improve-codebase-architecture

name	improve-codebase-architecture
description	Surface architectural friction in a codebase and apply deepening improvements.
disable-model-invocation	true

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.

Use this shared design vocabulary — stay on these terms, don't drift into "component," "service," "API," or "boundary":

• module — a unit of code with a single responsibility
• interface — what a module exposes; the surface callers depend on
• depth — how much complexity a module hides behind its interface; a deep module has a small interface hiding a large implementation
• seam — the place where two modules meet; their shared contract
• adapter — a module whose sole job is to translate between two seams
• leverage — how much work a module does relative to what a caller must understand to use it
• locality — how close related code lives; code that changes together should live together

Principles:

• Deletion test — would deleting this module concentrate complexity (good, it's deep), or just move it (bad, it's shallow)?
• The interface is the test surface — if a module is hard to test, its interface is wrong
• One adapter = hypothetical seam, two = real — the first adapter on a seam is scaffolding; the second proves the seam is right

The domain language in docs/architecture-notes.md and docs/business-rules.md names good seams. Create docs/glossary.md lazily when shared terminology is needed across docs.

Process

1. Explore

Read the project's architecture docs (docs/architecture-notes.md, docs/business-rules.md) first.

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:

• 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

Summarize the candidates directly in conversation. For each candidate, describe:

• 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 how tests would improve
• Recommendation strength — one of Strong, Worth exploring, Speculative

End the summary with a Top recommendation: which candidate you'd tackle first and why.

Use the domain vocabulary from docs/architecture-notes.md, docs/business-rules.md, and the terms defined above. If docs/glossary.md exists, use its terms — don't drift into "component," "service," "API," or "boundary."

Existing decision conflicts: if a candidate contradicts a decision recorded in an existing doc, only surface it when the friction is real enough to warrant revisiting that decision. Mark it clearly. Don't list every theoretical refactor a past decision forbids.

Ask the user: "Which of these would you like me to implement?"

3. Apply improvements

Once the user picks a candidate, apply the deepening improvement to the best of your ability. This means:

• Read the relevant code thoroughly before making changes. Understand the current interface, implementation, and callers.
• Design the deepened module: shrink the interface, expand the implementation behind it. Apply the deletion test — after the refactor, would deleting this module concentrate complexity or just move it?
• Implement the change: restructure files, extract and consolidate logic, tighten seams. Prefer locality — code that changes together should live together.
• Preserve existing tests and add new ones for the deepened interface. If a test was hard to write before and easy now, that's the signal you got it right.
• Run the full build and test suite (npm run build, npm run test:coverage). Fix any regressions. Maintain the minimum 80% coverage threshold in every reported Istanbul cell.
• Update relevant docs under docs/ when the change introduces a new concept, sharpens a fuzzy term, or alters a documented decision.

Record durable decisions as you go:

• Naming a deepened module after a concept not yet documented? Add the term to the relevant doc under docs/. Create docs/glossary.md lazily if a cross-cutting glossary is needed.
• Sharpening a fuzzy term during the conversation? Update the relevant doc right there.
• Want to explore alternative interfaces for the deepened module? Design two competing interfaces in parallel, compare them against the deletion test and leverage, and record the winner.

Completion Criterion

The architecture review is complete when:

• the candidates and trade-offs are clearly communicated and the top recommendation is explicit,
• the chosen deepening improvement is implemented, the build passes, and the test suite meets the 80% coverage threshold,
• and any durable architecture decisions that emerge are recorded in the relevant docs.