.
- **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:

- 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 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?

### 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. If `CONTEXT.md` defines "Order," talk
about "the Order intake module" — not "the FooBarHandler."

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.

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`. 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?"