Test strategy: prove behaviour at the cheapest useful layer

• Test observable behaviour and contracts, not private structure. A test that breaks on every refactor without catching a behaviour change is a maintenance tax.
• Choose the cheapest layer that gives confidence: a pure unit test for logic, an integration test for a boundary, end-to-end only for a small number of critical journeys.
• For a bug, write the failing test first. For legacy code, add characterization tests around the current behaviour before changing it.
• Cover the edges that matter: empty, invalid, unauthorized, duplicate, timeout, partial failure and retry. One test per behaviour, not one per line.
• Make failures diagnostic: assert the important value and include the input in the failure. Avoid sleeps, shared mutable state and dependence on execution order.
• Treat a flaky test as a defect. Quarantine, reproduce and fix the source; do not retry until green.
• Report coverage gaps honestly. A high percentage does not prove the risky paths were tested.

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NullByte skill pack. Original guidance for this tool; no third-party skill text is bundled.