Strategy

Push logic to common. Most tests live in common.

Layer	Volume	Coverage target
common	High	~80%+
model/common	Medium	Cross-cutting essentials
model/{model}	Low	Wiring, critical paths
pages / app	Minimal	Smoke / e2e

Rules

1. Test next to logic — lib/converter.test.ts
2. Test common heavily (fast, stable)
3. model tests wiring only — don't re-test logic
4. No business logic tests in pages / app

Extract to common

Move to common	Path
Validation, predicates, transforms	common/lib/
Date, string, URL formatting	common/lib/
Generic input UI (no domain names)	common/ui/

Before adding to model:

1. Does it mention project-specific names?
2. If not, put it in common
3. Pure functions in model → move to common/lib/ on review

model/view.tsx should wire OOUI props only. Logic goes to common.

TDD flow

1. Extract logic to common/lib
2. Write unit tests in common first
3. Model only wires common
4. Add one model integration test if needed

Sample

templates/common/lib/example.test.ts

Use the project's test runner. If none, prefer bun:test or vitest.

*.test.ts sits beside source — not in a tests/ folder (avoid path duplication).