# Common TDD

Guides quality-first TDD for new behavior, bug fixes, and test changes.

- Repository: https://github.com/HoangNguyen0403/agent-skills-standard (582 stars)
- Category: Testing
- License: MIT
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add HoangNguyen0403/agent-skills-standard --skill common-tdd -g -a claude-code -y`
Codex: `npx skills add HoangNguyen0403/agent-skills-standard --skill common-tdd -g -a codex -y`
Cursor: `npx skills add HoangNguyen0403/agent-skills-standard --skill common-tdd -g -a cursor -y`
Gemini CLI: `npx skills add HoangNguyen0403/agent-skills-standard --skill common-tdd -g -a gemini-cli -y`

## Description

Guides quality-first TDD for new behavior, bug fixes, and test changes. Selects the smallest test layer, proves a distinct regression risk, and runs bounded RED-GREEN-REFACTOR verification.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Quality-First TDD

## **Priority: P0 (CRITICAL)**

A passing test is insufficient; the test must prove an owned behavior and a distinct plausible fault.

## Choose the mode

- **New behavior:** strict RED -> GREEN -> REFACTOR. Do not write production code before the expected RED. If implementation preceded tests, report original RED as `NOT RUN`; never manufacture fake RED by deleting guards or falsifying logs.
- **Legacy or bug fix:** characterize only when needed, then reproduce the intended change as a failing regression (RED). Frozen old-revision proof against a pre-repair snapshot is valid regression evidence when explicitly labeled as `frozen_snapshot_red`. Preserve unrelated existing code; do not delete it merely because it predates the test.
## Before writing a test

Compare against the existing suite before adding or removing cases. Ensure the scenario is not already covered upstream or in nearby layers. Preserve justified cross-layer tests when layers catch distinct regression risks.

Create one Test Intent Record per behavior/risk (see `references/quality-contract.md` for schema):

- `id`: unique stable intent identifier
- `requirements`: traceable requirement IDs (at least one)
- `owner`: accountable owner
- `contract`: observable contract, application-owned result or outward side effect
- `fault`: distinct plausible fault this test catches (risk-sensitive, not blanket mutation quota)
- `layer`: smallest honest layer (`unit`, `integration`, `contract`, `e2e`)
- `cases`: minimal distinct equivalence classes; use a parameterized test for equivalent inputs
- `command`: exact focused single-run command
- `oracle`: independent oracle ref (path + sha256), basis (`approved_requirement`, `external_specification`, `independently_reviewed_example`), owner, and rationale; never copied solely from implementation
- `boundaries`: explicit `real` vs `mocked` boundaries with justification

The rest: https://github.com/HoangNguyen0403/agent-skills-standard/blob/HEAD/.agents/skills/common/common-tdd/SKILL.md