generate-tests

name	generate-tests
description	Generate test cases and test code for a function, component, endpoint, or feature.
argument-hint	[file, function, or component to test]
user-invocable	true
allowed-tools	Read, Write, Edit, Bash, Glob, Grep

Generate tests for $ARGUMENTS.

TDD Iron Law

No production code without a failing test first. This is not a suggestion. This is the process:

1. RED: Write a test that fails. Run it. Confirm exit code 1. Confirm the failure message is meaningful (not a syntax error or import error)
2. GREEN: Write the minimum code to make the test pass. Run it. Confirm exit code 0
3. REFACTOR: Clean up. Run tests again. Confirm still exit code 0

Vertical Slicing (MANDATORY)

RED-GREEN per feature slice, not all RED then all GREEN:

• CORRECT: test1 -> impl1, test2 -> impl2, test3 -> impl3
• WRONG: test1, test2, test3 ... impl1, impl2, impl3

Each slice must be a complete behaviour. Never leave more than one test failing at a time.

Failure Caps

• GREEN phase: If the same test fails 3 consecutive times after code changes, STOP. The test or the approach is wrong. Do not attempt a 4th fix. Step back, re-read the test, re-read the requirement, check assumptions. Escalate with the 3 failure messages as evidence
• Build/lint loop: If the same linter or type-checker error recurs after 3 fix attempts (same error code, same file), STOP. Report the error and the 3 attempts

Process

Step 1: Reconnaissance (MANDATORY before writing any test)

1. Read the code under test completely — inputs, outputs, side effects, error paths, edge cases
2. Identify the public API surface — what can callers invoke?
3. Find existing tests in the project to match conventions:
   • Test runner (Vitest, Jest, xUnit, pytest, pytest-bdd)
   • File location pattern (co-located vs separate tests/ directory)
   • Naming conventions (describe/it, test classes, BDD scenarios)
   • Assertion library (expect, Shouldly, assert, pytest assertions)
   • Mock strategy (vi.mock, NSubstitute, unittest.mock, pytest fixtures)
   • Factory patterns for test data

Step 2: Test Case Identification

For every function/component/endpoint, identify these categories:

Happy path (MUST have):

• Primary use case with valid inputs producing expected output
• Multiple valid input variations if the function branches on input type

Edge cases (MUST have):

• Empty inputs (null, undefined, empty string, empty array, zero)
• Boundary values (min, max, off-by-one, first, last)
• Single-element collections
• Unicode and special characters where strings are accepted

Error cases (MUST have):

• Invalid inputs that should throw or return errors
• Missing required fields
• Type mismatches (if dynamically typed)
• Network/IO failures (for async operations)
• Timeout and cancellation scenarios

State transitions (if applicable):

• Before/after side effects
• Idempotency — calling twice produces the same result
• Concurrent access (if relevant)

Step 3: Write Tests

Follow this structure for every test:

Arrange — set up preconditions and inputs
Act     — invoke the behaviour under test (ONE action)
Assert  — verify ONE expected outcome

Step 4: Run and Verify

Run tests using the correct runner in run mode (never watch mode):

# TypeScript/Vitest
CI=true npx vitest run path/to/file.test.ts

# TypeScript/Jest
CI=true npx jest path/to/file.test.ts

# .NET/xUnit
dotnet test --filter "FullyQualifiedName~ClassName"

# Python/pytest
pytest tests/path/to/test_file.py -v

After every test run, verify no orphaned processes:

pgrep -f "vitest|jest" || echo "Clean"

Stack-Specific Patterns

TypeScript / Vitest

import { describe, it, expect, vi, beforeEach } from 'vitest';

// Hoist mocks BEFORE imports
const mockDependency = vi.hoisted(() => ({
  doThing: vi.fn(),
}));

vi.mock('./dependency', () => ({ dependency: mockDependency }));

describe('MyFunction', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('returns null when user is not found', async () => {
    // Arrange
    mockDependency.doThing.mockResolvedValue(null);

