TDD Workflow

When to Use

• Building new features where correctness is critical
• Fixing bugs where a regression test should be written first
• Refactoring code that lacks test coverage
• Working on business logic, parsers, or data transformations

Prerequisites

• Test framework installed and configured (Jest, pytest, Go test, etc.)
• Ability to run tests from the command line
• Understanding of the feature requirements or bug to reproduce

Language	Common frameworks	Example commands
JavaScript / TypeScript	Jest, Vitest	npm test, npx vitest
Python	pytest	pytest
Go	go test	go test ./...
Ruby	RSpec, Minitest	bundle exec rspec, ruby -Itest test/
PHP	PHPUnit	./vendor/bin/phpunit
Rust	cargo test	cargo test

tdd-guard users: v1.7.0 improves SDK validation error handling, reduces response tokens for allow-operation, and refactors hook-payload parsing. After bundle add tdd-guard, re-check its framework detection before relying on automatic enforcement.

Workflow

0. Choose the Thinnest Slice First

Before you write or change anything, shrink the task to the smallest meaningful behavior that can go through one Red → Green → Refactor loop.

Good first slices usually have all three properties:

• one visible behavior change
• one main assertion path
• one reason it could fail

If the first test needs multiple branches, multiple modules, or a large fixture to make sense, shrink it again. After each Green phase, pause long enough to confirm the direction before stacking more behavior on top.

1. Pre-Check — Does a Test Already Exist?

Before writing a new test, verify you are not duplicating existing coverage.

glob pattern="**/*.{test,spec}.{js,jsx,ts,tsx}"
glob pattern="**/{test_*,*_test}.py"
glob pattern="**/*_test.go"
rg pattern="<function-or-behavior-name>" path="."

If an equivalent test already exists, extend or correct it instead of creating a duplicate. If not, proceed to write the failing test.

2. Red — Write a Failing Test

Identify the behavior to implement, then write a test that asserts it.

# Find existing test files to follow conventions

glob pattern="**/*.test.{ts,js,tsx}" # or **/*_test.go, **/test_*.py

Write a focused test using create or edit:

// example: src/utils/parse.test.ts
describe('parseConfig', () => {
  it('should return default values when input is empty', () => {
    expect(parseConfig({})).toEqual({ timeout: 30, retries: 3 });
  });
});

Run the test and confirm it fails:

npm test -- --testPathPattern="parse.test" 2>&1 | Select-Object -Last 20

Every test must name the break it catches: its name or a focused comment should answer which production regression makes it fail. Every test must also exercise the real behavior; a test that only verifies its mock can stay green while the implementation is broken.

Avoid change-detector tests that can fail only when an intentional constant, exact message, or private structure changes; they fire on redesign and sleep through behavioral bugs. Likewise, asserting that a script, skill, or config contains an exact string proves only that the source contains that string. Exercise the consuming behavior, output, side effect, or exit code instead.

Before finishing, perform a mutation check: deliberately make a safe, temporary break in the implementation and confirm that the relevant test fails, then restore it. If no test detects the mutation, the behavior is unprotected or the test is tautological.

3. Green — Write Minimal Code to Pass

Implement only enough code to make the failing test pass. Resist the urge to add extras.

// src/utils/parse.ts
export function parseConfig(input: Record<string, unknown>) {
  return { timeout: input.timeout ?? 30, retries: input.retries ?? 3 };
}

Run the test again and confirm it passes:

npm test -- --testPathPattern="parse.test"

4. Refactor — Clean Up Without Changing Behavior

Improve code structure, naming, and duplication while keeping all tests green.

Exception — type-only edits: Adding type annotations, cleaning up imports, or renaming for clarity without changing behavior does not require a prior failing test. Restrict this exception to structural changes only; any logic change must still go through Red → Green first.

# Run full test suite to ensure nothing else broke
npm test 2>&1 | Select-Object -Last 30

5. Repeat the Cycle

Add the next test case for edge cases or the next slice of behavior:

• Null/undefined inputs
• Boundary values
• Error conditions
• Integration with adjacent modules

Pick the smallest next slice that adds new information. If the product direction, API shape, or UX is still uncertain, stop after a thin vertical slice and get feedback before widening the change.

6. Check Coverage

npm test -- --coverage --collectCoverageFrom="src/utils/parse.ts"

Aim for ≥80% line coverage on new code, 100% branch coverage on critical paths.

Examples

Bug Fix TDD

# 1. Reproduce the bug as a test
#    grep for the failing function, write a test with the exact input that causes the bug
grep -n "calculateTotal" src/billing/*.ts

# 2. Run and see it fail
npm test -- --testPathPattern="billing"

# 3. Fix the code
# 4. Run and see it pass
# 5. Add edge-case tests around the fix

Multi-file Feature

# Use the task agent to run tests continuously while you code
# Start test watcher in async mode
npx jest --watch --testPathPattern="feature" # mode: async

Common Rationalizations

Rationalization	Reality
"This code is too simple to need tests"	Simple code still has edge cases. A 5-line function can have 3.
"I'll add tests later"	Later never comes. Untested code becomes legacy code overnight.
"It works, I can see it"	Manual verification is not reproducible. CI will catch what you missed.
"Copilot wrote it, so it must be right"	AI-generated code requires the same verification standards as human-written code.
"Tests still pass after refactoring"	Tests must exist before refactoring — otherwise they provide no safety net.

TDD Guardrail Enforcement

Prevent implementation code from being committed without a corresponding failing test. Add this pre-commit check to your workflow:

# Verify that implementation files have corresponding test files staged
$stagedSrc = git diff --cached --name-only | Where-Object { $_ -match "^src/" -and $_ -notmatch "\.test\." }
$stagedTests = git diff --cached --name-only | Where-Object { $_ -match "\.test\." }

if ($stagedSrc -and -not $stagedTests) {
    Write-Error "TDD violation: source files staged without corresponding test files."
    Write-Error "Write a failing test first, then implement."
    exit 1
}

To enforce this automatically, add it as a Git pre-commit hook:

# .git/hooks/pre-commit (chmod +x on Unix)
# See rules/common/testing.md for the full hook content

Red Flags

• Tests written after implementation with 100% pass rate (tests never saw a red state)
• Test files committed after source files
• Tests like it('works') with no meaningful assertions
• Tests that call internal implementations directly (testing private methods instead of public API)
• Tests written only to meet coverage numbers with no real assertions
• AI-generated implementation code committed before a failing test was written

Verification

• Confirmed tests started in a Red state (via commit history or direct observation)
• npm test (or equivalent) exits with code 0
• Line coverage ≥80% and branch coverage ≥90% on critical paths for new code
• Each test runs independently (no ordering dependencies)
• Tests exist for edge cases (null, empty values, boundary values)

Tips

• Write the assertion first, then work backward to the setup
• Start with the smallest meaningful behavior, not the full feature
• Each test should verify one behavior — keep tests small and descriptive
• Name tests as sentences: it('rejects negative quantities')
• Use task agent to run tests so output stays clean in your main context
• If you can't write a test easily, the code may need a better interface — that's valuable feedback
• Commit after each Green phase so you can safely revert a failed refactor
• Claude Code users can also study tdd-guard for hook-based TDD enforcement