| name | improve-codebase-architecture |
| description | Find deepening opportunities in a codebase and propose refactors that turn shallow modules into deep ones. Aim is testability and AI-navigability. Use when the user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, or make a codebase more testable. |
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.
Vocabulary
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. (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:
- 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.
Process
1. Explore
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.
If a CONTEXT.md exists at the repo root (or CONTEXT-MAP.md for multi-context repos), read it first — its vocabulary is the project's shared language and you should use it when naming modules. If docs/adr/ exists, scan ADRs in the area you're touching so you don't re-litigate decisions. If neither exists, proceed without them; just don't invent ceremony.
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 in how tests would improve
Use the project's actual domain vocabulary (from CONTEXT.md if present, otherwise the names already in the code) when describing modules — not generic terms like "FooBarHandler" or "the Order service" when "Order intake" is what the team would say.
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. Discuss the chosen candidate
Once the user picks a candidate, walk the design tree with them — constraints, dependencies, the shape of the deepened module, what sits behind the seam, what tests survive.
Stay in conversation; don't go off and refactor unilaterally. The output of this skill is a clear refactor plan the user can hand to /spec or /chunked-review, not the refactor itself.
Companion skills
/diagnose — when an architectural problem was uncovered while debugging ("no good test seam exists for this bug"), /diagnose Phase 6 hands the finding here. Treat that as a prioritised candidate./spec or /chunked-review — once a candidate is agreed, the actual implementation goes through one of those loops, not this skill./zoom-out — if the user is unfamiliar with the area you're proposing to refactor, run /zoom-out first to give them a map.
