Design

Part of the Plan/Build phases — see the workflow skill for tiers and sequencing.

Design deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. The aim is leverage for callers, locality for maintainers, and testability for everyone. Mostly T2/T3 territory.

Glossary

Use these terms exactly — 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, performance characteristics. Avoid: API, signature (too narrow — type-level surface only).

Implementation — what's inside a module. 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).

Depth — leverage at the interface: how much behaviour a caller (or test) can exercise per unit of interface they must learn. Deep = lots of behaviour behind a small interface; shallow = interface nearly as complex as the implementation.

Seam (Michael Feathers) — a place where you can alter behaviour without editing in that place; where 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 satisfying an interface at a seam. Describes role, not substance.

Leverage — what callers get from depth: 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. Fix once, fixed everywhere.

Deep vs shallow

Deep (aim for this):            Shallow (avoid):
┌───────────────┐               ┌───────────────────────────────┐
│ Small interface│              │        Large interface        │
├───────────────┤               ├───────────────────────────────┤
│               │               │      Thin implementation      │
│ Deep implemen- │              └───────────────────────────────┘
│ tation        │
└───────────────┘

When designing an interface, ask: can I reduce the number of methods? simplify the parameters? hide more complexity inside?

Principles

• Depth is a property of the interface, not the implementation. A deep module can be internally composed of small, swappable parts — they just aren't part of the interface. Internal seams (private, used by the module's own tests) can coexist with the external seam.
• The 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. Callers and tests cross the same seam. If you want to test past the interface, the module is probably the wrong shape.
• One adapter means a hypothetical seam; two adapters means a real one. Don't introduce a seam unless something actually varies across it.
• Design it twice. For a non-obvious interface, sketch 2–3 radically different shapes before committing, and compare on depth, locality, and seam placement. The first design is rarely the best one.

Designing for testability

1. Accept dependencies, don't create them.

   // Testable
   function processOrder(order, paymentGateway) {}
   // Hard to test
   function processOrder(order) { const gateway = new StripeGateway(); }

2. Return results, don't produce side effects.

   // Testable
   function calculateDiscount(cart): Discount {}
   // Hard to test
   function applyDiscount(cart): void { cart.total -= discount; }

3. Small surface area. Fewer methods = fewer tests; fewer params = simpler setup.

Relationships

A Module has exactly one Interface. Depth is measured against that interface. A Seam is where the interface lives; an Adapter sits at a seam and satisfies the interface. Depth produces Leverage for callers and Locality for maintainers.

Rejected framings

• Depth as implementation-lines ÷ interface-lines: rewards padding. Use depth-as-leverage.
• "Interface" = the TypeScript interface keyword / public methods: too narrow — interface includes every fact a caller must know.
• "Boundary": say seam or interface.

UI component checklist

A component isn't designed until, per component, you've named its states (default, hover, focus, active, disabled, loading, error, empty — each needs visual and a11y treatment), its variants as a managed enum set (not boolean-prop explosions), its props API (minimal required props; children over content-props; composition over configuration), and its a11y path (semantic elements, keyboard navigable, visible focus, ARIA only where HTML falls short).

Going deeper

When the work involves pinning down domain terminology, recording an architectural decision, or stress-testing concept boundaries, see references/domain-modeling.md.

To deepen a cluster of shallow modules safely, see references/deepening.md.

Adapted from mattpocock-skills (MIT, © 2026 Matt Pocock) and rampstack-skills (MIT, © 2026 RampStack Co.); see ATTRIBUTION.md.