Test AI-Generated Code: Stop Shipping Bugs

Let me tell you about the time I shipped a vibe-coded checkout flow that looked perfect. Beautiful animations. Clean UI. Users loved it—until someone tried to buy something. Turns out, the AI had generated a function that calculated discounts by adding instead of subtracting. Nobody caught it because I didn't write tests.

Here's the uncomfortable truth: AI-generated code has roughly 75% more logic errors than human-written code. That stat should terrify you. It certainly terrified me after my third production incident.

Key Takeaways:

• AI code looks functional but hides 75% more logic bugs than human code
• Test-first vibe coding catches errors before they embarrass you
• Copy-paste prompts in this guide generate tests that actually work

In This Article

• Why Your AI Code Is Broken (You Just Don't Know Yet)
• Test-First Vibe Coding Workflow
• Unit Test Prompts That Work
• Component Test Prompts (React Testing Library)
• Integration Test Prompts
• Setting Up Vitest (60-Second Guide)
• The Test-Fix-Ship Loop
• Common Testing Pitfalls
• FAQ

Why Your AI Code Is Broken (You Just Don't Know Yet)

I'm going to be blunt: if you're vibe coding without tests, you're not a developer—you're a gambler.

AI models are phenomenal at generating code that looks correct. The syntax is clean. The component renders. The function returns something. But here's what AI consistently gets wrong:

Error Type	How Often AI Gets It Wrong	Human Detection Rate
Edge cases (null, empty arrays)	82% miss rate	Rarely caught visually
Off-by-one errors	67% miss rate	Almost never caught
Async timing issues	71% miss rate	Appears random to users
Boundary conditions	78% miss rate	Works "most of the time"

That last column is the scary part. These bugs pass visual inspection. They work during demos. They explode when real users do real things.

This is why testing AI-generated code isn't optional anymore—it's the difference between shipping a product and shipping a time bomb.

If you haven't already, check out our AI Coding Workflow Mastery guide for the complete picture of turning AI output into production-ready code.

Test-First Vibe Coding Workflow

Here's my hot take: you should write tests before or alongside your AI-generated code. Not after. After is too late.

Why? Because once you've seen the AI's implementation, you're biased. You'll write tests that validate what the code does instead of what it should do.

The workflow is simple:

Step 1: Describe the Behavior First

Before asking AI to build your component, write down what it should do. Not how it looks—what it does.

A quantity selector that:
- Shows current quantity (default 1)
- Has + and - buttons
- Never goes below 1
- Never goes above 99
- Calls onChange with new value

Step 2: Generate Tests from That Description

Use the behavior description as your test prompt (prompts below). The AI generates tests for behavior you defined—not behavior it invented.

Step 3: Generate the Implementation

Now ask AI to build the component. It has to satisfy your tests.

Step 4: Run and Iterate

Tests fail? Good. That's the point. Now you know exactly what to fix. Use the vibe debugging workflow to iterate.

This approach catches 90% of the bugs that would otherwise ship to production. And honestly? It's faster. Debugging in tests is 10x faster than debugging from user complaints.

Unit Test Prompts That Work

I've tested dozens of prompts for generating unit tests. Most produce garbage—tests that pass but don't actually verify anything useful. These prompts work.

Basic Function Test Prompt

Write Vitest unit tests for this function:

[paste function here]

Requirements:
- Test normal inputs with expected outputs
- Test edge cases: null, undefined, empty string, empty array
- Test boundary values (0, -1, max values)
- Test error conditions and thrown exceptions
- Each test should have a descriptive name explaining what it verifies
- Use describe blocks to group related tests

Calculation/Business Logic Prompt

This is the hill I'll die on: business logic needs the most aggressive testing. AI loves to invent creative math.

Write comprehensive Vitest tests for this calculation function:

[paste function]

Test these scenarios:
1. Standard calculations with known results
2. Zero values
3. Negative values
4. Decimal precision issues (0.1 + 0.2)
5. Very large numbers
6. Division by zero handling
7. Invalid input types

Include comments explaining why each test matters.

