test-driven-development

A guide to test-driven development, or TDD, where a test is written before the code that makes it pass. Work proceeds through small cycles of failing test, working code, and cleanup.

In plain words
What is it for?
Use it when implementing changes test-first, especially when you need focused tests, small implementation steps, and repeated checks of the relevant test suite.
Why use it?
It makes each change tied to a specific expected behaviour and helps reveal whether a test fails for the intended reason.

Skill for Claude CodeCodex

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/intent-driven-dev/intent-driven-template/test-driven-development
Any agent
npx skills add intent-driven-dev/intent-driven-template --skill test-driven-development
Clone the repo
git clone --depth 1 https://github.com/intent-driven-dev/intent-driven-template

Made for: Claude Code, Codex.

Per session 26 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 333 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.

ModelPer sessionOnce invoked
Fable 5 $0.00026 $0.00333
Opus 5 $0.00013 $0.00167
Sonnet 5 $0.00005 $0.00067
Haiku 4.5 $0.00003 $0.00033

Measured 2d ago against content hash 8687c3dcc1d0, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

test-driven-development 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 2d 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.

.agents/skills/test-driven-development/SKILL.md · 41 lines

What it actually says

Test-Driven Development

Use the repository's existing test tooling and conventions.

One test, one behaviour, one reason to fail. A test name describes that one behaviour; never use joiners such as "and", "or", or "etc." to combine independent outcomes. Multiple assertions are acceptable only when together they establish that same behaviour.

Red

  • Select one observable behaviour from the requirement.
  • Write one test before changing implementation code.
  • Run the test and confirm it fails because the behaviour is missing, not because the test is broken.

Green

  • Write only enough implementation code to pass the failing test.
  • Run the test, then run the relevant suite.
  • Do not add behaviour without first seeing its test fail.

Refactor

  • Refactor only while the relevant suite is green, after completing a red-green slice.
  • Refactor either the tests or the implementation in one pass, not both.
  • Reuse shared setup, teardown, helpers, and parameterized structure when they clarify intent without combining behaviours.
  • Rerun the relevant suite after each refactor pass.

Patterns and anti-patterns

Read patterns and anti-patterns when choosing a test shape, writing examples, or deciding whether cases belong together.

Collaborators

Read collaborators before substituting a dependency. Mock system boundaries, not code owned by the application.

Files

What ships with it

2 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.

  1. 2d ago First seen · 41 lines · 26 tokens per session scan A 8687c3dcc1d0

Subscribe to this mod's changes

test-driven-development is a skill published in the GitHub repository intent-driven-dev/intent-driven-template (121 stars, last pushed 9d ago), licensed MIT. It adds 26 tokens to every session and 333 once invoked, about $0.0001 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.