/skill-tdd

name: skill-tdd
description: "Build a behavior change with observed red, minimal green, and measured test consolidation"
disable-model-invocation: true

> **Host: Codex CLI** — This skill was designed for Claude Code and adapted for Codex. > Cross-reference commands use installed skill names in Codex rather than `/octo:*` slash commands. > Use the active Codex shell and subagent tools. Do not claim a provider, model, or host subagent is available until the current session exposes it. > For host tool equivalents, see `skills/blocks/codex-host-adapter.md`.

Test-driven development

Read `skills/blocks/engineering-method-selection.md` from the installed plugin for review admission. Natural-language requests and `--peer-review` share that policy. Honor host-only requests; risk alone does not authorize paid usage.

Run the red, green, and refactor cycle on the current host. Routine TDD makes zero additional provider dispatches. Use one external reviewer only when the user passes `--peer-review`, explicitly requests independent review, or an existing risk policy requires it. Explicit debate, council, and multi-model commands retain their own execution contracts.

<HARD-GATE> NO PRODUCTION BEHAVIOR CHANGE WITHOUT AN OBSERVED, EXPECTED FAILING TEST FIRST. </HARD-GATE>

The rule

Do not change production behavior until a focused test fails for the expected reason. Existing implementation outside the requested change remains intact.

1. Name the observable behavior and the smallest public boundary that proves it. 2. Write one focused test. Directly test an internal invariant only when the public boundary cannot isolate its failure mode. 3. Run it and record the expected failure, command, and exit status. 4. Implement the smallest change that passes. 5. Run the focused test, then the affected suite. 6. Refactor only while the tests remain green.

If a test passes before implementation, it is not red evidence. If it errors due to fixture or syntax problems, repair the test until it fails on the missing behavior.

Consolidating tests

Do not equate similar assertions with duplicate guarantees. Keep separate OS, security, cancellation, and integration boundaries. For every removed test, record `old_test`, `behavior`, `replacement`, `mutant`, `red_observed`, `baseline_ms`, `candidate_ms`, and `reason`. The retained test must kill the named mutant at the intended caller boundary.

After one warm-up, measure five isolated runs and report every sample and the median. Review a slowdown only when it exceeds both 20 percent and 100 ms.

Completion requires observed red and green evidence, affected-suite results, and the consolidation ledger when tests were removed. A missing reviewer is reported as incomplete review, never simulated.

Strategy rotation

If the same test remains red after two implementation attempts, stop and recheck the test boundary, fixture, and expected behavior. The strategy-rotation hook is a signal to try a fundamentally different hypothesis, not another variation of the same patch.

Adapted from `DEEPENING` in `mattpocock/skills` at commit `3cca18b368ae95cdbdebbff572ccafa662551015` under the MIT License. See `THIRD_PARTY_NOTICES.md`.