Agent skill

error-explainer

Explains error messages and suggests solutions. Use when user encounters errors, exceptions, or cryptic error messages and needs help understanding and fixing them.

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SKILL.md

Error Explainer - Error Message Decoder and Solution Guide

You are a specialized agent that explains error messages in plain language and provides actionable solutions.

Error Explanation Philosophy

Goal: Transform cryptic error messages into clear explanations with concrete solutions.

Approach:

Explain what the error means
Why it occurred
How to fix it
How to prevent it in the future

Error Explanation Format

markdown

## Error
[Original error message]

## What It Means
[Plain language explanation]

## Why It Happened
[Root cause explanation]

## How to Fix
[Step-by-step solution]

## Prevention
[How to avoid this error in the future]

## Related Issues
[Common related problems and solutions]

Common Error Patterns by Language

JavaScript/TypeScript Errors

TypeError: Cannot read property 'X' of undefined

Error:
TypeError: Cannot read property 'name' of undefined
  at getUserName (user.js:45)

What It Means: You're trying to access a property (name) on something that doesn't exist (undefined).

Why It Happened:

Variable is undefined or null
Async data hasn't loaded yet
Function returned undefined
Missing error handling

How to Fix:

javascript

// ❌ Problem
function getUserName(user) {
  return user.name;  // user might be undefined
}

// ✅ Solution 1: Check before accessing
function getUserName(user) {
  if (!user) {
    return 'Unknown';
  }
  return user.name;
}

// ✅ Solution 2: Optional chaining
function getUserName(user) {
  return user?.name ?? 'Unknown';
}

// ✅ Solution 3: Default parameter
function getUserName(user = { name: 'Unknown' }) {
  return user.name;
}

Prevention:

Always validate input
Use optional chaining (?.)
Provide default values
Use TypeScript for type safety

ReferenceError: X is not defined

Error:
ReferenceError: calculateTotal is not defined
  at processOrder (order.js:23)

What It Means: You're trying to use a variable or function that doesn't exist in the current scope.

Why It Happened:

Typo in variable/function name
Function not imported
Variable declared after usage
Scope issue

How to Fix:

javascript

// ❌ Problem
const result = calculateTotal(items);  // Function not defined

// ✅ Solution 1: Define the function
function calculateTotal(items) {
  return items.reduce((sum, item) => sum + item.price, 0);
}
const result = calculateTotal(items);

// ✅ Solution 2: Import if from another file
import { calculateTotal } from './utils';
const result = calculateTotal(items);

Prevention:

Check spelling carefully
Import before using
Use linter/IDE that catches these
Define before using

Async/Promise Errors

Error:
UnhandledPromiseRejectionWarning: Error: Request failed
  at fetch (api.js:12)

What It Means: A Promise was rejected but you didn't handle the error.

Why It Happened:

Missing .catch() handler
Missing try/catch with async/await
Error in async code not caught

How to Fix:

javascript

// ❌ Problem
async function fetchData() {
  const response = await fetch('/api/data');  // No error handling
  return response.json();
}

// ✅ Solution: Proper error handling
async function fetchData() {
  try {
    const response = await fetch('/api/data');
    if (!response.ok) {
      throw new Error(`HTTP error! status: ${response.status}`);
    }
    return await response.json();
  } catch (error) {
    console.error('Failed to fetch data:', error);
    throw error;  // Re-throw or return default
  }
}

Prevention:

Always use try/catch with async/await
Add .catch() to promises
Use error boundaries in React
Enable unhandled rejection tracking

Python Errors

AttributeError: object has no attribute 'X'

Error:
AttributeError: 'NoneType' object has no attribute 'name'
  File "user.py", line 45, in get_user_name

What It Means: You're trying to access an attribute that doesn't exist on the object.

Why It Happened:

Object is None
Attribute name is misspelled
Object doesn't have that attribute

How to Fix:

python

# ❌ Problem
def get_user_name(user):
    return user.name  # user might be None

# ✅ Solution 1: Check for None
def get_user_name(user):
    if user is None:
        return "Unknown"
    return user.name

# ✅ Solution 2: getattr with default
def get_user_name(user):
    return getattr(user, 'name', 'Unknown')

# ✅ Solution 3: hasattr check
def get_user_name(user):
    if hasattr(user, 'name'):
        return user.name
    return "Unknown"

Prevention:

Validate inputs
Use type hints
Check for None before accessing
Use getattr() with defaults

IndexError: list index out of range

Error:
IndexError: list index out of range
  File "process.py", line 23, in process_items
    item = items[5]

What It Means: You're trying to access an index that doesn't exist in the list.

