Shape TDD

Original name

Original description

Drive an approved implementation plan to completion phase by phase, test-first, through the red→green→refactor cycle, but only for phases whose implementation does not exist yet. Reads a plan from context/changes/<change-id>/plan.md and the canonical Progress section, and for each phase first checks whether the phase is TDD'able and still unimplemented — if it is, you write a failing test (RED), make it pass with the minimal code (GREEN), then clean up (REFACTOR); if it is not TDD'able, you redirect that phase to /shape-implement; if implementation is already present, you stop and explain that TDD does not work for already existing code, then suggest /shape-implement for that phase. Mirrors /shape-implement (same plan, same Progress source of truth, same phase-end commit ritual and clipboard handoffs) but flips the order so the failing test always comes before the code. Assumes test infrastructure is already in place — it does NOT set up runners, configs, fixtures, or CI. Use this skill when the user says "tdd", "test-first", "red green refactor", "implement this plan test-first", "drive the plan with tests", or wants to execute an existing plan through a TDD loop. For plans where test-first does not fit, hand the phase to /shape-implement. For phases where the implementation already exists, stop instead of writing retroactive tests.