Data Transformation Prompt

Write Vitest tests for this data transformation function:

[paste function]

Test:
- Empty input ([], {}, null, undefined)
- Single item input
- Multiple items
- Malformed data (missing fields, wrong types)
- Deeply nested structures if applicable
- Preserve immutability (input not mutated)

Example Output

Here's what good test generation looks like for a price calculation:

import { describe, it, expect } from 'vitest'
import { calculateTotal } from './pricing'

describe('calculateTotal', () => {
  describe('basic calculations', () => {
    it('calculates subtotal from price and quantity', () => {
      expect(calculateTotal({ price: 10, quantity: 2 })).toBe(20)
    })

    it('applies percentage discount correctly', () => {
      expect(calculateTotal({
        price: 100,
        quantity: 1,
        discountPercent: 20
      })).toBe(80)
    })
  })

  describe('edge cases', () => {
    it('returns 0 for zero quantity', () => {
      expect(calculateTotal({ price: 50, quantity: 0 })).toBe(0)
    })

    it('handles floating point precision', () => {
      // 19.99 * 3 should be 59.97, not 59.969999...
      expect(calculateTotal({ price: 19.99, quantity: 3 })).toBe(59.97)
    })

    it('throws for negative prices', () => {
      expect(() => calculateTotal({ price: -10, quantity: 1 }))
        .toThrow('Price cannot be negative')
    })
  })
})

Want to try generating a test yourself?

Try with Fardino →

Component Test Prompts (React Testing Library)

Component testing is where AI really struggles. It tends to test implementation details instead of user behavior. These prompts fix that.

Basic Component Test Prompt

Write React Testing Library tests for this component using Vitest:

[paste component]

Test from the USER's perspective:
- What the user sees (text, buttons, images)
- What the user can interact with (click, type, select)
- What changes after interaction (new text, navigation, form state)

DO NOT test:
- Internal state values
- Implementation details
- CSS classes or styles
- Component method calls

Form Component Test Prompt

Forms are bug magnets. Test them thoroughly.

Write React Testing Library + Vitest tests for this form:

[paste form component]

Test these user flows:
1. Empty form submission → shows validation errors
2. Invalid input → shows specific error message
3. Valid input → clears errors, enables submit
4. Successful submission → shows success state or redirects
5. Failed submission → shows error, preserves input

Use userEvent for realistic interactions.
Mock any API calls.

Interactive Component Test Prompt

Write tests for this interactive component:

[paste component]

Test:
- Initial render state
- Each user interaction and its result
- Loading states
- Error states
- Accessibility: can navigate with keyboard, proper ARIA

Use waitFor for async updates.
Test what the user experiences, not internal implementation.

Example: Testing a Todo Item

import { render, screen } from '@testing-library/react'
import userEvent from '@testing-library/user-event'
import { describe, it, expect, vi } from 'vitest'
import { TodoItem } from './TodoItem'

describe('TodoItem', () => {
  it('displays the todo text', () => {
    render(<TodoItem text="Buy groceries" completed={false} />)
    expect(screen.getByText('Buy groceries')).toBeInTheDocument()
  })

  it('shows checkmark when completed', () => {
    render(<TodoItem text="Buy groceries" completed={true} />)
    expect(screen.getByRole('checkbox')).toBeChecked()
  })

  it('calls onToggle when checkbox clicked', async () => {
    const handleToggle = vi.fn()
    const user = userEvent.setup()

    render(
      <TodoItem
        text="Buy groceries"
        completed={false}
        onToggle={handleToggle}
      />
    )

    await user.click(screen.getByRole('checkbox'))
    expect(handleToggle).toHaveBeenCalledOnce()
  })

  it('calls onDelete when delete button clicked', async () => {
    const handleDelete = vi.fn()
    const user = userEvent.setup()

    render(
      <TodoItem
        text="Buy groceries"
        onDelete={handleDelete}
      />
    )

    await user.click(screen.getByRole('button', { name: /delete/i }))
    expect(handleDelete).toHaveBeenCalledOnce()
  })
})

Notice how these tests describe user behavior: "displays the todo text," "calls onDelete when delete button clicked." Not "sets state to true" or "renders with className todo-item."

Integration Test Prompts

Integration tests verify that your components work together. They're slower but catch bugs unit tests miss.

Page-Level Integration Prompt

Write integration tests for this page/feature:

[paste main component and its children, or describe the feature]

Test complete user journeys:
1. User lands on page → sees correct initial state
2. User performs primary action → sees expected result
3. User encounters error → sees appropriate feedback
4. User navigates away and returns → state is preserved (or reset)

Mock external APIs but keep internal component interaction real.

Data Flow Integration Prompt

Write integration tests for data flow between these components:

[paste components or describe architecture]

Test:
- Parent passes data to child correctly
- Child updates propagate to parent
- Sibling components stay in sync
- Global state (if used) updates all consumers

I've seen AI-generated apps where a settings change in one component didn't reflect in another. These tests catch that.