    // Act
    const result = await myFunction('nonexistent-id');

    // Assert
    expect(result).toBeNull();
  });
});

Rules:

• vi.hoisted() + vi.mock() for module mocks — never inline factory without hoisting
• No jsdom environment — test logic, not DOM
• Co-located test files: my-function.test.ts next to my-function.ts
• Use vi.fn() for function mocks, vi.spyOn() for partial mocks
• beforeEach(() => vi.clearAllMocks()) to prevent state leakage

.NET / xUnit

public class WhenCreatingANewSource
{
    [Fact]
    public async Task it_returns_the_created_source()
    {
        // Arrange
        var command = SourceFactory.CreateCommand();
        var handler = new CreateSourceHandler(
            Substitute.For<IDocumentSession>());

        // Act
        var result = handler.Handle(command);

        // Assert
        result.ShouldNotBeNull();
        result.Name.ShouldBe(command.Name);
    }
}

Rules:

• BDD-style class naming: WhenDoingSomething, GivenSomeState
• NSubstitute for mocks (Substitute.For<T>())
• Shouldly for assertions (ShouldBe, ShouldNotBeNull, ShouldThrow)
• Alba for HTTP integration tests (full pipeline, real routing)
• Factory classes for test data (e.g., SourceFactory.Create())
• Separate unit test project from integration test project

Python / pytest-bdd

# tests/features/user_login.feature
Feature: User login
  Scenario: Successful login with valid credentials
    Given a registered user with email "[e-mail]"
    When the user logs in with correct password
    Then the response contains a valid access token

# tests/step_defs/test_user_login.py
from pytest_bdd import given, when, then, scenarios

scenarios('../features/user_login.feature')

@given('a registered user with email "[e-mail]"')
def registered_user(db_session):
    return UserFactory.create(email="[e-mail]")

@when('the user logs in with correct password')
def login(client, registered_user):
    return client.post('/login', json={...})

@then('the response contains a valid access token')
def verify_token(login):
    assert login.status_code == 200
    assert 'access_token' in login.json()

Rules:

• Gherkin features for behaviour specs, step definitions for implementation
• Business language in .feature files — no infrastructure details
• Reuse step definitions across features via conftest.py
• Hypothesis for property-based testing alongside BDD scenarios
• @pytest.fixture for test data factories and shared setup

Anti-Patterns (NEVER do these)

• Multiple assertions per test — when it fails, you don't know which behaviour broke
• Testing implementation details — verifying mock call counts, internal state, private methods
• Shared mutable state — tests that depend on execution order or modify shared objects
• Inline test data — magic strings and numbers scattered through tests. Use factories
• Mocking what you own — mock at external boundaries only (HTTP, database, file system). Use real implementations for your own code
• Sleep in tests — use polling with timeout, fake timers, or async await. sleep(1000) is a race condition
• Testing the framework — don't test that React renders or that Express routes. Test your logic
• Snapshot overuse — snapshots are for catching unintended changes, not for replacing real assertions
• Ignoring flaky tests — a flaky test is a bug. Fix it or delete it. Never skip and forget

Evidence Requirements

Every test generation output MUST include:

### Evidence

| Test | Command | Exit | Result |
|---|---|---|---|
| [test name] | [exact command] | [0/1] | [PASS/FAIL: message] |

### Coverage Summary

- Happy path scenarios: [count] tested
- Edge cases: [count] tested
- Error cases: [count] tested
- Total tests: [count] passing, [count] failing, [count] skipped

"Tests pass" without an exit code is not evidence. Every claim requires the exact command and its exit code.

Output

Deliver:

1. Test file(s) following project conventions
2. Any required test fixtures or factories
3. Evidence table with commands and exit codes
4. List of any test cases intentionally deferred (with justification)

Related Skills

• /qa-engineer:write-bug-report — when tests reveal defects, write a structured bug report.
• /qa-lead:test-strategy — for defining the overall test strategy before generating individual tests.