rsun-tdd

Write one failing test for the smallest user-facing behavior, implement the minimal code to pass it, commit, then repeat for the next slice. Never batch multiple tests before implementations, since that produces tests describing imagined rather than real behavior.

Tracer bullet TDD builds a thin end-to-end slice through the real code path first, such as an API endpoint returning a hardcoded id, then thickens it slice by slice with persistence, validation, and side effects. Each slice is a working, committable increment.

Mock only external dependencies like third-party APIs, databases, time, and randomness, using factory functions that return default objects with overridable fields. Never mock internal collaborators, since that tests implementation details and breaks on harmless refactors.

Yes, the same RED-GREEN-commit discipline applies to Python pytest and TypeScript vitest. The skill includes examples for both, covering integration-style tests through public interfaces like HTTP endpoints.

Skip the TDD cycle for pure documentation or comment updates, configuration value changes, and static HTML content edits. Any production code, deliverable code, cron agent change, or new MCP tool must start with a failing test.

Tests that fail during behavior-preserving refactors are testing implementation details rather than public behavior. Rewrite them to exercise the public interface through real code paths instead of asserting on private methods or mocked internals.