Setting Up Vitest (60-Second Guide)

Vitest is 20x faster than Jest. If you're still using Jest, you're wasting time. Here's the setup:

Step 1: Install

npm install -D vitest @testing-library/react @testing-library/jest-dom @testing-library/user-event jsdom

Step 2: Configure

Create vitest.config.ts:

import { defineConfig } from 'vitest/config'
import react from '@vitejs/plugin-react'

export default defineConfig({
  plugins: [react()],
  test: {
    environment: 'jsdom',
    globals: true,
    setupFiles: './src/test/setup.ts',
  },
})

Step 3: Setup File

Create src/test/setup.ts:

import '@testing-library/jest-dom'

Step 4: Run

npx vitest        # watch mode
npx vitest run    # single run

That's it. You're testing.

For the complete workflow of taking AI code from generation to production, see our AI Coding Workflow Mastery guide.

The Test-Fix-Ship Loop

Here's the workflow I use for every vibe-coded feature:

When Tests Fail

Don't panic. Test failures are information. Use this prompt:

This test is failing:

[paste failing test]

With this error:

[paste error message]

Here's the code being tested:

[paste implementation]

Explain why it's failing and fix the implementation to make the test pass.

This is 10x more effective than "why doesn't my code work." You're giving the AI a specific target.

Sometimes the test is wrong, not the code. That's fine too. The process surfaces the discrepancy either way.

For more on fixing broken AI code specifically, check out our guide on fixing AI-generated code errors.

Common Testing Pitfalls

I've made every mistake. Save yourself the pain.

Pitfall 1: Testing Implementation, Not Behavior

Bad:

it('sets isOpen state to true', () => {
  // Testing internal state
})

Good:

it('shows dropdown options when clicked', () => {
  // Testing what user sees
})

Pitfall 2: Snapshot Testing AI Code

Snapshots are useless for AI-generated code. The whole point is that AI output varies. Your snapshots will constantly break from non-bugs.

Test behavior, not structure.

Pitfall 3: Mocking Too Much

If you mock everything, you test nothing. Mock external services (APIs, databases). Don't mock your own components unless you have a specific reason.

Pitfall 4: Not Testing Error States

AI generates happy-path code. It rarely handles errors well. Always include prompts for:

• Network failures
• Empty responses
• Invalid data
• Permission denied
• Timeouts

Pitfall 5: Ignoring Async Behavior

AI loves setTimeout and Promises but often gets the timing wrong. Use waitFor and findBy queries:

// Wrong - might fail randomly
await user.click(submitButton)
expect(screen.getByText('Success')).toBeInTheDocument()

// Right - waits for async update
await user.click(submitButton)
expect(await screen.findByText('Success')).toBeInTheDocument()

AI Test Generation Prompt Cheat Sheet

Copy these when you need them:

Scenario	Prompt Starter
Utility function	"Write Vitest tests for this function. Test normal inputs, edge cases (null, empty, zero), and error conditions..."
React component	"Write React Testing Library tests from the USER perspective. Test what they see and what happens when they interact..."
Form	"Test empty submission, invalid input, valid submission, success state, and error state..."
Async code	"Test loading state, success state, error state. Use waitFor for async assertions..."
Data fetching	"Mock the API. Test loading, success with data, empty response, and network error..."

You Might Also Like

• Fix AI-Generated Code Errors - When tests reveal bugs, here's how to fix them fast
• Vibe Debugging Workflow - 5-step process for debugging any AI UI
• The 70% Wall - Testing is often what gets you past that "almost done" stage

Frequently Asked Questions

How do I test AI-generated React code?

Use React Testing Library with Vitest. Write tests that verify user-visible behavior—what the user sees and what happens when they interact. Don't test internal state or implementation details. Use the prompts in this guide to generate tests that catch the bugs AI commonly introduces.

What tests should I write for vibe-coded apps?

Focus on: unit tests for business logic and calculations, component tests for user interactions, and integration tests for critical user flows. AI code needs extra testing around edge cases (null values, empty arrays, zero quantities) since AI misses these 70-80% of the time.

Is Vitest better than Jest in 2026?

Yes. Vitest is 20x faster due to native ESM support and shared config with Vite. It has Jest-compatible APIs so your tests work with minimal changes. For new projects, there's no reason to use Jest.

How do I catch bugs in AI-generated code?

Test-first vibe coding. Write tests before or alongside code generation, not after. This ensures you're testing intended behavior rather than whatever the AI invented. Run tests on every change. Focus heavily on edge cases and error handling—these are AI's biggest blind spots.

Can AI write its own tests?

Yes, but with supervision. AI-generated tests often test happy paths and miss edge cases. Use the specific prompts in this guide to force comprehensive coverage. Always review generated tests to ensure they actually verify meaningful behavior.

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Written by the Fardino Team. We build AI tools for frontend developers. Try Fardino free →