Fable TDD

Prove the behavior is missing or broken before changing production code, then make the smallest change that satisfies the right test at the right level.

Mission

TDD is not "write any failing test first." The red state must demonstrate the intended behavior gap through a trustworthy harness. A syntax error, broken fixture, stale build, or mock-only expectation does not earn the right to change production code.

The Skill optimizes for three things:

• causal confidence: the test fails because the behavior is wrong;
• minimal intervention: implementation changes only what the behavior requires;
• durable regression proof: the test would catch the bug if it returned.

Activate When

• fixing a reproducible bug or regression;
• adding behavior with a stable enough contract to assert;
• changing validation, state transitions, calculations, API behavior, persistence, or integration semantics;
• refactoring where an invariant needs executable characterization first.