testing-tierslisted

# Testing Tiers A decision discipline for keeping a test suite green **and** meaningful. A green suite that proves nothing is worse than a failing one you trust — this skill keeps the two apart. It applies to any project with tests; the tier names below generalize across frameworks (the concrete names like "vitest" or "jest" are placeholders for whatever the project uses). **The defining constraint:** tests must exercise the **real entry path** and **verify the world**, not the component's self-report. A mock-heavy unit suite can be 100% green while the shipped behavior is broken — the failure mode this skill exists to prevent. ## The tiers, and when each applies Pick the shallowest tier that would actually fail for the change's regression: | Tier | What it proves | Applies when | | --- | --- | --- | | **Unit** | A function/class behaves per its contract | The behavior is confined to one module | | **Integration** | Several modules cooperate | A shared contract or wiring changes | | **Real entry path** | The shipped entry (loader, binary, server, worker) actually works | A product-visible plugin, entrypoint, or composition changes | | **End-to-end** | The assembled system works against a real external boundary | The change touches provider, network, or cross-system behavior | | **Snapshot** | External contract / presentation output stays stable | Model-, protocol-, or human-visible output changes | Match evidence to the surface: focused unit tests for behavior, snapshot