---
name: improve-codebase-architecture
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.
---

# 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. (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.**

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 explore the codebase 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.** 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`.
- **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.
