# Tdd > Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", "strict TDD", "test-driven-development", or wants integration tests. - Skill: `netvar1337/tdd` (Agent Skill, multi-file: 5 files) - Install (CLI): `npx skillmds@latest add netvar1337/tdd` - Raw SKILL.md: https://api.skillmd.com/api/skills/netvar1337/tdd/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Integrations & APIs - Author: netvar1337 (https://skillmd.com/u/netvar1337) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/netvar1337/tdd --- # Test-Driven Development TDD is the red → green loop. This skill is the reference that makes that 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 behavior 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 it survives refactors because it doesn't care about internal structure. See [tests.md](tests.md) for examples and [mocking.md](mocking.md) for mocking guidelines. ## Seams: where tests go A **seam** is the public boundary you test at: the interface where you observe behavior 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, so 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?" When the shape of that interface is itself in question (how deep the module is, where the seam belongs, what the interface should expose), call the Skill tool with "codebase-design" for the vocabulary. It is the shared source of the module, interface, depth, seam, adapter, leverage and locality terms, and it is a reference to consult, not a session to run. ## Anti-patterns - **Implementation-coupled** — mocks internal interfaces, 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 behavior hasn't changed. - **Tautological** — the assertion recomputes the expected value the way the code does (`expect(add(a, b)).toBe(a + b)`, a snapshot derived by hand the same way, 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_ behavior: you test the _shape_ of things rather than user-facing behavior, the tests go insensitive to real changes, and you commit to test structure before understanding the implementation. Work in **vertical slices** instead: one test → one implementation → repeat, each test a **tracer bullet** that reflects 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. ## Enforcement When the user asks for strict TDD, the Iron Law applies: no production code without a failing test first, and you must watch each test fail before implementing. Full enforcement playbook, rationalization table, and completion checklist: [references/enforcement.md](references/enforcement.md). ## Consolidated strict-cycle checks For a strict TDD request, watch the focused test fail for the expected missing behavior before writing production code. A test that passes immediately is either covering existing behavior or asserting the wrong seam; correct the test before implementation. Then write the smallest production change that passes it, re-run the focused test, and run the relevant suite once to catch regressions. Keep each cycle vertical: one behavior, one public seam, one failing test, and one minimal implementation. Do not retain or adapt prewritten production code as a reference during the cycle. Prefer real collaborators over mocks; when a mock is unavoidable, preserve the behavior the test is intended to prove. Refactor only after green and keep the tests green while doing so. Before completion, confirm that each new behavior had an observed RED result, its GREEN result is clean, expected values came from an independent source of truth, and test names describe user-observable behavior rather than implementation details.