improve-codebase-architecture

name	improve-codebase-architecture
description	Find deepening opportunities in a codebase, informed by engineering plans in docs/plans/. 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." Full definitions in LANGUAGE.md.

• 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 (see LANGUAGE.md for the full list):

• 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 engineering plans. Existing plans give names to important seams, record decisions the skill should not re-litigate, and show which constraints matter in this codebase. Treat plans as implementation maps: resumable, sequenced, and explicit about boundaries.

Process

1. Explore

Read relevant engineering plans in docs/plans/ first. Prefer recent, active, similar-domain, and compact plans when the directory is large.

Use the plan docs the same way drafting-plans does:

• Treat an existing plan as the sequencing source of truth when the architecture question continues that work.
• Read relevant code, docs, schemas, runtime config, or issue/PR context before naming interfaces, examples, commands, or validation.
• Verify cheap drift-prone facts against the current repo before treating a plan statement as current state.
• Notice plan status (proposed, active, paused, landed, superseded) and avoid re-litigating landed or superseded decisions unless current code friction makes the revisit worthwhile.
• Pull forward the plan's goals, non-goals, invariants, delivery slices, verification strategy, and open questions when evaluating architecture candidates.

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

Present a numbered list of deepening opportunities. For each candidate:

• Files — which files/modules are involved
• Plan context — relevant docs/plans/ references, including conflicts or open questions when they matter
• 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 and plan verification would improve
• First slice — the smallest boring, useful step if the user chooses to explore it

Use docs/plans/ vocabulary for the product or system domain, and LANGUAGE.md vocabulary for the architecture. If plans define a concept such as "Order intake," use that name consistently instead of falling back to incidental implementation names.

Plan conflicts: if a candidate contradicts an existing plan decision, only surface it when the friction is real enough to warrant revisiting the plan. Mark it clearly (e.g. "contradicts docs/plans/example.md — but worth reopening because…"). Don't list every theoretical refactor a plan rules out.

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 that belongs in an existing plan? Update the relevant docs/plans/ document with the term, ownership seam, or decision.
• Sharpening a fuzzy term during the conversation? Record the clarified terminology in the relevant plan when it will help future architecture work.
• Changing scope, interface shape, risks, sequencing, or validation? Update the plan in the same pass. Treat the plan as the working map, not a changelog.
• Resolving an open question? Move it to resolved decisions or adjust goals/non-goals and delivery slices. Leave only genuinely unresolved questions.
• Adding material findings from the architecture review? Prefer a concise Key Learnings From Pressure-Testing or existing equivalent section over a detached critique.
• Updating a plan with YAML frontmatter? Update last_reviewed after material revisions or revalidation, and change status only for lifecycle changes.
• User rejects the candidate with a load-bearing reason? Offer to record the reason in the relevant plan so future architecture reviews don't re-suggest it. Only offer when the reason would actually be needed by a future explorer — skip ephemeral reasons ("not worth it right now") and self-evident ones.
• No relevant plan exists, but the candidate becomes real work? Offer to draft a new docs/plans/ plan using the repository's plan conventions. Do not create one just to record a casual idea.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.