test-driven-development

Test-driven development, or TDD, is a way to change code by first writing a test that fails, then making the smallest change that passes it, and finally cleaning up the code. The test checks visible behavior rather than private implementation details.

In plain words
What is it for?
Use it for bug fixes, behavior changes, refactoring, and code boundaries that need proof through unit, integration, property, concurrency, or browser tests.
Why use it?
It proves that a test can detect the bug or missing behavior before the fix is made. It also helps prevent fixes that only make tests pass without correcting the real problem.

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

Made for: Claude Code, Codex.

Per session 46 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 978 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 $0.00046 $0.00978
Opus 5 $0.00023 $0.00489
Sonnet 5 $0.00009 $0.00196
Haiku 4.5 $0.00005 $0.00098

Measured yesterday against content hash 25ea6b49e346, 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 yesterday.

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.

skills/test-driven-development/SKILL.md · 56 lines

How it starts

The opening of the file, as written. The whole thing — 56 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Test-Driven Development

Use a failing test to prove the test can detect the missing or broken behavior, then write the smallest trustworthy change.

Pragmatic Contract

  • Test observable behavior and contracts, not private implementation or mock choreography.
  • Scale the test level to risk. Prefer fast unit or contract tests; add integration, property, concurrency, or browser tests where the boundary demands them.
  • For legacy code, first write a characterization or regression test around the behavior being changed.
  • For a throwaway spike, generated code, or configuration-only change, state why test-first is inappropriate and define another failing validation signal. Do not silently call tests written afterward TDD.
  • Never delete or overwrite user work merely because implementation preceded the test. Preserve it, establish a red-capable test, and disclose the sequence honestly.
  • Never weaken an existing test, loosen a type, add a skip, or mock away the defect to manufacture green.

Cycle

  1. Discover the harness: Read repository instructions, existing tests, task definitions, and nearby patterns. Identify the smallest command that exercises the target behavior.
  2. State the contract: Name the production change that would make the test pass and a plausible regression that would make it fail.
  3. RED: Write one minimal test for one behavior. Run it and confirm it fails for the expected missing behavior, not a syntax, fixture, environment, or setup error.
  4. GREEN: Implement only enough production code to satisfy that test. Run the focused test and read the full output.
  5. Protect the neighborhood: Run the relevant package or subsystem tests. Fix production code when the new behavior breaks an existing valid contract; revisit the spec if contracts conflict.
  6. REFACTOR: Improve names, structure, duplication, and types only while all tests remain green. Do not add new behavior during refactoring.
  7. Repeat: Add the next smallest behavior, edge case, or failure path through a new red cycle.
  8. Prove the regression test: For a bug fix, temporarily reverse or bypass the fix when safe and confirm the regression test fails, then restore the fix and confirm green.
  9. Protect unrelated work: Before any full gate, inspect the full gate's task definition and working-tree state. If it runs whole-tree write-formatters and unrelated or user changes are present, validate the exact candidate in an isolated temporary worktree or run equivalent non-mutating checks; never reformat unrelated work.
  10. Run the full gate: Execute the repository-owned format, check, test, and build contract, normally mise run all, warning-free.

Read the full file on GitHub · 56 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.

  1. yesterday First seen · 56 lines · 46 tokens per session scan A 25ea6b49e346

Subscribe to this mod's changes

test-driven-development is a skill published in the GitHub repository fmind/dotfiles (4 stars, last pushed 2d ago), licensed MIT. It adds 46 tokens to every session and 978 once invoked, about $0.0002 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 2026-08-31.

Related

Other skills, from other repositories

dotfiles-bootstrap

Bootstrap a workstation with the dotfiles framework. Takes a GitHub user / owner+repo / explicit clone URL and runs dot init (which shells out to chezmoi) with the right safety prompts. Honors the active agent profile (ask / plan / apply / audit) so it defaults to dry-run in safer modes and full apply in apply.

sebastienrousseau/dotfiles · 88 tokens

vibe

Delegate a coding task to a cheap AI model (Mistral Vibe by default, but any provider Vibe knows about — DeepSeek, Gemini Flash, etc.) and supervise the result via git diff. Claude orchestrates, the cheap model codes. Claude consumes 500-1500 tokens per delegation regardless of how many file reads the delegate does…

sebastienrousseau/dotfiles · 137 tokens

aiq-research

Use when asked to run deep research or AI-Q research through a reachable NVIDIA AI-Q Blueprint backend.

laurigates/dotfiles · 25 tokens

obsidian-bases

Obsidian Bases database feature for YAML-based interactive note views. Use when creating .base files, writing filter queries, building formulas, configuring table/card views, or working with Obsidian properties and frontmatter databases.

laurigates/dotfiles · 49 tokens

telegram

Send notifications, interactive questions, or multiple-choice polls to the user via Telegram. Use when the user asks to be notified ("ping me", "notify me on Telegram", "ask me when..."), when a long-running task finishes and the user is likely away, when an irreversible action needs out-of-band confirmation, or when…

laurigates/dotfiles · 117 tokens

chezmoi-expert

Comprehensive chezmoi dotfiles management expertise including templates, cross-platform configuration, file naming conventions, and troubleshooting. Covers source directory management, reproducible environment setup, and chezmoi templating with Go templates. Use when user mentions chezmoi, dotfiles, cross-platform…

laurigates/dotfiles · 88 tokens