agentic-dev/.claude/skills/improve-codebase-architecture/INTERFACE-DESIGN.md

Interface Design

When the user wants to explore alternative interfaces for a chosen deepening candidate, use this parallel sub-agent pattern. Based on "Design It Twice" (Ousterhout) — your first idea is unlikely to be the best.

Uses the vocabulary in LANGUAGE.md — module (= feature by default in this repo), interface, seam, adapter, leverage.

Process

1. Frame the problem space

Before spawning sub-agents, write a user-facing explanation of the problem space for the chosen candidate:

• The constraints any new interface would need to satisfy
• The dependencies it would rely on, and which category they fall into (see DEEPENING.md)
• The hard constraints from SKILL.md that the new interface must respect (factory-function shape, per-feature DI, manifest-first, generator-first, brand wrappers, etc.)
• A rough illustrative code sketch to ground the constraints — not a proposal, just a way to make the constraints concrete

Show this to the user, then immediately proceed to Step 2. The user reads and thinks while the sub-agents work in parallel.

2. Spawn sub-agents

Spawn 3+ sub-agents in parallel using the Agent tool (subagent_type=general-purpose or a more specific type if appropriate). Each must produce a radically different interface for the deepened module.

Prompt each sub-agent with a separate technical brief (file paths, coupling details, dependency category from DEEPENING.md, what sits behind the seam). The brief is independent of the user-facing problem-space explanation in Step 1. Give each agent a different design constraint:

• Agent 1: "Minimise the interface — aim for 1–3 entry points max. Maximise leverage per entry point."
• Agent 2: "Maximise flexibility — support many use cases and extension."
• Agent 3: "Optimise for the most common caller — make the default case trivial."
• Agent 4 (if applicable): "Design around ports & adapters for cross-seam dependencies."

Every brief MUST also include:

• This repo's vocabulary from docs/glossary.md (use case, manifest, feature, slice, etc.)
• The architecture vocabulary from LANGUAGE.md
• The hard constraints from SKILL.md — sub-agents must not propose interfaces that violate ADR-006 (boundaries), ADR-008 (per-feature DI), ADR-012 (factory shape, one controller per use case), ADR-013 (schemas in use-case file), ADR-014/017 (vendor isolation), ADR-015 (events for cross-feature), ADR-020 (manifest-driven coverage bands), or ADR-021 (versioning by commit-path).
• The relevant feature feature.manifest.ts shape so the proposed interface aligns with manifest-first ordering.

Each sub-agent outputs:

1. Interface (types, methods, params — plus invariants, ordering, error modes, schemas if applicable)
2. Usage example showing how callers in this repo would use it (use real file paths and real existing feature names)
3. What the implementation hides behind the seam
4. Dependency strategy and adapters (see DEEPENING.md) — which existing ports/adapters get reused, which (if any) are new
5. Manifest + binder impact — which feature.manifest.ts entries and which bind-production.ts / bind-dev-seed.ts files change
6. Trade-offs — where leverage is high, where it's thin
7. ADR conflicts (if any) — call out by ADR number with rationale, or state "none"

3. Present and compare

Present designs sequentially so the user can absorb each one, then compare them in prose. Contrast by:

• Depth (leverage at the interface)
• Locality (where change concentrates)
• Seam placement (which existing *.interface.ts survives, which gets replaced, which is new)
• Conformance impact (how many manifests change, how many binders change, how many tests rewrite)
• Coverage delta (cumulative L0 band impact — does any layer drop below its declared 100% / 95%?)

After comparing, give your own recommendation: which design you think is strongest and why. If elements from different designs would combine well, propose a hybrid. Be opinionated — the user wants a strong read, not a menu.

If the chosen design crosses a feature-package boundary (e.g. moves a use case from @repo/blog to @repo/media), state explicitly:

• Which feature.manifest.ts files lose / gain entries
• Which package versions will bump on the next release-please PR (per ADR-021 commit-path bump targeting)
• Whether the migration needs an intermediate compatibility seam to keep pnpm conformance green during the transition

The implementation lands via the manifest-first ordering: (1) update the manifests in both packages, (2) write the new contracts in the use-case file, (3) write the failing tests, (4) implement until green. Don't skip the order even when the move feels mechanical.