error-explanation-generator

Error Explanation Generator

Analyze test failures and provide clear explanations with actionable fixes.

Core Capabilities

This skill helps debug failed tests by:

1. Parsing error messages - Extract key information from test output
2. Identifying root causes - Determine why tests fail
3. Explaining clearly - Translate technical errors into understandable language
4. Providing fixes - Suggest concrete, actionable solutions
5. Recognizing patterns - Detect common error categories across frameworks

Error Analysis Workflow

Step 1: Gather Error Context

Collect all relevant information:

Essential Information:

• Complete error message and stack trace
• Test framework being used (pytest, jest, junit, etc.)
• Test code that failed
• Code being tested (if accessible)
• Programming language

Additional Context (if available):

• Recent code changes
• Environment details (OS, language version, dependencies)
• Test configuration files

How to gather:

# Python pytest
pytest -v --tb=long

# JavaScript/TypeScript jest
npm test -- --verbose

# Java junit
mvn test -X

# Go tests
go test -v

Step 2: Parse and Categorize the Error

Identify the error category using references/error_patterns.md:

Common Categories:

1. Assertion Failures - Expected vs actual value mismatch
2. Exceptions/Errors - Runtime errors during test execution
3. Timeout Errors - Tests taking too long
4. Setup/Teardown Failures - Fixture or initialization issues
5. Import/Dependency Errors - Missing modules or broken imports
6. Type Errors - Type mismatches in typed languages
7. Mock/Stub Issues - Problems with test doubles
8. Configuration Errors - Test framework or build config issues
9. Compilation Errors - Syntax or build failures
10. Flaky Test Issues - Intermittent failures

Step 3: Extract Key Information

Pull out critical details:

From Error Message:

• Error type (AssertionError, TypeError, NullPointerException, etc.)
• Expected vs actual values
• Error description
• Line numbers where error occurred

From Stack Trace:

• Failure point in test code
• Failure point in source code
• Call chain leading to error

Example Extraction:

FAILED tests/test_user.py::test_create_user - AssertionError: assert 201 == 200

Extracted:

• Test file: tests/test_user.py
• Test name: test_create_user
• Error type: AssertionError
• Expected: 200
• Actual: 201

Step 4: Identify Root Cause

Analyze why the error occurred:

Questions to Ask:

• What was the test expecting?
• What actually happened?
• Why did the actual behavior differ from expected?
• Is this a test bug or a code bug?

Common Root Causes:

• Logic error in implementation
• Incorrect test expectations
• Missing or wrong test data
• Race conditions or timing issues
• Environmental differences
• Outdated mocks or fixtures
• Dependency version conflicts

Step 5: Provide Explanation

Structure the explanation clearly:

## Error Summary

[One-sentence description of what went wrong]

## What Happened

[Explain the error in plain language]

**Expected:** [What the test expected to happen]
**Actual:** [What actually happened]

## Root Cause

[Explain why this happened - the underlying issue]

## How to Fix

### Option 1: [Most likely fix] ⭐

[Specific code change or action to take]

// Before [problematic code]

// After [fixed code]

**Why this works:** [Brief explanation]

### Option 2: [Alternative fix]

[Another possible solution if Option 1 doesn't apply]

## Verification

[How to verify the fix works]

[command to run test]

## Prevention

[How to avoid this error in the future]

Step 6: Suggest Next Steps

Provide actionable guidance:

If fix is clear:

• Show exact code change
• Provide test command to verify
• Mention related tests to check

If diagnosis needs more info:

• Suggest debugging steps
• Recommend logging/print statements to add
• Propose code to inspect

If issue is environmental:

• Suggest dependency checks
• Recommend configuration verification
• Propose environment comparison

Error Pattern Examples

Example 1: Assertion Failure (Python/pytest)

Error Output:

FAILED tests/test_calculator.py::test_divide - AssertionError: assert 2.5 == 2

Analysis:

## Error Summary
Test expected integer division but got float division result.

## What Happened

**Expected:** `2` (integer)
**Actual:** `2.5` (float)

The test `test_divide` called a divide function with arguments that produce
a decimal result (e.g., 5 / 2 = 2.5) but expected an integer result.

## Root Cause

The implementation uses true division (`/`) which returns floats in Python 3,
but the test expects integer division behavior.

## How to Fix

### Option 1: Update implementation to use integer division ⭐

Before

def divide(a, b): return a / b

After

def divide(a, b): return a // b # Integer division

**Why this works:** The `//` operator performs floor division, returning
integers when both operands are integers.

### Option 2: Update test to expect float result

Before

assert divide(5, 2) == 2

After

assert divide(5, 2) == 2.5

Use this if float division is the intended behavior.

## Verification

pytest tests/test_calculator.py::test_divide -v

## Prevention

- Clarify division requirements in function documentation
- Use type hints: `def divide(a: int, b: int) -> int:`
- Add test cases for both integer and float division scenarios

Example 2: TypeError (JavaScript/Jest)

Error Output:

TypeError: Cannot read property 'name' of undefined
    at getUserName (src/user.js:5:20)
    at Object.<anonymous> (tests/user.test.js:10:22)

Analysis:

## Error Summary
Attempted to access property 'name' on undefined user object.

## What Happened

**Expected:** User object with 'name' property
**Actual:** `undefined`

Function `getUserName()` tried to access `.name` property on a value that
was undefined, causing a runtime error.

## Root Cause

The user object passed to `getUserName()` was undefined. This commonly
happens when:
- Function called without argument
- Async data not yet loaded
- Database/API query returned no result

## How to Fix

### Option 1: Add null/undefined check ⭐

// Before function getUserName(user) { return user.name; }

// After function getUserName(user) { if (!user) { return null; // or throw error, or return default } return user.name; }

**Why this works:** Defensively handles cases where user might be undefined.

### Option 2: Fix the test to provide valid user object

// Before const name = getUserName(); // Called without argument

// After const name = getUserName({ name: 'Alice', id: 1 });

### Option 3: Use optional chaining (modern JS)

function getUserName(user) { return user?.name ?? 'Unknown'; }

## Verification

npm test -- user.test.js

## Prevention

- Add input validation to functions
- Use TypeScript for type safety
- Define clear function contracts
- Use optional chaining for property access

Example 3: Timeout Error (E2E/Cypress)

Error Output:

Timed out retrying after 4000ms: Expected to find element: [data-testid="submit-btn"],
but never found it.

Analysis:

## Error Summary
Test couldn't find submit button within timeout period.

## What Happened

**Expected:** Element with `data-testid="submit-btn"` to appear on page
**Actual:** Element never appeared within 4 seconds

## Root Cause

Possible reasons:
1. Element takes longer than 4s to render
2. Element has different selector/ID
3. Element only appears under certain conditions
4. JavaScript error preventing element from rendering

## How to Fix

### Option 1: Increase timeout for slow-loading elements ⭐