Deepen Architecture

Deepen Architecture

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.

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 — 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.

Process

1. Explore

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.

2. Module Depth score

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

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