Codebase Design

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.

This skill is reference, not a procedure — it holds the vocabulary that codebase-improve-architecture, codebase-plan-refactor, and tdd all speak. Read it when you need the definitions; those skills carry the process.

Stopping rule

A reference has no checkpoints, no one-question-at-a-time gate, and no stopping condition of its own — so an agent pointed at it and told to "go" will improvise a process out of the most action-shaped content it can find (usually the parallel design pass in INTERFACE-DESIGN.md) and run a long way before asking anything.

When this skill is invoked on its own: state the vocabulary that bears on the question, answer the design question asked, and stop. Reading files, surveying the codebase, and proposing refactors are not this skill's work.

Don't know which module to redesign yet? That's codebase-improve-architecture — the survey that finds candidates.
Want the design argued with rather than named? That's grill-with-docs or grill-me.
Have a concrete behaviour to build? That's tdd.

Run one of those as the driver and let this skill sit underneath it as vocabulary.

Glossary

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 — applies equally to a function, class, package, or tier-spanning slice. Avoid: unit, component, service.

Interface Everything a caller must know to use the module correctly. Includes the type signature, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics. Avoid: API, signature (too narrow — those 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. A module is shallow when the interface is nearly as complex as the implementation.

Seam (from Michael Feathers) A place where you can alter behaviour without editing in that place. The location at which a module's interface lives. Choosing 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 have to learn. One implementation pays back across N call sites and M tests.

Locality What maintainers get from depth. Change, bugs, knowledge, and verification concentrate at one place rather than spreading across callers. Fix once, fixed everywhere.

Deep vs shallow

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):

┌─────────────────────────────────┐
│       Large Interface           │  ← Many methods, complex params
├─────────────────────────────────┤
│  Thin Implementation            │  ← Just passes through
└─────────────────────────────────┘

When designing an interface, ask:

Can I reduce the number of methods?
Can I simplify the parameters?
Can I hide more complexity inside?

Principles

Depth is a property of the interface, not the implementation. A deep module can be internally composed of small, mockable, swappable parts — they just aren't part of the interface. A module can have internal seams (private to its implementation, used by its own tests) as well as the external seam at its interface.
The deletion test. Imagine deleting the module. If complexity vanishes, the module wasn't hiding anything (it was a pass-through). If complexity reappears across N callers, the module 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.

Designing for testability

Accept dependencies, don't create them.

// Testable
function processOrder(order, paymentGateway) {}

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

Return results, don't produce side effects.

// Testable
function calculateDiscount(cart): Discount {}

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

Small surface area. Fewer methods means fewer tests; fewer params means simpler test setup.

Relationships

A Module has exactly one Interface (the surface it presents to callers and tests).
Depth is a property of a Module, measured against its Interface.
A Seam is where a Module's 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 ratio of implementation-lines to interface-lines (Ousterhout): rewards padding the implementation. We use depth-as-leverage instead.
"Interface" as the TypeScript interface keyword or a class's public methods: too narrow — interface here includes every fact a caller must know.
"Boundary": overloaded with DDD's bounded context. Say seam or interface.

Going deeper

Deepening a cluster given its dependencies — DEEPENING.md: dependency categories, seam discipline, and replace-don't-layer testing.
Exploring alternative interfaces — INTERFACE-DESIGN.md: design the interface several radically different ways in parallel, then compare on depth, locality, and seam placement.
Naming the modules after the domain — the domain-modeling skill owns CONTEXT.md and the ADR discipline. A deepened module should be named in the project's ubiquitous language, not invented vocabulary.

Delegation Map

User need	Delegate via	Notes
Find which modules to deepen across a codebase	Invoke `codebase-improve-architecture`	This skill supplies its vocabulary; that skill supplies the scan and the grilling loop
Turn an agreed deepening into ordered commits	Invoke `codebase-plan-refactor`	Plan first, then implement in tiny steps
Put behaviour tests at the new interface	Invoke `tdd`	The interface is the test surface — test through it, not past it
The new module needs a domain-accurate name	Invoke `domain-modeling`	Resolve the term, write it to `CONTEXT.md`, then name the module after it

Loop eligibility: false — reference vocabulary, consulted on demand.

Host portability: this skill is pure reference — no host-specific tools. See PORTABILITY.md.

Self-Evolve Loop

Journal ~/.ink-and-agency/learnings/codebase-design.md (or workspace-local .ink-and-agency/ where the sandbox confines writes). Read it first; append what the run taught — SELF-EVOLVE.md.

codebase-design

SKILL.mdskills/codebase-design/SKILL.md