| 1 | --- |
| 2 | name: improve-codebase-architecture |
| 3 | description: Find deepening opportunities in a codebase, informed by the domain language in CONTEXT.md and the decisions in docs/adr/. Use when the user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, or make a codebase more testable and AI-navigable. |
| 4 | --- |
| 5 | |
| 6 | # Improve Codebase Architecture |
| 7 | |
| 8 | Surface architectural friction and propose **deepening opportunities** — refactors that turn shallow modules into deep ones. The aim is testability and AI-navigability. |
| 9 | |
| 10 | ## Glossary |
| 11 | |
| 12 | 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). |
| 13 | |
| 14 | - **Module** — anything with an interface and an implementation (function, class, package, slice). |
| 15 | - **Interface** — everything a caller must know to use the module: types, invariants, error modes, ordering, config. Not just the type signature. |
| 16 | - **Implementation** — the code inside. |
| 17 | - **Depth** — leverage at the interface: a lot of behaviour behind a small interface. **Deep** = high leverage. **Shallow** = interface nearly as complex as the implementation. |
| 18 | - **Seam** — where an interface lives; a place behaviour can be altered without editing in place. (Use this, not "boundary.") |
| 19 | - **Adapter** — a concrete thing satisfying an interface at a seam. |
| 20 | - **Leverage** — what callers get from depth. |
| 21 | - **Locality** — what maintainers get from depth: change, bugs, knowledge concentrated in one place. |
| 22 | |
| 23 | Key principles (see [LANGUAGE.md](LANGUAGE.md) for the full list): |
| 24 | |
| 25 | - **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. |
| 26 | - **The interface is the test surface.** |
| 27 | - **One adapter = hypothetical seam. Two adapters = real seam.** |
| 28 | |
| 29 | 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. |
| 30 | |
| 31 | ## Process |
| 32 | |
| 33 | ### 1. Explore |
| 34 | |
| 35 | Read the project's domain glossary and any ADRs in the area you're touching first. |
| 36 | |
| 37 | 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: |
| 38 | |
| 39 | - Where does understanding one concept require bouncing between many small modules? |
| 40 | - Where are modules **shallow** — interface nearly as complex as the implementation? |
| 41 | - Where have pure functions been extracted just for testability, but the real bugs hide in how they're called (no **locality**)? |
| 42 | - Where do tightly-coupled modules leak across their seams? |
| 43 | - Which parts of the codebase are untested, or hard to test through their current interface? |
| 44 | |
| 45 | 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. |
| 46 | |
| 47 | ### 2. Present candidates |
| 48 | |
| 49 | Present a numbered list of deepening opportunities. For each candidate: |
| 50 | |
| 51 | - **Files** — which files/modules are involved |
| 52 | - **Problem** — why the current architecture is causing friction |
| 53 | - **Solution** — plain English description of what would change |
| 54 | - **Benefits** — explained in terms of locality and leverage, and also in how tests would improve |
| 55 | |
| 56 | **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." |
| 57 | |
| 58 | **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. |
| 59 | |
| 60 | Do NOT propose interfaces yet. Ask the user: "Which of these would you like to explore?" |
| 61 | |
| 62 | ### 3. Grilling loop |
| 63 | |
| 64 | 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. |
| 65 | |
| 66 | Side effects happen inline as decisions crystallize: |
| 67 | |
| 68 | - **Naming a deepened module after a concept not in `CONTEXT.md`?** Add the term to `CONTEXT.md` — same discipline as `/grill-with-docs` (see [CONTEXT-FORMAT.md](../grill-with-docs/CONTEXT-FORMAT.md)). Create the file lazily if it doesn't exist. |
| 69 | - **Sharpening a fuzzy term during the conversation?** Update `CONTEXT.md` right there. |
| 70 | - **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 [ADR-FORMAT.md](../grill-with-docs/ADR-FORMAT.md). |
| 71 | - **Want to explore alternative interfaces for the deepened module?** See [INTERFACE-DESIGN.md](INTERFACE-DESIGN.md). |
| 72 |