tdd

name	tdd
description	Implements features using strict test-driven development — red-green-refactor cycle. Writes a failing test first, then minimal code to pass, then refactors. Use when building features that need high reliability.
argument-hint	[feature or function to implement]
allowed-tools	Read, Grep, Glob, Bash, Write, Edit

Test-Driven Development

Detect Test Environment

!ls package.json pytest.ini pyproject.toml go.mod Cargo.toml *.csproj Gemfile mix.exs 2>/dev/null !find . -type f \( -name "*.test.*" -o -name "*.spec.*" -o -name "test_*" -o -name "*_test.*" \) -not -path "*/node_modules/*" -not -path "*/.git/*" 2>/dev/null | head -5

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The TDD Cycle

For the requested feature, repeat this cycle until complete:

RED — Write a Failing Test

1. Write the smallest possible test that describes the next piece of behavior
2. The test must import/reference the function or module (even if it doesn't exist yet)
3. Run the test — it MUST fail
4. Verify the failure message is meaningful (not just a compilation error)

GREEN — Make It Pass

1. Write the absolute minimum code to make the test pass
2. Hardcoding is acceptable at this stage — it will be cleaned up in refactor
3. Don't add functionality the test doesn't require
4. Run the test — it MUST pass
5. Run all related tests — nothing else should break

REFACTOR — Clean Up

1. Remove duplication in both production and test code
2. Improve naming, extract methods, simplify logic
3. Run tests after every refactoring step — they must stay green
4. Stop when the code is clean

Test Progression

Start simple, add complexity gradually:

1. Null/empty/zero case
2. Single item / simplest valid input
3. Multiple items / typical case
4. Edge cases / boundary values
5. Error cases / invalid input
6. Async / concurrent behavior (if applicable)

Output

• Show each cycle clearly: RED (test code + failure), GREEN (impl + pass), REFACTOR
• Display test runner output at each step
• Final summary: all tests, coverage of the feature

Rules

• NEVER write implementation before a failing test
• NEVER skip the refactor step
• One behavior per cycle — don't batch multiple features
• If the test framework isn't set up, set it up first