Testing Discipline

Trigger phrases: "write a test", "run the tests", "coverage", "are the tests green", "unit test", "integration test", "tests pass locally", "flaky test", "test is flaky"

Goal: behavior correctness — test real behavior without breaking product code just to make a test pass.

Principles

• Pyramid: many unit, fewer integration, few end-to-end (e2e). Limit e2e to critical flows.
• AAA: Arrange-Act-Assert; one test = one behavior.
• Isolation & determinism: external dependencies are mocked/faked; time and randomness are fixed; test order is independent.
• Risk-coverage: risk, not metrics. Critical path + boundary + negative + authorization (IDOR/404) scenarios.
• Naming: what_it_tests_under_which_condition_what_it_expects — on failure it is clear what broke.
• Red-green: first a failing test, then the implementation.

Watch out

• Flaky = bug: an occasionally failing test is not tolerated; it is fixed at the root.
• In snapshot/golden-file tests, avoid needless brittleness (assert only the meaningful output).

Tests that cannot fail

Green is evidence only if the test could have gone red. Shapes that check nothing:

• Assertion restates the code: it recomputes the implementation's own expression, so it moves with every change.
• Setup guarantees the result: arrange plants the value the assert reads back; it holds even if the act never ran.
• Expected value derived the way the code derives it: same formula, query or parser — the same wrong assumption on both sides still matches.
• A double asserted against its own stub: the fake is told to return X and the test asserts X; only the fake runs.
• No assertion: it passes because nothing threw. If not throwing is the behavior under test, assert that.

Prove it can fail: break what the test covers — invert a condition, return a wrong constant — confirm it goes red, then restore. Still green means the test is documentation, not a gate: fix it or delete it, never count it as coverage.

Flaky triage

An intermittent failure gets exactly one outcome, decided the day it is seen: fix it, quarantine it (out of the gating lane, never out of the suite, with an owner and a date), or escalate it as the product defect it signals. Infrastructure flakes may get a bounded, logged retry; product flakes never do. Classifying them, the retry rule, and what a quarantine entry must carry: references/flaky-triage.md.

DoD (this skill's contribution)

• The project's own test command is green — read it off the manifest/CI (dotnet test · npm test · pytest · go test ./...), don't assume one; critical paths are covered; no empty/meaningless tests.