From 02ecce3e3e14cb29133b9b1c83abb5d054934126 Mon Sep 17 00:00:00 2001
From: Kimi Code <kimi@cloudhost.es>
Date: Fri, 7 Aug 2026 18:58:34 +0200
Subject: [PATCH] feat: add tdd skill

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+---
+name: tdd
+description: Test-driven development. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration tests.
+license: MIT
+source: https://github.com/mattpocock/skills
+---
+
+# 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 survives refactors because it doesn't care about internal structure.
+
+Use the project's existing test conventions for examples and 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 — 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 — consult the [`codebase-design`](../codebase-design/SKILL.md) skill for the vocabulary.
+
+## 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 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 responds 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.