---
title: "Debugging in Claude Code"
description: "How to find and fix bugs with Claude Code—from error messages to complex logical issues. The systematic approach."
pillar: "Claude Code"
level: "intermediate"
date: "2026-01-20"
url: "https://theglitch.ai/academy/claude-code/debugging-claude-code"
---

# Debugging in Claude Code

How to find and fix bugs with Claude Code—from error messages to complex logical issues. The systematic approach.


# Debugging in Claude Code

> **The Glitch's Take:** "AI debugging isn't magic. It's systematic investigation with a faster investigator."

**Part of:** [The Complete Guide to Claude Code](/articles/claude-code/claude-code-complete-guide)
**Level:** Intermediate
**Reading Time:** 11 minutes

---

## The Point

Claude Code doesn't debug by guessing repeatedly. Well-structured debugging prompts get to root causes faster than trial-and-error. This guide covers the systematic approach to debugging with AI—when it works, when it doesn't, and how to do it right.

---

## TL;DR

- **Share error messages** — Full context, not just the summary
- **Provide reproduction steps** — What triggers the bug?
- **Use model matching** — Opus for complex bugs, Sonnet for simple
- **Iterate with hypotheses** — Not "fix it" but "test this theory"
- **Verify systematically** — Confirm fix actually addresses root cause

---

## Debugging Workflow Overview

### The Pattern

```
1. Gather context (error, reproduction, expected behavior)
2. Share with Claude Code
3. Review hypothesis
4. Test fix
5. Verify root cause resolved (not just symptom)
```

### What Makes AI Debugging Different

| Traditional | With Claude Code |
|-------------|------------------|
| Read code manually | Claude scans codebase |
| Form hypothesis alone | Collaborative hypothesis |
| Write fix, test | Claude writes fix, you test |
| Google the error | Claude explains the error |
| Hope it works | Systematic verification |

---

## Bug Types and Approaches

### Type 1: Crash with Stack Trace

**Context to provide:**
```
Bug: Application crashes on startup.

Stack trace:
[Full stack trace here]

This started after [recent change].
```

**What Claude Code does:**
1. Reads stack trace
2. Identifies crash location
3. Traces causation
4. Proposes fix

**Prompt pattern:**
```
The app crashes with this error:

[paste full error]

Started after I [describe recent change].

Find the root cause and fix it.
```

### Type 2: Wrong Output (No Crash)

**Context to provide:**
```
Bug: User registration saves but email is wrong.

Steps:
1. Fill form with email "test@example.com"
2. Submit
3. Check database

Expected: email = "test@example.com"
Actual: email = "test@example"

No error messages.
```

**What Claude Code does:**
1. Traces data flow from input to storage
2. Identifies transformation point
3. Finds where data gets corrupted
4. Proposes fix

**Prompt pattern:**
```
Bug: [Description]

Steps to reproduce:
1. [Step]
2. [Step]

Expected: [What should happen]
Actual: [What happens]

No error messages. The relevant code paths are:
- [File 1]
- [File 2]
```

### Type 3: Intermittent / Race Condition

**Context to provide:**
```
Bug: Sometimes the dashboard shows stale data.

Happens maybe 1 in 5 times.
More common when clicking fast.
Refresh fixes it temporarily.

Suspected timing issue.
```

**What Claude Code does:**
1. Identifies async operations
2. Analyzes timing dependencies
3. Looks for missing await, race conditions
4. Proposes synchronization fix

**Prompt pattern:**
```
Intermittent bug: [Description]

Frequency: [How often]
Trigger pattern: [What seems to cause it]
Workaround: [What temporarily fixes it]

I suspect [your hypothesis if any].
```

### Type 4: Performance / Slow

**Context to provide:**
```
Bug: Page load takes 8 seconds.
It was fast before we added the user list feature.

The slow request is GET /api/dashboard.
Network tab shows 8s server response time.
```

**What Claude Code does:**
1. Identifies slow operation
2. Analyzes complexity (loops, queries)
3. Finds N+1 queries or O(n²) operations
4. Proposes optimization

**Prompt pattern:**
```
Performance issue: [What is slow]

Metrics:
- [Response time / duration]
- [When it started]
- [What changed]

Profile data (if available):
[Any profiling output]
```

### Type 5: "It Worked Before"

**Context to provide:**
```
Bug: Login stopped working.

Last working commit: abc1234
Current commit: def5678

Nothing in login code changed (I think).
```

**What Claude Code does:**
1. Compares commits
2. Identifies changes that could affect login
3. Traces dependency changes
4. Pinpoints regression cause

**Prompt pattern:**
```
Regression: [What stopped working]

Last known good: [commit/date/version]
First known bad: [commit/date/version]

Git diff shows:
[summary of changes, or ask Claude to run git diff]
```

---

## Debugging Prompts That Work

### The Minimal Effective Prompt

```
Bug: [One sentence description]

Error:
[Full error message/stack trace]

Reproduce:
[Steps to trigger]

Find and fix.
```

### The Detailed Investigation Prompt

```
Investigating: [Issue description]

Context:
- [Relevant background]
- [Recent changes]
- [Environment details]

Error/Symptom:
[What goes wrong]

What I've tried:
- [Attempt 1 - result]
- [Attempt 2 - result]

My hypothesis: [What I think is wrong]

Please:
1. Validate or refute my hypothesis
2. Find the actual root cause
3. Propose a fix
4. Explain why this fix addresses the root cause
```

