improve-codebase-architecture

name	improve-codebase-architecture
description	Find deepening opportunities in a codebase — refactors that turn shallow modules into deep ones for better testability and AI-navigability. Use when the user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, or make a codebase more testable. NOT for code review before merge (use /sr). NOT for spec/requirement gaps (use /ct). Reads but does not write code.
allowed-tools	["Read","Grep","Glob","Bash","Task","Skill"]

Improve Codebase Architecture

Upstream: Adapted from mattpocock/skills/improve-codebase-architecture. Vocabulary and principles draw on Ousterhout (A Philosophy of Software Design) and Feathers (Working Effectively with Legacy Code). Adapted to use this repo's tasks/ and product-docs/ conventions instead of upstream's CONTEXT.md + docs/adr/.

Surface architectural friction and propose deepening opportunities — refactors that turn shallow modules into deep ones. The aim is testability and AI-navigability.

Vocabulary

This skill uses the canonical architecture vocabulary in .claude/skills/architecture-language/LANGUAGE.md. Load it before suggesting candidates. Use the terms exactly — module, interface, seam, adapter, depth, leverage, locality. Don't drift into "component", "service", "API", or "boundary".

Key principles (full list in architecture-language/LANGUAGE.md):

• 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.

Process

1. Explore

Read existing context first:

• product-docs/UBIQUITOUS_LANGUAGE.md — domain terms (use them when naming candidates)
• product-docs/PRD/*.md and product-docs/JTBD/*.md — which features are load-bearing
• Any active tasks/task-*/tech-decomposition-*.md — recent decisions in the area
• CLAUDE.md and {{DOCS_DIR}}/project-structure.md — architecture conventions

If any of these are missing, proceed silently — don't flag absence or suggest creating them upfront.

Then use the Explore agent (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 how tests would improve
• Dependency category — see DEEPENING.md (in-process / local-substitutable / remote-but-owned / true-external)

Use UBIQUITOUS_LANGUAGE.md vocabulary for the domain, and architecture-language/LANGUAGE.md for the architecture. If UBIQUITOUS_LANGUAGE.md defines "Order", talk about "the Order intake module" — not "the FooBarHandler", and not "the Order service".

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 (invoke /grill-me). 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 UBIQUITOUS_LANGUAGE.md? Add the term via /ubiquitous-language right there.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.
• User rejects the candidate with a load-bearing reason? Note it in the relevant tasks/ doc or product-docs/ so future architecture passes don't re-suggest the same thing. Skip ephemeral reasons ("not worth it right now") and self-evident ones.

4. Output

This skill READS and REPORTS — it does not write code or commit changes. The output is:

• A list of candidates (Step 2)
• Optionally, a deepened-module design (Step 3, after grilling)

If the user wants to act on a candidate:

• Small refactor → handoff to /quick
• Larger refactor → handoff to /ct to produce a tech-decomposition

Scope boundaries

• Does NOT propose interfaces upfront — that comes only after the user picks a candidate
• Does NOT re-litigate decisions already documented in product-docs/ or active tasks/ docs
• Does NOT write code or modify files outside its own outputs
• Does NOT expand scope to include unrelated tech debt