1---
2name: improve-codebase-architecture
3description: 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 
8Surface 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 
12Use 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 
23Key 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 
29This skill is _informed_ by the project's domain model — `CONTEXT.md` and any `docs/adr/`. The domain language gives names to good seams; ADRs record decisions the skill should not re-litigate. See [CONTEXT-FORMAT.md](../domain-model/CONTEXT-FORMAT.md) and [ADR-FORMAT.md](../domain-model/ADR-FORMAT.md).
30 
31## Process
32 
33### 1. Explore
34 
35Read existing documentation first:
36 
37- `CONTEXT.md` (or `CONTEXT-MAP.md` + each `CONTEXT.md` in a multi-context repo)
38- Relevant ADRs in `docs/adr/` (and any context-scoped `docs/adr/` directories)
39 
40If any of these files don't exist, proceed silently — don't flag their absence or suggest creating them upfront.
41 
42Then 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:
43 
44- Where does understanding one concept require bouncing between many small modules?
45- Where are modules **shallow** — interface nearly as complex as the implementation?
46- Where have pure functions been extracted just for testability, but the real bugs hide in how they're called (no **locality**)?
47- Where do tightly-coupled modules leak across their seams?
48- Which parts of the codebase are untested, or hard to test through their current interface?
49 
50Apply 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.
51 
52### 2. Present candidates
53 
54Present a numbered list of deepening opportunities. For each candidate:
55 
56- **Files** — which files/modules are involved
57- **Problem** — why the current architecture is causing friction
58- **Solution** — plain English description of what would change
59- **Benefits** — explained in terms of locality and leverage, and also in how tests would improve
60 
61**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."
62 
63**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.
64 
65Do NOT propose interfaces yet. Ask the user: "Which of these would you like to explore?"
66 
67### 3. Grilling loop
68 
69Once 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.
70 
71Side effects happen inline as decisions crystallize:
72 
73- **Naming a deepened module after a concept not in `CONTEXT.md`?** Add the term to `CONTEXT.md` — same discipline as `/domain-model` (see [CONTEXT-FORMAT.md](../domain-model/CONTEXT-FORMAT.md)). Create the file lazily if it doesn't exist.
74- **Sharpening a fuzzy term during the conversation?** Update `CONTEXT.md` right there.
75- **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](../domain-model/ADR-FORMAT.md).
76- **Want to explore alternative interfaces for the deepened module?** See [INTERFACE-DESIGN.md](INTERFACE-DESIGN.md).
77