tdd

name	tdd
description	Test-driven development — the red → green loop and the rules that make its tests worth keeping. Use when the user wants to build a feature or fix a bug test-first, mentions "red-green-refactor", TDD, or integration tests.
codex-short-description	The red → green loop, done so tests are worth keeping
compatibility	claude-code codex opencode

Test-Driven Development

TDD is the red → green loop. This skill is the reference that makes the loop produce tests worth keeping: what a good test is, where tests go, the anti-patterns, and the rules of the loop. Every section applies on every cycle — consult them before and during the loop, not after.

When exploring the codebase, read CONTEXT.md (if it exists) so test names and interface vocabulary match the project's domain language, and respect ADRs in the area you're touching.

What a good test is

Tests verify behaviour through public interfaces, not implementation details. Code can change entirely; tests shouldn't. A good test reads like a specification — "user can checkout with valid cart" tells you exactly what capability exists — and survives refactors because it doesn't care about internal structure.

See tests.md for good/bad examples and mocking.md for mocking guidelines.

Seams — where tests go

A seam is the public boundary you test at: the interface where you observe behaviour without reaching inside. Tests live at seams, never against internals.

Test only at pre-agreed seams. Before writing any test, write down the seams under test and confirm them with the user. No test is written at an unconfirmed seam. You can't test everything — agreeing the seams up front is how testing effort lands on the critical paths and complex logic instead of every edge case.

Ask: "What's the public interface, and which seams should we test?"

Anti-patterns

• Implementation-coupled — mocks internal collaborators, tests private methods, or verifies through a side channel (querying the database instead of using the interface). The tell: the test breaks when you refactor but behaviour hasn't changed.
• Tautological — the assertion recomputes the expected value the way the code does (expect(add(a, b)).toBe(a + b), a hand-derived snapshot, a constant asserted equal to itself), so it passes by construction and can never disagree with the code. Expected values must come from an independent source of truth — a known-good literal, a worked example, the spec.
• Horizontal slicing — writing all tests first, then all implementation. Bulk tests verify imagined behaviour: you test the shape of things rather than user-facing behaviour, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in vertical slices — one test → one implementation → repeat, each test a tracer bullet responding to what the last cycle taught you.

Rules of the loop

• Red before green. Write the failing test first, then only enough code to pass it. Don't anticipate future tests or add speculative features.
• One slice at a time. One seam, one test, one minimal implementation per cycle.
• Refactoring is not part of the loop. It belongs to the review stage (see the code-review skill), not the red → green implementation cycle.

Host portability: tool names follow Claude Code conventions; on other hosts map by intent — see PORTABILITY.md.

Self-Evolve Loop

This skill learns across invocations — the full contract is SELF-EVOLVE.md. Start: read the learnings journal — ~/.ink-and-agency/learnings/tdd.md and/or the workspace-local .ink-and-agency/learnings/tdd.md — if present, and apply its guidance. End: self-evaluate the results; optionally ask the user for feedback (never block on it); append signal-bearing learnings to the journal (user-global when the sandbox allows writing there, workspace-local otherwise); route skill-improvement ideas per the contract's tiers — edit the canonical source when one is present, never the plugin cache.