Test-Driven Development

Drive implementation through one behavioral test at a time. For test necessity or quality without a test-first workflow, read test-judgment.md instead.

Preconditions

Use the public interface and target behaviors from the request or accepted plan. Ask only when a missing decision blocks a meaningful test; do not reconfirm settled choices.

Before each new test, use the test-necessity gate to name the regression it catches and check for duplicate coverage. If it rejects a test, complete that slice without manufacturing RED and report the exemption. A file category alone never grants an exemption.

Red-Green-Refactor

1. Write one focused test for an observable behavior. Run it and confirm RED for the expected reason, not an unrelated compile or setup failure.
2. Write the minimal production change that makes it GREEN, then run the test.
3. Refactor while GREEN, rerunning affected tests. Repeat for the next behavior until the accepted target is covered.

Do not write all tests first and then all implementation. Vertical slices keep each test grounded in what the previous cycle revealed rather than an imagined implementation.

Reading, baseline checks, and accepted formal model edits may precede RED. Behavior-changing production edits may not. Do not pause for user approval between cycles unless a new scope, design, or safety decision arises.