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Official agent skill

TDD

by sanity-io in sanity-io/sanity

Test-driven development with red-green-refactor loop. An agent skill from sanity-io/sanity.

OfficialMITAuto-check passedTesting & QA

Install TDD

skills CLI
$ npx skills add sanity-io/sanity --skill tdd -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install sanity-io/sanity tdd --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/sanity-io/sanity.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/skills/tdd .claude/skills/tdd && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
tdd
GitHub stars
6.4k
Used in
20 other repos
Token cost
~1k tokens
SKILL.md length
460 words
Files
6
Skills in repo
34
Repo updated
First seen
Licence
MIT

At a glance

Test-driven development with red-green-refactor loop. An agent skill from sanity-io/sanity.

  • Works in 4 steps: Planning → Tracer Bullet → Incremental Loop → …
  • User wants to build features
  • SKILL.md covers Philosophy, Anti-Pattern: Horizontal Slices, Workflow and Checklist Per Cycle
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

TDD is an agent skill from sanity-io/sanity, published by the product's own GitHub organization. Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.

Its SKILL.md is about 1k tokens, which your agent loads only when the skill is triggered. The skill folder holds 5 other files (for example `deep-modules.md`, `interface-design.md` and `mocking.md`).

It sits in Testing & QA, covering Test-driven development. The repository describes itself as: Sanity Studio – Rapidly configure content workspaces powered by structured content. The licence is MIT.

When your agent uses it

  • User wants to build features
  • Fix bugs using TDD
  • Mentions red-green-refactor
  • Wants integration tests

Example prompts

  • “red-green-refactor”
  • “/tdd”

Workflow steps

4 steps, taken from the step headings in SKILL.md.

  1. Planning
  2. Tracer Bullet
  3. Incremental Loop
  4. Refactor

What it can do on your machine

Read from SKILL.md and the folder at commit 982525c. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md.

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

TDD loads about 1k tokens when it runs. Until then it costs about 53 tokens; SKILL.md has 460 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~53
When it runs · the whole SKILL.md, loaded when a task matches
~1k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from sanity-io/sanity at commit 982525c, republished under its MIT licence (© sanity-io). 460 words, ~1,047 tokens.

Download SKILL.mdSave it as .claude/skills/tdd/SKILL.md (or your agent's skills folder). This skill also uses 5 other files; get the full folder from GitHub.
name
tdd
description
Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.

Test-Driven Development

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.

See tests.md for examples and mocking.md for mocking guidelines.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code."

This produces crap tests:

  • Tests written in bulk test imagined behavior, not actual behavior
  • You end up testing the shape of things (data structures, function signatures) rather than user-facing behavior
  • Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
  • You outrun your headlights, committing to test structure before understanding the implementation

Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Workflow

Show full SKILL.md (193 more words)Show less
1. Planning

Before writing any code:

  • Confirm with user what interface changes are needed
  • Confirm with user which behaviors to test (prioritize)
  • Identify opportunities for deep modules (small interface, deep implementation)
  • Design interfaces for testability
  • List the behaviors to test (not implementation steps)
  • Get user approval on the plan

Ask: "What should the public interface look like? Which behaviors are most important to test?"

You can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes

This is your tracer bullet - proves the path works end-to-end.

3. Incremental Loop

For each remaining behavior:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

  • One test at a time
  • Only enough code to pass current test
  • Don't anticipate future tests
  • Keep tests focused on observable behavior
4. Refactor

After all tests pass, look for refactor candidates:

  • Extract duplication
  • Deepen modules (move complexity behind simple interfaces)
  • Apply SOLID principles where natural
  • Consider what new code reveals about existing code
  • Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added

© sanity-io, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 5 other files in .agents/skills/tdd of sanity-io/sanity.

  • SKILL.md
  • deep-modules.md
  • interface-design.md
  • mocking.md
  • refactoring.md
  • tests.md

Open the folder on GitHubat commit 982525c

Used in 21 other repositories

We found 23 copies of this SKILL.md (exact, near-identical or edited) in other folders, from 20 other GitHub owners. This page covers the copy in sanity-io/sanity, which our catalogue first saw on October 7, 2026.

Compare with similar skills

TDD next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

TDD compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
TDD this skillsanity-io/sanity6.4k20 repos~1kAutomated safety check: PassMIT
TDDfossasia/eventyay-interpretation1.6k28 repos~1.1kAutomated safety check: PassApache-2.0
TDD WorkflowhellangleZ/burn-in-cceverywhere-ralph11211 repos~2.4kAutomated safety check: PassNone
Test Driven Developmentfarm-fe/farm5.6k49 repos~2.5kAutomated safety check: PassMIT
Tapd Story PipelineTencentBlueKing/bk-bcs840—~2.6kAutomated safety check: PassCustom licence
Absolute Initmaddhruv/absolute2181 repos~3kAutomated safety check: PassMIT

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Categories

Questions about TDD

What does TDD do?

Test-driven development with red-green-refactor loop. An agent skill from sanity-io/sanity. TDD is an agent skill from sanity-io/sanity, published by the product's own GitHub organization. Test-driven development with red-green-refactor loop.

When should I use TDD?

TDD fits situations like: user wants to build features; fix bugs using TDD; mentions red-green-refactor; wants integration tests.

How do I install TDD in Claude Code?

Run `npx skills add sanity-io/sanity --skill tdd -a claude-code`. Or copy the skill folder (.agents/skills/tdd in sanity-io/sanity) into .claude/skills/tdd in your project. Claude Code loads it when a task matches its description.

How do I install TDD in Codex?

Run `npx skills add sanity-io/sanity --skill tdd -a codex`. Or copy the skill folder (.agents/skills/tdd in sanity-io/sanity) into .agents/skills/tdd in your project. Codex loads it when a task matches its description.

Can I use TDD in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add sanity-io/sanity --skill tdd -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/tdd, .gemini/skills/tdd, .github/skills/tdd and .opencode/skills/tdd in your project.

What does TDD need to run?

SKILL.md names no scripts, command-line tools or credentials: TDD is instructions for the agent only.

Does TDD access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is TDD safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does TDD use?

TDD is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does TDD use?

About 1k tokens (SKILL.md is roughly 4.2k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to TDD?

Skills that share tags, products or a category with TDD: TDD (fossasia/eventyay-interpretation, 1.6k stars), TDD Workflow (hellangleZ/burn-in-cceverywhere-ralph, 112 stars), Test Driven Development (farm-fe/farm, 5.6k stars) and Tapd Story Pipeline (TencentBlueKing/bk-bcs, 840 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains TDD?

sanity-io (a GitHub organization, an official publisher) maintains it in sanity-io/sanity, which has 6,352 GitHub stars. The repository holds 34 skills in this directory. The repository was last updated on October 7, 2026.

Source: sanity-io/sanity on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.