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conventions

Apply these opinionated testing conventions whenever writing, reviewing, or planning tests: what earns a test and what is coverage theatre, asserting outputs rather than mock calls, the three modes (functional, performance, security), naming and isolation rules, where to mock, and reporting findings for coverage rather than self-filtering. Includes house Jest standards for NestJS unit and contract tests, TDD discipline, and test-report structure.

plugins/engineering/cortex-testing/skills/conventions/SKILL.md

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# Testing

House conventions for writing tests and test strategy.

## What a test is for

A test earns its place by failing when the behaviour breaks. That single criterion resolves most arguments about what to test: if you cannot name the bug a test would catch, it is coverage theatre — it costs maintenance on every refactor and reports success while the system is broken.

Two consequences run through everything below:

- **Assert on outputs — return values, thrown errors, persisted state.** A test whose only assertion is `expect(mock).toHaveBeenCalledWith(…)` verifies the wiring you just wrote, not the behaviour. Rewrite the implementation correctly in a different shape and it fails; break the behaviour while keeping the calls and it passes. Both are the wrong way round.
- **Test decisions, not lines.** Branches, error translation, state transitions, boundary conditions. A pass-through delegator or a field mapping with no conditional has nothing to get wrong, and a test for it only breaks when someone renames a method.

Coverage percentage is a floor signal — it finds untested files. It cannot tell a valuable test from a vacuous one, so treat a high number as "nothing is empty", never as "the tests are good".