/debug

/debug - Debug Command

Purpose

Analyze and debug errors, exceptions, or unexpected behavior with systematic investigation and root cause analysis. This is a streamlined version of /fix focused on diagnosis and investigation.

Usage

/debug [error message or description]

Arguments

• [error message] - Stack trace or error output
• [description] - Natural language description of unexpected behavior

How It Works

The /debug command follows a systematic 3-step debugging process:

Step 1: Analyze Error

1. Parse Error Message and Stack Trace

   • Extracts error type (TypeError, ValueError, etc.)
   • Identifies file and line number
   • Reads stack trace for execution path

2. Identify Error Location

   • Pinpoints the exact failing line
   • Examines surrounding code
   • Checks function context

3. Understand Error Type

   • Categorizes error (logic, runtime, type, etc.)
   • Identifies common causes for this error type
   • Notes relevant language-specific behavior

Step 2: Investigate

1. Trace Execution Path

   • Follows code flow from entry to error
   • Maps variable states at each step
   • Identifies where state becomes invalid

2. Check Related Code

   • Examines caller functions
   • Reviews recent changes to area
   • Searches for similar patterns

3. Form Hypotheses

   • Lists possible root causes
   • Ranks by likelihood
   • Identifies tests to validate each hypothesis

Step 3: Fix

1. Implement Minimal Fix

   • Addresses root cause
   • Keeps changes focused
   • Preserves existing behavior

2. Verify Fix Works

   • Tests the specific error case
   • Runs related tests
   • Checks for side effects

3. Add Regression Test

   • Creates test that would catch this bug
   • Documents the error scenario
   • Ensures test fails without fix

Debug Output Format

## Debug Report

### Error

TypeError: Cannot read property ‘email’ of undefined at getUserEmail (src/services/UserService.ts:45) at processUser (src/api/users.ts:23)

### Analysis
The error occurs when trying to access `user.email` but `user` is undefined.

### Location
**File**: `src/services/UserService.ts`
**Line**: 45
**Function**: `getUserEmail`

### Execution Trace
1. API receives request → `processUser()`
2. Calls `getUserEmail(userId)`
3. Inside getUserEmail:
   ```typescript
   const user = this.fetchUser(userId); // Missing await!
   return user.email; // user is a Promise, not User object

Root Cause

Missing await keyword - fetchUser() is async but wasn’t awaited, so user is a Promise object instead of a User object.

Hypotheses Tested

1. ✅ Missing await - Confirmed by checking function signature
2. ❌ User doesn’t exist - Would be null, not undefined
3. ❌ Database error - Would throw different error

Fix

// Before
async getUserEmail(userId: string) {
  const user = this.fetchUser(userId);
  return user.email;
}

// After
async getUserEmail(userId: string) {
  const user = await this.fetchUser(userId);
  if (!user) {
    throw new NotFoundError(`User ${userId} not found`);
  }
  return user.email;
}

Verification

# Test the fix
pnpm test src/services/UserService.test.ts

# Run full suite
pnpm test

Prevention

Added regression test: test_getUserEmail_awaits_user_fetch

## Common Debug Patterns

### Null/Undefined Errors

```typescript
// Symptom
Cannot read property 'X' of undefined

// Common Causes
- Missing await on async function
- Object not initialized
- API returned null
- Array/object access out of bounds

// Investigation
1. Check if value should exist
2. Trace where it's set
3. Verify async operations

Type Errors

# Symptom
TypeError: unsupported operand type(s) for +: 'int' and 'str'

# Common Causes
- Mixed types in operation
- Missing type conversion
- Unexpected input type

# Investigation
1. Print types at error line
2. Check input sources
3. Verify type conversions

Race Conditions

// Symptom
Intermittent failures, works sometimes

// Common Causes
- Async operations out of order
- Shared state without locking
- Component unmounted during async

// Investigation
1. Add logging to trace timing
2. Check for missing awaits
3. Look for shared mutable state

Infinite Loops/Recursion

# Symptom
RecursionError: maximum recursion depth exceeded

# Common Causes
- Missing base case
- Base case never reached
- Infinite loop condition

# Investigation
1. Check base/termination condition
2. Verify condition can be met
3. Add iteration counter logging

Flags

Flag	Description	Example
--depth=[1-5]	Investigation depth (1=quick, 5=exhaustive)	--depth=4
--trace	Show detailed execution trace	--trace
--hypothesis	Show all hypotheses considered	--hypothesis
--format=[fmt]	Output format (concise/detailed)	--format=concise

Flag Examples

# Quick debug with minimal output
/debug --depth=1 --format=concise "error message"

# Deep investigation with full trace
/debug --depth=5 --trace --hypothesis "intermittent failure"

# Detailed analysis
/debug --format=detailed "TypeError in auth.ts"

Examples

Runtime Error

Input:

/debug IndexError: list index out of range in process_items at line 42

Output:

1. Identifies array access issue
2. Traces loop bounds
3. Finds off-by-one error
4. Suggests fix with correct range

Unexpected Behavior

Input:

/debug "User authentication succeeds but session not created"

Output:

1. Traces auth flow
2. Finds session creation after response sent
3. Identifies async timing issue
4. Recommends await before response

Intermittent Issue

Input:

/debug --depth=5 "Test fails randomly, passes on retry"

Output:

1. Analyzes for race conditions
2. Checks for time-dependent logic
3. Reviews shared state access
4. Identifies missing await in test setup

When to Use

Use /debug when you need to:

• Investigate an error without immediately fixing it
• Understand why something breaks
• Diagnose intermittent issues
• Trace execution paths
• Form hypotheses about root causes

Use /fix instead when you want to:

• Fix the issue immediately
• Add regression tests
• Complete the full fix workflow

Deliverables

After running /debug, you receive:

1. Error Analysis - Detailed breakdown of the error
2. Execution Trace - Step-by-step flow to error
3. Root Cause - Explanation of why it fails
4. Hypotheses - Tested theories about cause
5. Fix Recommendation - Suggested solution (optional)
6. Prevention Strategy - How to avoid similar issues

Related Commands

• /fix - Complete fix workflow with tests
• /test - Generate tests after debugging
• /review - Review code for potential issues
• /refactor - Improve code after fix