/testing-principles

name: testing-principles
description: Language-agnostic testing principles including TDD, test quality, coverage standards, and test design patterns. Use when writing tests, designing test strategies, or reviewing test quality.

Language-Agnostic Testing Principles

Test-Driven Development (TDD)

Use this cycle for new or changed executable behavior and reproducible bug fixes. For a behavior-preserving refactor, first confirm existing tests pass or add passing characterization tests, then refactor and rerun the same regression evidence.

RED: confirm the new test fails for the intended reason. GREEN: implement the smallest passing change. REFACTOR: improve structure while the test remains green. VERIFY: run the repository's applicable regression checks.

Quality Requirements

• Treat coverage as a diagnostic signal for finding untested areas, not a target — a target gets gamed into trivial tests (Goodhart's Law)
• Concentrate tests on critical paths, business logic, and behavior whose regression would matter
• Prioritize meaningful assertions over the coverage number; any CI threshold is the project's config, not a quality goal in itself
• Use project-configured speed budgets when present. Otherwise investigate test speed only when observed feedback or CI cost is material to the current outcome; retain slower tests when their proof boundary requires it

Test Design Rules

• Structure each test as Arrange, one Act, and Assert; multiple assertions may prove one behavior.
• Follow the repository's test naming convention and name the condition and observable outcome.
• Exercise behavior through a public or integration boundary. Assertions verify return values, outputs, errors, or state changes rather than private implementation.
• Use independently derived literal, property, approved snapshot, or fixture expectations. An implementation-derived oracle cannot detect the same implementation defect.
• Keep each test's expected outcome unconditional. Table-driven or property-based cases are acceptable when each case is reported distinctly and uses an independent oracle.
• Cover accepted boundary and error behavior; derive cases from the contract instead of adding generic edge-case permutations.
• Each test creates and cleans up its own state, passes in isolation and any order, and controls time or randomness that affects its result.
• Keep tests executable. Fix or remove tests that no longer describe accepted behavior; restore tests disabled only to bypass a failure.

Mock and Boundary Rules

• Mock direct external I/O boundaries; keep internal business logic and the boundary under test real.
• Use the existing application-owned adapter as the mock boundary. Introduce an adapter only when external I/O, an unstable contract, or required substitution cannot be controlled through the current design.
• Keep mock behavior limited to the contract needed by the test.

Data Layer Testing

Mock-based tests are sufficient when data access is only a dependency of the behavior under test. Verify against the project's real database engine or its accepted equivalent when the subject is a query, repository implementation, schema constraint, or migration compatibility. Resolve the test environment from repository configuration; when no representative environment exists and adding one is outside the approved work, report the missing verification decision.

Cross-check data-access code against the schema source named in the Design Doc or repository configuration. Schema-source verification is required to prove table, column, type, constraint, or dialect compatibility; successful mocks prove behavior only at the mocked boundary.

Verification Requirements

Capability Probe Postconditions

A capability probe passes when it uses the consumer's boundary and asserts the exact property that consumer needs. Command success, import success, or object existence is setup evidence.

Test Organization

Follow the repository's established test paths, runner routing, and naming. When establishing an approved new convention, separate test types only when their setup, runner, or environment differs.

Regression Testing

• Add a regression test for every reproducible behavior bug fix. When executable reproduction is impossible, record the reason and the alternative static, contract, or environment evidence that prevents recurrence.
• Before behavior-preserving changes to uncharacterized legacy code, establish passing characterization evidence and rerun it after the change.