Test-Driven Development

TDD is the fundamental practice. Every line of production code must be written in response to a failing test.

For how to write good tests, load the testing skill. This skill focuses on the TDD workflow/process. For mutation-aware test planning, load the mutation-testing skill and use its resources/mutator-rules.md resource as the source of truth.

RED-GREEN-MUTATE-KILL MUTANTS-REFACTOR Cycle

RED: Write Failing Test First

• NO production code until you have a failing test
• Test describes desired behavior, not implementation
• Test should fail for the right reason
• Before finalizing the test, scan the intended behavior against the mutator rules: boundaries, boolean combinations, equality, arithmetic identities, array/string operations, optional chaining, and side effects
• Add obvious missing cases immediately; use the harness's ask-question facility when the expected behavior is a product/domain judgment