### The "Help Me Debug" Prompt

```
I'm stuck on a bug. Let's debug together.

The problem: [Description]

Show me how to:
1. Add logging to trace the issue
2. Identify where the problem originates
3. Test potential fixes

I'll run the code and share results.
```

---

## When to Use Which Model

### Use Sonnet For:

- Clear error messages
- Obvious bugs (typos, missing imports)
- Performance issues with clear metrics
- Bugs in code Claude recently wrote

### Use Opus For:

- Complex logic bugs
- Race conditions / async issues
- Bugs with no clear error
- "Impossible" bugs that don't make sense
- Security vulnerabilities

### The Switch Point

Start with Sonnet. Switch to Opus when:
- Sonnet suggests same fix repeatedly without success
- The bug seems to defy obvious explanation
- Multiple components might be involved

---

## Debugging Session Structure

### Phase 1: Context Gathering

```
Before we debug, I need to understand the system.

Please read:
- [Relevant file 1]
- [Relevant file 2]

Summarize how [feature] works currently.
```

### Phase 2: Hypothesis Formation

```
Based on the error and your understanding, what are the
possible causes? Rank by likelihood.
```

### Phase 3: Investigation

```
Let's test hypothesis 1 first. Add logging to [location]
to confirm [expected observation].
```

### Phase 4: Fix Implementation

```
The logs confirm [root cause]. Implement a fix that:
- Addresses the root cause (not just symptoms)
- Doesn't break [related functionality]
- Includes a test to prevent regression
```

### Phase 5: Verification

```
Run the tests. Verify the bug is fixed.
Also verify [related functionality] still works.
```

---

## Common Debugging Mistakes

### Mistake 1: Hiding Error Details

**Bad:**
```
The login doesn't work. Fix it.
```

**Good:**
```
Login fails with:
"TypeError: Cannot read property 'email' of undefined
    at AuthService.login (auth.ts:42)"
```

### Mistake 2: "Just Fix It" Without Context

**Bad:**
```
Fix the bug in the user service.
```

**Good:**
```
Bug in user service: getUser returns null for existing users
when called immediately after createUser. Works if I wait
1 second between calls.
```

### Mistake 3: Not Providing Reproduction Steps

**Bad:**
```
The payment form sometimes fails.
```

**Good:**
```
Payment form fails when:
1. Add item to cart
2. Start checkout
3. Enter invalid card (4000...)
4. See error "invalid card"
5. Enter valid card (4242...)
6. Submit
7. Fails with "payment already attempted"
```

### Mistake 4: Ignoring Claude's Questions

When Claude asks clarifying questions, answer them. They're not stalling—they're gathering necessary context.

---

## Debugging Tools Claude Code Uses

### Log Analysis

Claude Code can:
- Add console.log/print statements
- Interpret log output you share
- Suggest logging strategy

### Test Isolation

Claude Code can:
- Write minimal reproduction tests
- Create unit tests for suspected components
- Isolate failure to specific function

### Git History

Claude Code can:
- Run git diff between commits
- Analyze what changed
- Identify regression points

### Static Analysis

Claude Code can:
- Read code and identify issues
- Check for common bug patterns
- Trace data flow through code

---

## When AI Debugging Fails

### Situations Where It Struggles

| Situation | Why | Alternative |
|-----------|-----|-------------|
| Environment-specific | Can't access your env | Share env details explicitly |
| External service issues | Can't reach services | Test services independently |
| Data-specific bugs | Can't see your data | Share anonymized examples |
| Hardware/network | Physical layer issues | Traditional debugging |

### Escalation Path

1. **Add more context** — More logs, more details
2. **Switch to Opus** — Better reasoning
3. **Pair debug** — Interactive back-and-forth
4. **Manual investigation** — Some bugs need human intuition

---

## Quick Reference

### Debugging Prompt Template

```
Bug: [Brief description]

Error/Symptom:
[What happens - full details]

Reproduction:
1. [Step 1]
2. [Step 2]
3. [Bug occurs]

Expected: [What should happen]
Actual: [What happens]

Context:
- [Recent changes]
- [Relevant files]

Find root cause and fix.
```

### Debugging Checklist

- [ ] Full error message shared
- [ ] Reproduction steps provided
- [ ] Expected vs actual behavior stated
- [ ] Recent changes mentioned
- [ ] Relevant files identified
- [ ] Model selected appropriately
- [ ] Fix verified to address root cause

### Model Selection

| Bug Type | Model |
|----------|-------|
| Obvious error | Sonnet |
| Logic bug | Opus |
| Race condition | Opus |
| Performance | Sonnet first, then Opus |
| Security | Opus |
| Regression | Sonnet |

---

## Next Steps

- [10 Mistakes Everyone Makes with Claude Code](/articles/claude-code/claude-code-mistakes)
- [Building Your Context Layer](/articles/claude-code/building-context-layers)
- [Back to Claude Code Guide](/articles/claude-code/claude-code-complete-guide)

---

## Sources

- [Anthropic Claude Documentation](https://docs.anthropic.com)
- Debugging patterns from The Glitch's production use

---

*Last verified: 2026-01-20. Tested debugging workflows on TypeScript, Python, and Go codebases.*