🐛Debugging with Claude Code

Debugging with Claude Code

Debugging is one of Claude Code's most powerful use cases. Instead of spending hours hunting for the problem, you can simply describe the issue or paste the error message and let Claude Code find the solution.

Paste the Error Message Directly

The simplest debugging approach:

> I got this error: 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:

1. Read the UserList.tsx file
2. Understand line 15 and its context
3. Identify that a variable is undefined
4. Suggest the fix (e.g., adding a null check or default value)

Reading Stack Traces

> Here's the full stack trace — analyze the problem: Error: ECONNREFUSED 127.0.0.1:5432 at TCPConnectWrap.afterConnect [as oncomplete] at /app/src/db/connection.ts:23:15 at /app/src/services/userService.ts:45:10 at /app/src/routes/api.ts:12:5

Claude Code reads every file in the trace and follows the error from its source.

Fixing Failing Tests

> This test is failing — fix it: FAIL src/utils/__tests__/formatDate.test.ts ✕ formats date correctly (5ms) Expected: "2024-01-15" Received: "1/15/2024"

Claude Code will read both the test file and the function being tested, identifying why the output doesn't match.

Debugging Across Multiple Files

> Users can't log in — the frontend sends a POST request > but the server responds with 401. Trace the issue from frontend to backend.

Claude Code will trace the data flow across files:

1. Reads frontend code (e.g., login.ts)
2. Reads API route (e.g., auth.route.ts)
3. Reads auth middleware
4. Reads user service
5. Identifies where the flow breaks

Common Debugging Patterns

1. "Why isn't this working?"

> This code should send an email but it does nothing. > The sendEmail function is in mailer.ts — check it.

2. "Performance is slow"

> The products page takes 8 seconds to load. > Check the database queries in productService.ts

3. "Works locally but not in production"

> Everything works on my machine but I get 500 in production. > Here's the error log from production: [paste log]

4. "Unexpected behavior"

> The button appears twice sometimes. > Check the ActionButton.tsx component

Advanced Debugging Techniques

Adding Strategic Logging

> Add console.log at every step of the processPayment function > to trace the execution path

Claude Code adds logging, helps you identify the issue, and then removes the debug logs when done.

Checking Environment Variables

> Database connection is failing. > Check the .env file and verify the environment variables

Dependency Analysis

> I'm getting an import error. > Check package.json and make sure all dependencies are installed

Type Errors in TypeScript

> TypeScript is complaining about this: Type 'string | undefined' is not assignable to type 'string'. at src/utils/parser.ts:42:5

Claude Code understands TypeScript's type system and provides precise fixes.

Tips for Effective Debugging with Claude Code

1. Be specific — "The button doesn't work" is less useful than "The submit button on the registration form doesn't call the API"
2. Paste full error messages — Don't truncate stack traces
3. Describe the steps — "The error happens after logging in and then clicking on the profile"
4. Mention what you tried — "I tried restarting the server and clearing the cache"
5. Point to files — "The problem is probably in src/auth/ or src/middleware/"

Complete Example: Debugging an API Error

> A user reported that when uploading an image larger than 5MB they get > a 413 error. But the settings say the max is 10MB. > > Here's the error: > PayloadTooLargeError: request entity too large > at readBody (node_modules/raw-body/index.js:155:15) > at /app/src/middleware/upload.ts:28:5 > at /app/src/routes/profile.ts:55:3

Claude Code will examine:

• Upload middleware settings
• Express/body-parser configuration
• Nginx/proxy settings if applicable
• And find that the limit in middleware differs from the declared settings

Debugging Workflow

Step 1: Describe or paste the error Step 2: Claude Code reads relevant files Step 3: Claude Code identifies root cause Step 4: Claude Code proposes a fix Step 5: You approve the fix Step 6: Claude Code applies it Step 7: You test again

When Claude Code Can't Find the Bug

Sometimes the issue is environmental or requires runtime data:

> I've described the problem but you can't find it. > What additional information would help you debug this?

Claude Code will ask for:

• Runtime logs
• Network request/response details
• Browser console output
• Environment-specific configuration

Summary

Claude Code excels at debugging because it can:

• Read multiple files in seconds
• Trace error paths across application layers
• Understand the full project context
• Suggest precise fixes with explanations
• Handle TypeScript, JavaScript, Python, and many other languages

Next: We'll learn how to use streaming responses with the Claude API.