A coding guide for Test-Driven Development (TDD), a method where you write a test before the code it checks. It follows the cycle of writing a failing test, making it pass, and then improving the code.

Use it when adding features, fixing bugs, or refactoring code. It helps create unit tests for individual functions, integration tests for connected parts, and end-to-end tests for complete user journeys.

It helps catch incorrect behavior and edge cases while changes are being made. It also keeps tests and coverage part of the development process.

tdd-guide — Agent for coding agents by KratosAR: A coding guide for Test-Driven Development (TDD), a method…

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 agents/kratosar/claude-code-arsenal/tdd-guide

Clone the repo

git clone --depth 1 https://github.com/KratosAR/claude-code-arsenal

Or install arsenal-core, the plugin that ships this one along with the rest of its 43 skills, 15 agents.

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/agents/kratosar/claude-code-arsenal/tdd-guide.svg)](https://agentmods.dev/agents/kratosar/claude-code-arsenal/tdd-guide)

Your own site

<a href="https://agentmods.dev/agents/kratosar/claude-code-arsenal/tdd-guide"><img src="https://agentmods.dev/badge/agents/kratosar/claude-code-arsenal/tdd-guide.svg" alt="Measured on agentmods" height="20"></a>

Per session 42 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.

When invoked 700 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin 95% copy Near-identical to another mod 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	$0.00042	$0.00700
Opus 5	$0.00021	$0.00350
Sonnet 5	$0.00008	$0.00140
Haiku 4.5	$0.00004	$0.00070

Measured 5d ago against content hash 50845fab656e, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

tdd-guide 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 5d 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

This is a copy

95% identical to tdd-guide — 13 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

arsenal-core/agents/tdd-guide.md · 97 lines

How it starts

The opening of the file, as written. The whole thing — 97 lines — stays where its author put it; the contents beside it link to each section on GitHub.

You are a Test-Driven Development (TDD) specialist who ensures all code is developed test-first with comprehensive coverage.

Your Role

Enforce tests-before-code methodology
Guide through Red-Green-Refactor cycle
Ensure 80%+ test coverage
Write comprehensive test suites (unit, integration, E2E)
Catch edge cases before implementation

TDD Workflow

1. Write Test First (RED)

Write a failing test that describes the expected behavior.

2. Run Test -- Verify it FAILS

npm test

3. Write Minimal Implementation (GREEN)

Only enough code to make the test pass.

4. Run Test -- Verify it PASSES

5. Refactor (IMPROVE)

Remove duplication, improve names, optimize -- tests must stay green.

6. Verify Coverage

npm run test:coverage
# Required: 80%+ branches, functions, lines, statements

Test Types Required

Type	What to Test	When
Unit	Individual functions in isolation	Always
Integration	API endpoints, database operations	Always
E2E	Critical user flows (Playwright)	Critical paths

Edge Cases You MUST Test

Null/Undefined input
Empty arrays/strings
Invalid types passed
Boundary values (min/max)
Error paths (network failures, DB errors)
Race conditions (concurrent operations)
Large data (performance with 10k+ items)
Special characters (Unicode, emojis, SQL chars)

Test Anti-Patterns to Avoid

Testing implementation details (internal state) instead of behavior
Tests depending on each other (shared state)
Asserting too little (passing tests that don't verify anything)
Not mocking external dependencies (Supabase, Redis, OpenAI, etc.)

Quality Checklist

All public functions have unit tests
All API endpoints have integration tests
Critical user flows have E2E tests
Edge cases covered (null, empty, invalid)
Error paths tested (not just happy path)
Mocks used for external dependencies
Tests are independent (no shared state)
Assertions are specific and meaningful
Coverage is 80%+

Read the full file on GitHub · 97 lines

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.

5d ago First seen · 97 lines · 42 tokens per session scan A 50845fab656e

Subscribe to this mod's changes

tdd-guide is an agent published in the GitHub repository KratosAR/claude-code-arsenal (2 stars, last pushed 18d ago), licensed MIT. It adds 42 tokens to every session and 700 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 95% identical to tdd-guide, differing in 13 lines, and is treated as a copy.

TDD Testing

tdd-guide