A guide to Test-Driven Development, or TDD, a way of writing a failing test before writing the code that should pass it. It follows the red-green-refactor cycle: fail first, make it pass, then clean up the code.

Use it when writing tests first, implementing a new behavior with TDD, designing test fixtures, or reviewing a test-first approach.

It gives a structured way to define expected behavior before implementation and helps catch regressions while code changes. The guidance also covers test design, isolation, test data, and assertions.

tdd — Skill for Claude Code and Codex by pproenca: A guide to Test-Driven Development, or TDD, a way…

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add skills/pproenca/dot-skills/tdd

Any agent

npx skills add pproenca/dot-skills --skill tdd

Clone the repo

git clone --depth 1 https://github.com/pproenca/dot-skills

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/pproenca/dot-skills/tdd.svg)](https://agentmods.dev/skills/pproenca/dot-skills/tdd)

Your own site

<a href="https://agentmods.dev/skills/pproenca/dot-skills/tdd"><img src="https://agentmods.dev/badge/skills/pproenca/dot-skills/tdd.svg" alt="Measured on agentmods" height="20"></a>

Per session 96 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,358 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5	$0.00096	$0.01358
Opus 5	$0.00048	$0.00679
Sonnet 5	$0.00019	$0.00272
Haiku 4.5	$0.00010	$0.00136

Measured 4d ago against content hash f1bd0e195037, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

tdd scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 4d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

Copies of this mod

1 near-identical copy found in the catalogue:

tdd — 98% identical, 3 lines differ

skills/.curated/tdd/SKILL.md · 124 lines

How it starts

The opening of the file, as written. The whole thing — 124 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Community Test-Driven Development Best Practices

Comprehensive guide to Test-Driven Development practices, designed for AI agents and LLMs. Contains 42 rules across 8 categories, prioritized by impact to guide test writing, refactoring, and code generation.

When to Apply

Reference these guidelines when:

Writing new tests using TDD workflow
Implementing the red-green-refactor cycle
Designing test structure and organization
Creating test data and fixtures
Reviewing or refactoring existing test suites

TDD Workflow

RED: Write a failing test that defines desired behavior
GREEN: Write minimal code to make the test pass
REFACTOR: Clean up code while keeping tests green
Repeat for each new behavior

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Red-Green-Refactor Cycle	CRITICAL	`cycle-`
2	Test Design Principles	CRITICAL	`design-`
3	Test Isolation & Dependencies	HIGH	`isolate-`
4	Test Data Management	HIGH	`data-`
5	Assertions & Verification	MEDIUM	`assert-`
6	Test Organization & Structure	MEDIUM	`org-`
7	Test Performance & Reliability	MEDIUM	`perf-`
8	Test Pyramid & Strategy	LOW	`strat-`

Quick Reference

1. Red-Green-Refactor Cycle (CRITICAL)

cycle-write-test-first - Write the Test Before the Implementation
cycle-minimal-code-to-pass - Write Only Enough Code to Pass the Test
cycle-refactor-after-green - Refactor Immediately After Green
cycle-verify-test-fails-first - Verify the Test Fails Before Writing Code
cycle-small-increments - Take Small Incremental Steps
cycle-maintain-test-list - Maintain a Test List

2. Test Design Principles (CRITICAL)

design-test-behavior-not-implementation - Test Behavior Not Implementation
design-one-assertion-per-test - One Logical Assertion Per Test
design-descriptive-test-names - Use Descriptive Test Names
design-aaa-pattern - Follow the Arrange-Act-Assert Pattern
design-test-edge-cases - Test Edge Cases and Boundaries
design-avoid-logic-in-tests - Avoid Logic in Tests

Read the full file on GitHub · 124 lines

Files

What ships with it

47 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

4d ago First seen · 124 lines · 96 tokens per session scan A f1bd0e195037

Subscribe to this mod's changes

tdd is a skill published in the GitHub repository pproenca/dot-skills (201 stars, last pushed 19d ago), licensed MIT. It adds 96 tokens to every session and 1,358 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

Productivity Testing Writing and docs

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brainstorming

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auto-perf-optimize

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chat-perf

Run chat perf benchmarks and memory leak checks against the local dev build or any published VS Code version. Use when investigating chat rendering regressions, validating perf-sensitive changes to chat UI, or checking for memory leaks in the chat response pipeline.

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chat-pet-sprite-creation

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cpu-profile-analysis

Analyze V8/Chrome CPU profiles (.cpuprofile) and DevTools trace files (Trace-.json). Use when: profiling performance, investigating slow functions, comparing code paths, finding bottlenecks, analyzing timeToRequest, understanding call trees from sampling profiler data, analyzing layout/paint/rendering, investigating…

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