Test-Driven Development

TDD with AI: Writing Specs That Generate Tests

AI tools excel at generating tests from precise specifications — combine human spec-writing with AI test generation.

AI + TDD: The Powerful Combination

AI coding tools are excellent at generating test code — when given the right specifications. The combination of human-written specifications and AI-generated tests is more powerful than either alone.

Without specs: AI generates tests that cover the happy path but miss edge cases, error conditions, and security-relevant scenarios.

With specs: AI generates comprehensive tests because the specifications tell it exactly what behavior to verify.

How to Prompt for Test Generation

Effective prompts include:

The function signature or component interface
The expected behavior for each scenario
Edge cases and error conditions
The testing framework to use

The Spec-to-Test Pipeline

Write requirement in clear format
Translate to test case descriptions
AI generates test code from descriptions
Human reviews: Are assertions meaningful? Are edge cases covered?
Implement code to pass the tests

Reviewing AI-Generated Tests

AI tests need human review. A checklist:

Does it test behavior, not implementation?
Are assertions meaningful (not just "doesn't throw")?
Are error paths tested, not just happy paths?
Are edge cases covered (empty input, boundary values, null)?
If a bug existed, would this test catch it?

What AI Misses (Add These Manually)

Security edge cases: script injection, SQL injection
Race conditions
Business-specific domain rules
Authorization failures
Error propagation when dependencies fail

Key Takeaways

AI generates tests best when given precise specifications — vague prompts produce vague tests
The pipeline: write requirement → define test cases → AI generates test code → human reviews
Review checklist: behavior not implementation, meaningful assertions, error paths, edge cases
AI generates excellent happy-path tests; humans must add edge cases, security checks, and domain-specific scenarios

Example

typescript

// Effective prompt → quality AI-generated tests
// Prompt: "Write Vitest tests for validateEmail(email: string): boolean
// Rules: must have @, domain must have dot, no spaces, max 254 chars"

describe('validateEmail', () => {
  it('accepts valid email addresses', () => {
    expect(validateEmail('user@example.com')).toBe(true);
    expect(validateEmail('user.name+tag@sub.domain.com')).toBe(true);
  });

  it('rejects emails without @', () => {
    expect(validateEmail('userexample.com')).toBe(false);
  });

  it('rejects emails with domain missing dot', () => {
    expect(validateEmail('user@example')).toBe(false);
  });

  it('rejects emails with spaces', () => {
    expect(validateEmail('user @example.com')).toBe(false);
    expect(validateEmail('user@ example.com')).toBe(false);
  });

  it('rejects emails exceeding 254 characters', () => {
    const longEmail = 'a'.repeat(250) + '@b.com';
    expect(validateEmail(longEmail)).toBe(false);
  });

  it('rejects empty string and null-like inputs', () => {
    expect(validateEmail('')).toBe(false);
  });
});

Example — TYPESCRIPT

// Effective prompt → quality AI-generated tests
// Prompt: "Write Vitest tests for validateEmail(email: string): boolean
// Rules: must have @, domain must have dot, no spaces, max 254 chars"

describe('validateEmail', () => {
  it('accepts valid email addresses', () => {
    expect(validateEmail('user@example.com')).toBe(true);
    expect(validateEmail('user.name+tag@sub.domain.com')).toBe(true);
  });

  it('rejects emails without @', () => {
    expect(validateEmail('userexample.com')).toBe(false);
  });

  it('rejects emails with domain missing dot', () => {
    expect(validateEmail('user@example')).toBe(false);
  });

  it('rejects emails with spaces', () => {
    expect(validateEmail('user @example.com')).toBe(false);
    expect(validateEmail('user@ example.com')).toBe(false);
  });

  it('rejects emails exceeding 254 characters', () => {
    const longEmail = 'a'.repeat(250) + '@b.com';
    expect(validateEmail(longEmail)).toBe(false);
  });

  it('rejects empty string and null-like inputs', () => {
    expect(validateEmail('')).toBe(false);
  });
});