Debugging with Claude Code

Debugging is where Claude Code truly shines. It can analyze error messages, trace through code paths, identify root causes, and apply fixes — all through natural language conversation.

Sharing an Error Message

The simplest way to debug is to paste an error directly:

> I'm getting this error when I run my app:
TypeError: Cannot read properties of undefined (reading 'map')
at UserList (src/components/UserList.jsx:15)

Claude Code will:

1. Read the referenced file (UserList.jsx)
2. Identify that the data being mapped is undefined
3. Suggest adding a null check or default value
4. Offer to apply the fix directly

Debugging a Failing Test

Point Claude Code at a failing test:

> Run pytest tests/test_auth.py and fix any failures

Claude Code will run the test, read the output, analyze the failure, and make corrections.

Debugging with Stack Traces

For complex stack traces, just paste the entire thing:

> Help me understand this Java stack trace and fix the issue:

java.lang.NullPointerException
    at com.app.service.UserService.getUser(UserService.java:42)
    at com.app.controller.UserController.show(UserController.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

Finding Bugs Without Error Messages

Sometimes you know something is wrong but don’t have an error:

> The login form submits but the user never gets redirected to the dashboard. Can you trace through the login flow and find the issue?

Claude Code will trace the code path from form submission through authentication to redirection, identifying where the flow breaks down.

Performance Debugging

Claude Code can help with performance issues too:

> This API endpoint /api/users is taking 5 seconds to respond. Can you analyze the code and find what's slow?

Common issues Claude Code identifies:

• N+1 database queries
• Missing database indexes
• Inefficient loops or algorithms
• Unnecessary API calls
• Large data transfers

Debugging Strategy Tips

1. Include the full error — Don’t summarize, paste the complete error output
2. Describe what you expected — “I expected X but got Y” helps narrow down the issue
3. Mention what you’ve tried — This prevents Claude from suggesting things you’ve already done
4. Let Claude run commands — Allow it to run tests, check logs, or print debug info

Summary

Claude Code is an excellent debugging partner. It can read errors, trace code paths, run tests, and apply fixes. Next, we’ll explore how Claude Code helps with refactoring.

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Next: Refactoring Code with Claude Code