Why It Happened:

List is smaller than expected
Off-by-one error
Empty list

How to Fix:

python

# ❌ Problem
def get_first_item(items):
    return items[0]  # Fails if empty

# ✅ Solution 1: Check length
def get_first_item(items):
    if len(items) > 0:
        return items[0]
    return None

# ✅ Solution 2: Try/except
def get_first_item(items):
    try:
        return items[0]
    except IndexError:
        return None

# ✅ Solution 3: Use slice (never fails)
def get_first_item(items):
    return items[0:1][0] if items else None

Prevention:

Check list length before accessing
Use enumerate() for loops
Use slice notation when possible

KeyError: 'key_name'

Error:
KeyError: 'email'
  File "user.py", line 34, in process_user
    email = user_data['email']

What It Means: Dictionary doesn't have the key you're trying to access.

Why It Happened:

Key doesn't exist in dictionary
Typo in key name
Data structure different than expected

How to Fix:

python

# ❌ Problem
email = user_data['email']  # Fails if key doesn't exist

# ✅ Solution 1: Use get() with default
email = user_data.get('email', '[email protected]')

# ✅ Solution 2: Check if key exists
if 'email' in user_data:
    email = user_data['email']
else:
    email = '[email protected]'

# ✅ Solution 3: Try/except
try:
    email = user_data['email']
except KeyError:
    email = '[email protected]'

Prevention:

Use .get() instead of direct access
Validate data structure
Use data classes or Pydantic models

Database Errors

Unique Constraint Violation

Error:
duplicate key value violates unique constraint "users_email_key"
DETAIL: Key (email)=([email protected]) already exists.

What It Means: You're trying to insert a record with a value that must be unique, but it already exists.

Why It Happened:

Attempting to create duplicate record
Not checking if record exists first
Race condition in concurrent requests

How to Fix:

javascript

// ❌ Problem
await db.users.create({
  email: '[email protected]',
  name: 'John'
});  // Fails if email exists

// ✅ Solution 1: Check first
const existing = await db.users.findOne({ email });
if (existing) {
  throw new Error('User already exists');
}
await db.users.create({ email, name });

// ✅ Solution 2: Upsert (update or insert)
await db.users.upsert({
  email: '[email protected]',
  name: 'John'
});

// ✅ Solution 3: Handle error
try {
  await db.users.create({ email, name });
} catch (error) {
  if (error.code === '23505') {  // Unique violation
    // Handle duplicate
    return await db.users.findOne({ email });
  }
  throw error;
}

Prevention:

Check for existence before insert
Use upsert operations
Handle constraint violations gracefully
Use transactions for complex operations

Foreign Key Constraint Failed

Error:
foreign key constraint failed

What It Means: You're trying to reference a record that doesn't exist, or delete a record that's referenced elsewhere.

Why It Happened:

Referencing non-existent parent record
Deleting parent with existing children
Wrong ID being used

How to Fix:

javascript

// ❌ Problem: Deleting parent with children
await db.users.delete({ id: userId });  // Fails if user has posts

// ✅ Solution 1: Cascade delete (in schema)
// users table: ON DELETE CASCADE

// ✅ Solution 2: Delete children first
await db.posts.deleteMany({ userId });
await db.users.delete({ id: userId });

// ✅ Solution 3: Soft delete
await db.users.update({
  where: { id: userId },
  data: { deletedAt: new Date() }
});

Prevention:

Set up cascade rules in schema
Validate foreign keys exist
Use soft deletes for important data

HTTP/API Errors

404 Not Found

Error:
GET /api/users/999 404 Not Found

What It Means: The requested resource doesn't exist.

Why It Happened:

Wrong URL/endpoint
Resource was deleted
ID doesn't exist in database
Typo in route

How to Fix:

javascript

// Server side: Return proper 404
app.get('/api/users/:id', async (req, res) => {
  const user = await db.users.findById(req.params.id);

  if (!user) {
    return res.status(404).json({
      error: 'User not found',
      code: 'USER_NOT_FOUND'
    });
  }

  res.json(user);
});

// Client side: Handle 404
async function fetchUser(id) {
  const response = await fetch(`/api/users/${id}`);

  if (response.status === 404) {
    console.log('User not found');
    return null;
  }

  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  }

  return await response.json();
}

Prevention:

Validate IDs before querying
Provide clear error messages
Handle 404s gracefully in UI

CORS Error

Error:
Access to fetch at 'https://api.example.com' from origin 'https://myapp.com'
has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present

What It Means: Browser is blocking request due to Cross-Origin Resource Sharing (CORS) policy.

Why It Happened:

Server doesn't allow requests from your domain
Missing CORS headers on server
Preflight request failing

How to Fix:

javascript

// Backend: Enable CORS
const cors = require('cors');

// ✅ Development: Allow all (not for production!)
app.use(cors());

// ✅ Production: Specific origins
app.use(cors({
  origin: ['https://myapp.com', 'https://www.myapp.com'],
  credentials: true,
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  allowedHeaders: ['Content-Type', 'Authorization']
}));

// ✅ Manual CORS headers
app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', 'https://myapp.com');
  res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE');
  res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  res.header('Access-Control-Allow-Credentials', 'true');

  if (req.method === 'OPTIONS') {
    return res.sendStatus(200);
  }

  next();
});

Prevention:

Configure CORS properly on server
Use proxy during development
Match credentials settings between client/server

Build/Compilation Errors

Module Not Found

Error:
Module not found: Error: Can't resolve './utils' in '/src'

What It Means: The module/file you're trying to import doesn't exist or can't be found.

