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Shared vocabulary for designing deep modules.
Passed 21 of 22 test cases with the skill, and 15 of 22 without it.
and 10 more in the full report
6 fixed15 passed either way1 still failing0 broke
Tokens, turns and time describe resource use, not a dollar price.
One of 6 test cases the skill fixed. Same model, same prompt, with and without the skill.
I have a payment handling module that currently exposes over twenty public methods, making unit testing complicated and integration messy. Could you review its structural design and suggest a refactored version? Please analyze where to place proper seams, how to minimize the exposed interface while keeping rich internal behavior, and explain how the change improves leverage for callers and locality for maintainers.
| Checked | Without skill | With skill |
|---|---|---|
| Avoids forbidden terminology such as component, service, API, and boundary. | Missed | Passed |
| Defines leverage as capability gained per unit of interface complexity learned. | Missed | Passed |
| Defines locality as concentrating maintenance changes and verification in one place. | Missed | Passed |
The instructions your model reads when the skill loads.
Design deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. Use this language and these principles wherever code is being designed or restructured. The aim is leverage for callers, locality for maintainers, and testability for everyone.
Use these terms exactly: don't substitute "component," "service," "API," or "boundary." Consistent language is the whole point.
Module: anything with an interface and an implementation. Deliberately scale-agnostic: a function, class, package, or tier-spanning slice. Avoid: unit, component, service.
Interface: everything a caller must know to use the module correctly: the type signature, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics. Avoid: API, signature (too narrow, they refer only to the type-level surface).
Implementation: what's inside a module, its body of code. Distinct from Adapter: a thing can be a small adapter with a large implementation (a Postgres repo) or a large adapter with a small implementation (an in-memory fake). Reach for "adapter" when the seam is the topic; "implementation" otherwise.
Depth: leverage at the interface. The amount of behaviour a caller (or test) can exercise per unit of interface they have to learn. A module is deep when a large amount of behaviour sits behind a small interface, shallow when the interface is nearly as complex as the implementation.
Seam (Michael Feathers): a place where you can alter behaviour without editing in that place; the location at which a module's interface lives. Where to put the seam is its own design decision, distinct from what goes behind it. Avoid: boundary (overloaded with DDD's bounded context).
Adapter: a concrete thing that satisfies an interface at a seam. Describes role (what slot it fills), not substance (what's inside).
Leverage: what callers get from depth. More capability per unit of interface they learn. One implementation pays back across N call sites and M tests.
Locality: what maintainers get from depth. Change, bugs, knowledge, and verification concentrate in one place rather than spreading across callers. Fix once, fixed everywhere.
Deep module = small interface + lots of implementation:
┌─────────────────────┐
│ Small Interface │ ← Few methods, simple params
├─────────────────────┤
│ │
│ Deep Implementation│ ← Complex logic hidden
│ │
└─────────────────────┘
Shallow module = large interface + little implementation (avoid):
test cases passed, vs 15 / 22 without
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