Deep Modules

When to use

• Lens mode — apply the vocabulary and checks below to judge or shape one interface, in a design, a review, or another skill that loads this one. No subagents. A single file or function is always lens mode.
• Audit mode — scan many modules you can't eyeball in one read for shallow ones, and return a ranked findings table. Dispatches subagents; runs Audit mode.

The lens

Use these terms exactly — don't substitute "component," "service," "API," or "boundary."

• Module — anything with an interface and an implementation; scale-agnostic (function, class, package, tier-spanning slice).
• Interface — everything a caller must know to use it correctly: the signature, plus invariants, ordering, error modes, required config, performance characteristics. Broader than "API/signature."
• Implementation — the body inside the module. Distinct from Adapter: a concrete thing satisfying an interface at a seam (a Postgres repo; an in-memory fake).
• Depth — leverage at the interface: behaviour a caller or test can exercise per unit of interface it must learn. Deep = lots of behaviour behind a small interface; shallow = interface nearly as complex as the implementation.
• Seam (Feathers) — a place you can change behaviour without editing in place; where the interface lives. Where to put it is its own decision.
• Leverage — callers' payoff: one implementation pays back across N call sites and M tests.
• Locality — maintainers' payoff: change, bugs, and knowledge concentrate in one place. Fix once, fixed everywhere.