/common-tdd

name: common-tdd
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."
metadata:
  triggers:
    files:
      - "**/*.test.ts"
      - "**/*.spec.ts"
      - "**/*_test.go"
      - "**/*Test.java"
      - "**/*_test.dart"
      - "**/*_spec.rb"
    keywords:
      - tdd
      - unit test
      - write test
      - red green refactor
      - failing test
      - test coverage

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.
• **Legacy or bug fix:** characterize only when needed, then reproduce the intended change as a failing regression (RED). Preserve unrelated existing code; do not delete it merely because it predates the test.

Before writing a test

Create one Test Intent Record per behavior/risk:

• `contract`: observable contract — an application-owned result or side effect
• `fault`: distinct fault — a distinct plausible regression this test would catch
• `layer`: smallest honest unit, component, contract, integration, or E2E layer
• `cases`: minimal distinct equivalence classes; use a parameterized test for equivalent inputs
• `command`: exact focused single-run command

Reject tests that duplicate an existing fault, assert implementation detail or mock choreography, depend on time/network/order, or force a broader behavior into a unit.

Bounded loop

1. Run configured lint/type checks, inspect nearby tests, and derive the smallest command. 2. **RED:** add one intent group and run it in the foreground, sequentially, single-run mode. 3. Classify RED as `expected_red`, `invalid_red`, `unexpected_green`, or `verification_infra_failed`. If it is `unexpected_green`, inspect existing coverage and remove a redundant or weak case before implementing production code. 4. **GREEN:** implement only enough to satisfy `expected_red`; rerun the same command. 5. **REFACTOR:** improve structure without changing behavior; rerun the same command. 6. Escalate only when evidence requires it: related unit target, integration/contract target, then explicit release/full-suite gate.

Execution safety

• Honor project timeouts; otherwise use a 120-second fallback to bound a focused command.
• On timeout, terminate only the agent-owned process group and verify child cleanup.
• Never watch, blanket-kill, or retry an unchanged failure. Record the new hypothesis or corrective change first.
• Coverage is repository-configured, project-owned evidence. Without a configured threshold, report risk gaps and never add padding tests for a percentage.

Red flags and rationalizations

• Stop on: `add tests after`, `too small`, `passed first run`, `run the full suite again`, or `mock every collaborator`.
• Urgency, manual testing, test count, or a coverage target never bypasses the intent record, expected RED, bounded command, or fault proof.

Test shape

• Use clear Arrange, Act, Assert phases; comments are optional.
• Assert observable outcomes. Assert an interaction only when that interaction is the contract.
• Mock external boundaries only when isolation requires it; prefer real pure/domain behavior and simple fakes.
• Keep test names behavior-focused, without ticket IDs or TODO/FIXME markers.

See `references/quality-contract.md` for the intent record, failure taxonomy, layer routing, and runner examples.