Prompt Engineering for Debugging

Debugging is one of the most valuable use cases for AI assistance. The right prompts can transform AI into a powerful debugging partner that helps you find and fix issues faster.

The Debugging Mindset

When debugging with AI, think of it as consulting an expert who:

• Wasn't there when you wrote the code
• Doesn't know your local environment
• Needs explicit context about what you've tried

The more context you provide, the better the diagnosis.

Essential Debugging Information

Always include these elements in debugging prompts:

Basic Debugging Prompt Template

I'm encountering an error that I need help debugging.

**Error Message:**
[paste exact error message and stack trace]

**Relevant Code:**
```[language]
[paste the code where error occurs]

Expected Behavior: [what should happen]

Actual Behavior: [what actually happens]

Environment:

• Node.js: v20.10.0
• Framework: Next.js 14
• OS: macOS 14.2

What I've Tried:

• [attempt 1]
• [attempt 2]

Please help me:

1. Understand why this error occurs
2. Find the root cause
3. Suggest a fix

## Error Type-Specific Prompts

### Runtime Errors

Debug this runtime error:

Error: TypeError: Cannot read properties of undefined (reading 'map') at ProductList (ProductList.tsx:23:18) at renderWithHooks (react-dom.development.js:14985:18)

Code:

const ProductList = ({ products }) => {
  return (
    <div>
      {products.map(product => (
        <ProductCard key={product.id} product={product} />
      ))}
    </div>
  );
};

Context:

• Products are fetched from API in parent component
• API sometimes returns null instead of empty array
• Error happens intermittently

Analyze:

1. Why does this error occur?
2. What are all possible causes?
3. What's the most robust fix?

### Async/Timing Errors

I have a race condition or timing issue.

Symptom: Data sometimes shows stale values after update.

Code:

const updateUser = async (userId: string, data: UpdateData) => {
  await api.updateUser(userId, data);
  const updatedUser = await api.getUser(userId);
  setUser(updatedUser);
};

Environment:

• React 18 with concurrent features enabled
• Using React Query for data fetching

Questions:

1. Is this actually a race condition?
2. What could cause stale data?
3. How should I restructure this?

### Build/Compilation Errors

Getting a TypeScript compilation error I don't understand.

Error:

TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.

Code:

const processConfig = (config: Config) => {
  const apiUrl = config.apiUrl; // string | undefined
  fetchData(apiUrl); // fetchData expects string
};

I understand the error message, but I'm not sure the best way to handle this. What are my options and which is recommended?

### Memory Leaks

I suspect a memory leak in my React component.

Symptoms:

• Memory usage grows over time
• Performance degrades after extended use
• Chrome DevTools shows detached DOM nodes

Code:

useEffect(() => {
  const socket = new WebSocket(WS_URL);

  socket.onmessage = (event) => {
    setMessages(prev => [...prev, JSON.parse(event.data)]);
  };

  return () => socket.close();
}, []);

Questions:

1. Is there a memory leak in this code?
2. What's causing it?
3. How do I properly clean up?

## Progressive Debugging Strategy

### Step 1: Understand the Error

First, help me understand this error message:

Error: ENOENT: no such file or directory, open '/app/config/production.json'

What does this error mean, and what are the common causes?

### Step 2: Trace the Problem

Now I understand the error. Help me trace where it originates.

The error occurs when running: npm run build

Relevant files:

• webpack.config.js (uses config file)
• package.json scripts
• Directory structure

[paste relevant code]

Where in the build process does this fail?

### Step 3: Fix and Verify

Based on our analysis, the issue is [X].

Proposed fix: [your proposed fix]

Questions:

1. Will this fix actually solve the problem?
2. Are there edge cases I'm missing?
3. Could this introduce new issues?

## Rubber Duck Debugging with AI

Use AI as an intelligent rubber duck:

I'm stuck on a bug and want to think through it out loud.

The Problem: Users report that sometimes their session expires immediately after login.

What I Know:

• Sessions are stored in Redis
• JWT tokens have 24-hour expiry
• Issue happens intermittently
• More common in production than staging

Let me walk through the login flow:

1. User submits credentials
2. Server validates and creates JWT
3. JWT is stored in Redis with session data
4. JWT is sent to client in HTTP-only cookie

Ask me questions to help me find the bug.

## Debugging Complex Systems

### Distributed System Debugging

I'm debugging an issue across multiple services.

Architecture:

• API Gateway (Kong)
• Auth Service (Node.js)
• User Service (Go)
• Database (PostgreSQL)

Problem: Intermittent 401 errors that don't reproduce locally.

Request Flow:

1. Client -> Gateway (auth check passes)
2. Gateway -> Auth Service (token validation)
3. Auth Service -> User Service (get user details)
4. Returns 401 randomly at step 3

Logs show no errors. What should I check?

### Database Query Debugging

This query is slow in production but fast locally.

Query:

SELECT u.*, COUNT(o.id) as order_count
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE u.created_at > '2024-01-01'
GROUP BY u.id
ORDER BY order_count DESC
LIMIT 100;

Execution times:

• Local (1000 rows): 50ms
• Production (2M rows): 45 seconds

EXPLAIN shows: [paste explain output]

What indexes should I add and why?

## Debugging Prompt Patterns

### The "What Changed" Pattern

This code was working yesterday, now it's broken.

What changed:

• Updated React from 18.2 to 18.3
• Added new ESLint rules
• Teammate merged PR #123

Error: [error message]

Which change likely caused this?

### The "Works/Doesn't Work" Pattern

Same function, different results:

This WORKS:

const result = await fetchData('users');
console.log(result); // [user1, user2]

This DOESN'T WORK:

const result = await fetchData('products');
console.log(result); // undefined

Both endpoints return valid JSON when tested with curl. Why does one work and not the other?

### The "Minimal Reproduction" Pattern

Help me create a minimal reproduction of this bug.

Original complex code that has the bug: [paste complex code]

I want to isolate the issue. What's the smallest piece of code that would reproduce this behavior?

## Common Debugging Mistakes to Avoid

### 1. Too Little Context

**Bad**:

Why is this broken? [single line of code]

**Good**:

This function throws an error when called with undefined. [full function with context about how/where it's called]

### 2. Omitting Error Messages

**Bad**:

My code doesn't work.

**Good**:

Error: TypeError: Cannot read properties of undefined Stack trace: [full stack trace]

### 3. Not Sharing What You've Tried

**Bad**:

How do I fix this null pointer error?

**Good**:

How do I fix this null pointer error? I've tried:

• Adding null checks (still occurs)
• Using optional chaining (different error)
• Checking API response (data exists)

## Practice Exercise

Create debugging prompts for these scenarios:

1. **A React component renders infinitely** - useEffect with dependencies issue
2. **API returns 500 but only on certain inputs** - Data validation problem
3. **Tests pass locally but fail in CI** - Environment difference
4. **CSS styles not applying** - Specificity or cascade issue

For each, include:
- Error/symptom description
- Relevant code
- Environment details
- Specific questions to ask

## Summary

- Always include error messages, code, expected/actual behavior
- Provide environment details and what you've tried
- Use progressive debugging: understand, trace, fix
- Create minimal reproductions for complex issues
- Be specific about what kind of help you need

## Next Steps

Now that you can debug effectively with AI, let's explore prompt engineering for refactoring—transforming working code into better code.