improve-codebase-architecture

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.

SKILL: improve-codebase-architecture

Bot: developer · operator · any Role: Surface architectural friction in a codebase and propose deepening opportunities — refactors that turn shallow modules into deep ones (high leverage at a small interface). Uses a strict glossary (Module / Interface / Implementation / Depth / Seam / Adapter / Leverage / Locality) — consistent language is the point. Informed by CONTEXT.md + docs/adr/. Ug-ug mode: lite — output structured findings, not prose Model: sonnet — judgment-heavy: identifying genuine depth opportunities vs. premature abstraction Tool compatibility: Claude Code · Cursor · Codex Status: beta Parallelizable: yes — no shared mutable state detected (auto-inferred; verify)

Origin: Imported from mattpocock/skills (engineering/improve-codebase-architecture), MIT licensed. Source files preserved at _source_*.md. Companion docs (DEEPENING.md, INTERFACE-DESIGN.md, LANGUAGE.md) copied into this folder so the SKILL.md's relative links resolve.

When to invoke

• User asks: "improve architecture", "find refactoring opportunities", "consolidate this", "make this more testable"
• Onboarding: producing the first architectural map for a codebase
• Pre-feature: identifying seams a new feature needs before writing it
• Periodic: scheduled architecture review (quarterly per project)

Don't invoke for:

• Single-file edits — too zoomed in
• Codebases without a CONTEXT.md or ADRs — the skill leans on them; run arch-decision first
• Greenfield projects with no code yet

Handoffs

Lambda candidates

Input / Output

Input: repo root with CONTEXT.md (domain glossary) + docs/adr/ (decisions). Optionally a specific module or area to focus on. Output: list of deepening proposals, each with: shallow-module identified, deletion-test reasoning, suggested seam, adapter count today vs. needed, proposed interface, callers affected, ADR reference.

Improve Codebase Architecture (source body)

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 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 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
• 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, and 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."

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 — same discipline as /grill-with-docs (see CONTEXT-FORMAT.md). Create the file lazily if it doesn't exist.
• 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. See ADR-FORMAT.md.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.

Permissions