claude-skills/skills/tdd-criteria/SKILL.md

---
name: tdd-criteria
version: 0.2.0
description: >
  TDD by default with four bright-line carve-outs. Applies before any code
  change. Triggers: "TDD", "test-driven", "следуй TDD", "use TDD",
  "should I write tests", "skip tdd", "[skip-tdd: ...]",
  "[test-modify: ...]", "tdd-criteria". Cross-agent policy — no tool refs.
  Full rationale: .wiki/concepts/tdd-criteria-design.md
---

# tdd-criteria

> TDD by default. Skip only with a bright-line marker. Tests defend code from silent deletion; rule 4 defends tests from silent rewriting.

## When this runs

**At session start** — when `CLAUDE.md` contains the line `follow tdd-criteria`.

**Before any code change** — touching a `*.ts`, `*.js`, `*.py`, `*.go`, `*.rs`, `*.java`, `*.rb`, `*.ex`, `*.swift`, `*.kt`, `*.cs`, `*.php`, or similar source file triggers the decision algorithm below.

**On explicit reference** — when the user says "TDD", "test-driven", "следуй TDD", "use TDD", "should I write tests", "skip tdd", "tdd-criteria", or includes `[skip-tdd: ...]` or `[test-modify: ...]` in a commit subject.

## Default mode

TDD by default: write a failing test first, then write the minimum code to make it pass, then refactor. Skip only if one of four bright-line carve-outs matches and is marked in the commit subject.

## Decision algorithm (8 questions, top-down)

Walk through in order. First «yes» determines mode. All «no» → TDD by default.

1. **Bug fix?** → TDD (red-test first)
2. **Consuming external contract** (SDK, REST API, foreign schema)? → TDD (contract-test)
3. **Security / auth / money / identifiers?** → TDD
4. **Pure logic** — function (input → output) without I/O, global state, bounded inputs? → TDD
5. **Visual / config** — CSS, layout, design tokens, `.env.example`, prompts, wiki, README? → SKIP, `[skip-tdd: visual]`
6. **Spike** — explicit POC «throwaway» in commit/PR/task subject? → SKIP, `[skip-tdd: spike]` + spike-survivor task if code survives
7. **One-shot** — migration, ETL backfill, ad-hoc cleanup, runs once? → SKIP, `[skip-tdd: oneshot]`
8. **Transit wrapper** ≤10 non-blank non-comment lines — no branching, re-export / glue? → SKIP, `[skip-tdd: wrapper]`

(default) → TDD

**Composite tasks.** A task that doesn't fit one category is composite — break it down per artefact type. The criterion applies per artefact, not per task. Example: a settings page = CSS layout `[skip-tdd: visual]` + validation logic `[TDD]` + API wrapper `[TDD]`.

**Refactoring** — restructuring code without changing observable behaviour, with existing tests covering it — does not require new tests. Existing tests must still pass. If refactoring introduces new behaviour, that part is subject to the decision algorithm as a separate artefact.

## Ironclad rules (TDD obligatory)

1. **Bug fix** — if there's an issue / failure log / repro, write a red-test codifying the repro. Marginal cost: 5 min. Marginal benefit: regression test forever.
2. **Pure logic, bounded inputs** — no fixtures, no mocks, no setup. Test = input/output pair. Skipping is gratuitous.
3. **Third-party contract** — SDK bumps change signatures silently. Contract-test pins known input → known shape. Catches break at first install, not days later.
4. **Security / auth / money / IDs** — asymmetric blast radius: false positive ≪ false negative. TDD = insurance.

In all four, recovery cost from silent deletion is high. The test is the only artefact that makes deletion visible.

## Permissive carve-outs (skip + marker required)

| # | Category | Trigger | Marker |
|---|----------|---------|--------|
| 5 | Visual / config | CSS, layout, tokens, `.env.example`, prompts, wiki | `[skip-tdd: visual]` |
| 6 | Spike | «POC, throwaway» in commit/PR/task subject | `[skip-tdd: spike]` |
| 7 | One-shot | Migration, ETL, ad-hoc cleanup; runs once | `[skip-tdd: oneshot]` |
| 8 | Wrapper | ≤10 non-blank non-comment lines, no branching, re-export / glue | `[skip-tdd: wrapper]` |

These are **accepted-risk zones** — you accept that an agent can vandalise without immediate signal, because recovery is cheap (eyeball next render; throwaway by contract; runs once; reconstruct ≤ delete).

## Anti-loophole

1. **Skip without category is invalid.** One of four explicit categories required — not «other reasons». No marker = violation.
2. **Spike survivor rule.** Merged spike → same merge-commit creates `[backfill-tests-<slug>]` task (in `.tasks/` if available, otherwise a TODO comment or GitHub issue). Otherwise «spike» becomes «skipped tests forever».
3. **Friction is the point.** `[skip-tdd: visual]` 50× in a design-system rework is irritating — that's the fence. Re-evaluate after ≥2 weeks, not before.
4. **Tests are append-only by default.** Modifying an assertion, deleting a test, or disabling it (`it.skip`/`xit`/`@pytest.mark.skip`/`@Disabled`) requires:
   - **Marker in commit subject:** `[test-modify: <test-name>: was <X>; is <Y>; reason: <Z>]` — `<X>` and `<Y>` are **literal assertion expressions**, not paraphrased. Example: `[test-modify: validates email: was expect(isValid("a@b")).toBe(true); is expect(isValid("a@b.com")).toBe(true); reason: tightened spec to require TLD]`
   - **Separate commit from impl changes.** A commit must not modify both `*.test.*` and `src/*` files (or project-equivalents). `git log --grep '\[test-modify'` must show a clean test-only audit trail.
   - **Why literal was/is:** an agent forced to write the literal assertion publishes exactly what they're rewriting. «Updated to match new behaviour» hides everything — agents will use that whenever allowed.

## Cross-agent applicability

Pure policy — no agent-specific tool references in this body. Works on Claude, Gemini, Copilot, or any future agent. Hermes mapping: `mode: auto`, `category: software-development`, no replace-rules.

## Out of scope

- Does not enforce via git hooks (separate optional task: `tdd-criteria-precommit-hook`).
- Does not modify project `CLAUDE.md` (that's `project-bootstrap`'s job).
- Does not run tests.
- Does not apply rule 4 retroactively to tests written before the rule was adopted.

## Why this exists

Tests make behaviour an invariant; without them, code is an artefact silent-deletable by agents. AND: the test itself must be defended too (rule 4) — otherwise the contract collapses back into an artefact when the agent rewrites the failing test. Full rationale: `.wiki/concepts/tdd-criteria-design.md`.