Codebase Design

Design deep modules: a lot of behavior behind a small interface, placed at a clean seam, testable through that interface. Use this language wherever code or a factory issue is being designed. The aim is leverage for callers, locality for maintainers, and testability for everyone.

Glossary

Use these terms exactly — don't substitute component, service, boundary, or API when you mean module or interface; don't say task or ticket for issue, or test file for frozen check. Consistent language is the whole point.

Design (Pocock, adapted):

• Module - anything with an interface and an implementation, at any scale: a function, a package, or a whole vertical slice.
• Interface - everything a caller must know to use a module correctly: types, invariants, ordering, error modes, config, performance - not just a signature.
• Implementation - the body of code behind the interface; distinct from adapter, which names role at a seam, not substance.
• Depth - leverage at the interface: behavior reached per unit of interface learned. Deep means a small interface over a big implementation; shallow is the opposite, and to avoid.
• Seam (Feathers) - the place a module's interface lives; where behavior can change without editing there.
• Adapter - a concrete thing satisfying an interface at a seam. One adapter is a hypothetical seam; two make it real.
• Leverage - what callers get from depth: one implementation pays off across many call sites and tests.
• Locality - what maintainers get from depth: change and bugs concentrate in one place instead of spreading.

Factory (one line each; matches docs/spec/skill-library.md Target flow):

• Run - one factory build from intake to the closing PR, on its own factory branch.
• Tracking issue - the run's parent issue: dashboard, digest, preflight record.
• Issue - one vertical-slice unit of work: one builder job, one disjoint file set, one change-skeleton.
• Slice - the vertical cut an issue implements; slice names the cut, issue names its tracker record - same unit, two angles.
• Frozen check - the committed, read-only acceptance check a builder's work is graded against.
• Check-runner - the deterministic script that grades a frozen check's RUN items and exits typed (0/2/5).
• Strategist - the fresh high-judgment subagent that drafts specs, cuts issues, stress-tests plans, and runs full-lane reviews.
• Builder - the fresh, worktree-isolated agent that implements one issue and never commits.
• Orchestrator - the one session that grounds, dispatches strategist and builder work, freezes, and integrates; never writes implementation code; in the fast lane (/architect-fast) it also runs the closing test pass whose output feeds the builder review - a recorded relaxation.
• Factory branch - the run's integration branch (factory/<run>) that job branches merge into.
• Worktree - the isolated git checkout a builder or reviewer works in, verified against the freeze commit.
• Job report - a builder's raw-evidence artifact, ending in one STATUS line.
• Verdict - the grading record posted on an issue at close: checkrun summary plus typed exit, postflight result, slice call, decisive reason; the closing review's run-level verdict goes on the tracking issue.
• Ruling - an orchestrator decision recorded post-freeze: a PHASE-0 execution conflict, boundary amendment, or respawn answer.
• Digest - the shipped-issues, diffstat, rulings, and domain-language summary handed to the integrate subagent's docs pass.
• Hard stop - an irreversible or destructive action the loop refuses without a human ruling.

Deep vs. shallow

A deep module hides a lot of implementation behind a small interface: few methods, simple params, most of the complexity out of sight. A shallow module is the opposite - an interface almost as complex as what it wraps, often a near-passthrough. When shaping an interface, ask whether the method count and params can shrink, and whether more logic can move inside.

Principles

• Depth is a property of the interface, not the implementation - a deep module can be built from small internal seams its own tests use, without exposing them at the external interface.
• The deletion test: imagine deleting the module. If complexity vanishes, it was a pass-through; if it 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.

Designing for testability

• Accept dependencies, don't create them - a function that takes its gateway is testable; one that builds a StripeGateway() internally is not.
• Return results, don't produce side effects - a function that returns a computed discount is testable; one that mutates a cart in place is not.
• Small surface area: fewer methods = fewer tests needed; fewer params = simpler test setup.

Going deeper

• 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: dispatch parallel subagents to design the interface several radically different ways, then compare on depth, locality, and seam placement.