Agent Skills: Codebase Design

Codebase Design

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.

Why

Complexity is the enemy. It shows up as:

• Change amplification: a simple change touches many places.
• Cognitive load: you must hold a lot in your head to change anything safely.
• Unknown unknowns: it is not obvious what you need to change or know.

It has two causes: dependencies and obscurity. Good design removes both.

Work strategically, not tactically. Working code is not enough. Each change is a small, continual investment in the design, not a patch on top of it.

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: 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.

Project names

When the project's own pattern names a layer (Router, Service, Repository, Entity), keep that name for the concrete thing: say "the OrderService". Describe its design with the glossary: is that module deep, where is its seam, what is its interface?

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?
• Can I define an error out of existence?

Principles

Details, rationale, and examples in principles.md.

• Deep modules and the deletion test. If deleting the module makes complexity vanish, it was a pass-through.
• Test at the correct level. The interface is the test surface. Test at seams, not every function; replace tests, don't layer them.
• Parse, don't validate. Turn untrusted input into domain types at the external seam. The core never re-checks.
• Make illegal states unrepresentable. Shape the types so invalid combinations can't be built.
• Functional core, imperative shell. Pure decisions inside, I/O at the edges.
• Errors as values. Expected failures are part of the interface and are returned, not thrown.
• Define errors out of existence. Change the semantics so the error cannot occur. Do this first.
• Information hiding. Each design decision lives in one module. Duplicated knowledge is leakage.
• Pull complexity downward. A simple interface matters more than a simple implementation.
• Somewhat general-purpose modules are deeper. Keep general and special-purpose code apart.
• Different layers, different abstractions. No pass-through methods or variables.
• Together or apart. Combine what shares information; split general mechanism from specific use.
• Make the common case simple. Defaults over configuration.
• Comments and names carry the design. Describe what isn't obvious; write interface comments first.
• Seam discipline. One adapter means a hypothetical seam, two means a real one. Accept dependencies, don't create them.

Red flags

Before finalising a design, check it against red-flags.md: shallow module, information leakage, pass-through method, vague name, and the rest.

Names and domain language

Name modules with the project's domain terms from CONTEXT.md. A name that is hard to pick or hard to describe is a design smell: the concept is probably unclear. Resolve it with the oracle-domain-modelling skill, and take contested design decisions through oracle-grill-me.

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

• Principles with examples, see principles.md.
• Red-flags checklist, see red-flags.md.
• Deepening a cluster given its dependencies, see deepening.md: dependency categories, seam discipline, and replace-don't-layer testing.
• Exploring alternative interfaces, see design-it-twice.md: spin up parallel sub-agents to design the interface several radically different ways, then compare.