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

Distinct from define-language and model-domain: Use this skill to find module-level refactoring opportunities in the codebase. Use define-language to produce a canonical glossary of terms. Use model-domain to stress-test a plan through a domain-model interview.

HARD GATE — Deep modules must solve a forcing function, not just be “nice abstractions.” If you cannot articulate why the abstraction exists, it is premature.

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 — specs/tech-architecture/tech-stack.md and any specs/adr/. The domain language gives names to good seams; ADRs record decisions the skill should not re-litigate. See CONTEXT-FORMAT.md and ADR-FORMAT.md.

Read existing documentation first:

• specs/tech-architecture/tech-stack.md (or specs/tech-architecture/tech-stack.md + each specs/tech-architecture/tech-stack.md in a multi-context repo)
• Relevant ADRs in specs/adr/

If any of these files don’t exist, proceed silently — don’t flag their absence or suggest creating them upfront.

Look-here-first (churn heuristic): Before organic exploration, rank candidate modules by recent commit frequency. High-churn files are architectural friction magnets — start there.

bash scripts/bp-churn-rank.sh --since 90.days --limit 20

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?
• 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.

For each candidate module, assign a Module Depth score (1–5, Ousterhout):

Include the score in each candidate row. Prioritize score ≤ 2 for deepening.

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

Use specs/tech-architecture/tech-stack.md vocabulary for the domain, and LANGUAGE.md vocabulary for the architecture.

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. 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?”

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 specs/tech-architecture/tech-stack.md? Add the term to specs/tech-architecture/tech-stack.md — same discipline as model-domain (see CONTEXT-FORMAT.md). Create the file lazily if it doesn’t exist.
• Sharpening a fuzzy term during the conversation? Update specs/tech-architecture/tech-stack.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. See ADR-FORMAT.md.
• Want to explore alternative interfaces for the deepened module? See INTERFACE-DESIGN.md.

When a deepening move splits or merges modules, update specs/import-boundaries.json (Playwright DEPS.list pattern) — declare which scripts/lib/*.sh files may source which peers. CI enforces via:

bash scripts/check-import-boundaries.sh

Run the check before proposing cross-module source edges. Convention docs alone do not authorize new imports; the allowlist must list them.

→ verify: test -f specs/import-boundaries.json && bash scripts/check-import-boundaries.sh && echo OK || echo FAIL

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