Instruction file imported from timander/copilot-instructions-starter-kit (.github/instructions/xp.instructions.md). Copyright stays with the author.

Testing Discipline

Auto-loaded only on test files. The full red-green-refactor protocol lives in /tdd.

Principles

• Test behavior, not implementation. A test that breaks during a refactor was testing the wrong thing.
• One concept per test. Multiple asserts on the same object is fine; multiple concepts in one test is not.
• Descriptive names that document behavior. shouldReturnEmptyList_whenInputIsNull over test1.
• Arrange–Act–Assert structure, with blank lines between phases when it aids reading.
• Edge cases: null, empty, boundary, exhausted iterator, max-size, negative.
• Fast. Mock or fake external dependencies. A slow test suite gets skipped.
• Tests are documentation. A reader should understand the unit by reading its tests.

Discipline

• No production code without a failing test.
• If you can't write a test, the design needs to change — that's the feedback. Don't power through it.
• If a step feels big, break it smaller.
• Tests are first-class design artifacts, not afterthoughts. They get refactored too.