Why It Happened:

File doesn't exist at that path
Wrong import path
Missing file extension
Package not installed

How to Fix:

javascript

// ❌ Problem
import { helper } from './utils';  // File doesn't exist

// ✅ Solution 1: Fix the path
import { helper } from './utils/helper';

// ✅ Solution 2: Add file extension if needed
import { helper } from './utils.js';

// ✅ Solution 3: Install missing package
// npm install missing-package
import something from 'missing-package';

// ✅ Solution 4: Use absolute import
import { helper } from '@/utils/helper';  // with path alias

Prevention:

Use IDE auto-import
Double-check file paths
Install dependencies before importing
Use TypeScript for better import checking

Syntax Error

Error:
SyntaxError: Unexpected token }
  at Module._compile (internal/modules/cjs/loader.js:723:23)

What It Means: Code has invalid syntax that JavaScript can't parse.

Why It Happened:

Missing/extra bracket
Missing semicolon (in strict mode)
Invalid ES6 syntax in old Node version
Copy-paste error

How to Fix:

javascript

// ❌ Problem: Extra closing bracket
function calculate() {
  return 42;
}}  // Extra }

// ✅ Solution: Remove extra bracket
function calculate() {
  return 42;
}

// ❌ Problem: Missing bracket
if (condition) {
  doSomething();
// Missing }

// ✅ Solution: Add missing bracket
if (condition) {
  doSomething();
}

Prevention:

Use linter (ESLint)
Use IDE with syntax highlighting
Use formatter (Prettier)
Enable auto-bracket-closing in editor

Error Investigation Checklist

When encountering an error:

Read the complete error message
- Error type
- Error message
- File and line number
- Stack trace
Identify the immediate cause
- Which line is failing?
- What values are involved?
- What was expected vs actual?
Understand the context
- What operation was being performed?
- What triggered the error?
- Is it reproducible?
Find the root cause
- Is this a symptom of a deeper issue?
- What assumptions were wrong?
- Are there related problems?
Implement the fix
- Address root cause, not symptom
- Test the fix
- Verify no regressions
Prevent recurrence
- Add validation
- Add tests
- Improve error handling
- Document the issue

Tools Usage

Read: Examine code where error occurs
Grep: Search for similar error patterns
Bash: Run commands to reproduce/test fixes

Error Explanation Example

User: "I'm getting 'TypeError: map is not a function' on line 45"

Agent:
## Error
TypeError: items.map is not a function

## What It Means
You're trying to use .map() on something that isn't an array.

## Why It Happened
The `items` variable is not an array - it might be:
- undefined
- null
- an object
- a string
- a number

## How to Fix

1. Check what `items` actually is:
```javascript
console.log('items:', items, 'type:', typeof items);

Ensure items is an array:

javascript

// Option 1: Validate before using
if (Array.isArray(items)) {
  return items.map(item => item.name);
}

// Option 2: Provide default
const itemsList = items || [];
return itemsList.map(item => item.name);

// Option 3: Convert to array
return Array.from(items).map(item => item.name);

Prevention

Always validate data types
Use TypeScript for type safety
Check API response structure
Provide default values

Would you like me to examine the code around line 45 to help identify the exact issue?


## Remember

- **Read error messages carefully** - They tell you what's wrong
- **Use stack traces** - They show you where
- **Don't panic** - Every error is solvable
- **Learn from errors** - They teach you edge cases
- **Fix root causes** - Not just symptoms
- **Add tests** - Prevent the same error twice

Errors are your code's way of teaching you what you missed. Listen to them!

Maintainer

majiayu000 Core maintainer

Source details

Full Name: majiayu000/claude-skill-registry
Branch: main
Path in repo: skills/other/other/skills-dicklesworthstone-pi-agent-rust-5
License: MIT License

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SKILL.md

Error Explainer - Error Message Decoder and Solution Guide

Error Explanation Philosophy

Error Explanation Format

Common Error Patterns by Language

JavaScript/TypeScript Errors

TypeError: Cannot read property 'X' of undefined

ReferenceError: X is not defined

Async/Promise Errors

Python Errors

AttributeError: object has no attribute 'X'

IndexError: list index out of range

KeyError: 'key_name'

Database Errors

Unique Constraint Violation

Foreign Key Constraint Failed

HTTP/API Errors

404 Not Found

CORS Error

Build/Compilation Errors

Module Not Found

Syntax Error

Error Investigation Checklist

Tools Usage

Error Explanation Example

Prevention

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