The Complete Guide to Debugging with Claude Code: From Bug Detection to Fix

How Claude Code Transforms Debugging

Bug detection and fixing is one of the most time-consuming tasks for developers. Claude Code can handle everything from parsing error messages, tracing stack traces, to identifying root causes — drastically reducing your debugging time.

If you’re not yet familiar with the basics, check out our Claude Code Getting Started Guide first.

Technique 1: Paste the Error Message Directly

The simplest and most effective method is to pass the error message as-is.

> I'm getting the following error. Investigate the cause and fix it.
>
> TypeError: Cannot read properties of undefined (reading 'map')
>   at UserList (src/components/UserList.tsx:15:23)
>   at renderWithHooks (node_modules/react-dom/...)

Claude Code will open the relevant file, understand the code context, and suggest a fix.

Actual Fix Example

// Before: crashes when users is undefined
function UserList({ users }: { users: User[] }) {
  return (
    <ul>
      {users.map(user => <li key={user.id}>{user.name}</li>)}
    </ul>
  );
}

// After: optional chaining and fallback
function UserList({ users = [] }: { users?: User[] }) {
  if (users.length === 0) {
    return <p>No users found</p>;
  }
  return (
    <ul>
      {users.map(user => <li key={user.id}>{user.name}</li>)}
    </ul>
  );
}

Technique 2: Share Reproduction Steps

For complex bugs, providing reproduction steps improves accuracy.

> The bug can be reproduced with the following steps. Identify the cause and fix it.
> 1. Navigate to /dashboard
> 2. Select "2026-03" in the filter
> 3. Click the "Export" button
> 4. The CSV file is empty
>
> Expected behavior: The filtered data should be included in the CSV

Technique 3: Identify Failures Through Tests

Pass test failure results to let Claude Code pinpoint the issue.

> Run npm test, check which tests fail,
> then fix the code so all tests pass.

Claude Code runs the tests, traces back from the failing test content to the problematic code, and fixes it. For more on test integration, see TDD and Claude Code.

Technique 4: Investigation via Log Injection

> Add console.log statements to the fetchUserData function's processing flow
> to determine at which stage the data is being lost.

async function fetchUserData(userId: string): Promise<UserData> {
  console.log("[DEBUG] fetchUserData called with:", userId);

  const response = await api.get(`/users/${userId}`);
  console.log("[DEBUG] API response status:", response.status);
  console.log("[DEBUG] API response data:", JSON.stringify(response.data));

  const transformed = transformUser(response.data);
  console.log("[DEBUG] Transformed data:", JSON.stringify(transformed));

  return transformed;
}

If you also instruct it to remove the logs after investigation, your code stays clean.

> Once you've found the cause, fix it and remove all the console.log statements you added.

Technique 5: Batch-Fix Type Errors

When TypeScript throws a flood of type errors, have Claude Code handle them all at once.

> Run npx tsc --noEmit and fix all type errors.
> Maintain type safety in each fix. Do not use the any type.

Technique 6: Investigating Performance Issues

Claude Code can also tackle “it works but it’s slow” problems.

> This function is slow. Analyze its time complexity and suggest improvements.

// Before: O(n^2) inefficient implementation
function findDuplicates(items: string[]): string[] {
  return items.filter((item, index) =>
    items.indexOf(item) !== index
  );
}

// After: improved to O(n)
function findDuplicates(items: string[]): string[] {
  const seen = new Set<string>();
  const duplicates = new Set<string>();
  for (const item of items) {
    if (seen.has(item)) {
      duplicates.add(item);
    }
    seen.add(item);
  }
  return [...duplicates];
}

For detailed performance improvement techniques, also check out our Performance Optimization Guide.

CLAUDE.md Settings to Boost Debugging Efficiency

Writing debugging rules in your project’s CLAUDE.md reduces the instructions needed each time.

## Debugging Rules
- Always run related tests after fixing errors
- Always remove debug code like console.log after investigation
- Do not bypass type errors with the any type
- Show code before and after the fix

Summary

The key to debugging with Claude Code is providing specific error messages and reproduction steps. Vague instructions reduce accuracy, so provide as much information as possible. If you want to improve your error handling design itself, also check out Error Handling Design Patterns.

For detailed feature information, refer to the official Anthropic documentation.