bug-fixing

name	bug-fixing
description	Test-driven bug fixing workflow. Activates when: fixing bugs, debugging issues, resolving defects, investigating errors, or when user mentions: bug, fix, broken, not working, error, issue, defect, regression.
argument-hint	["bug description or issue reference"]

Test-Driven Bug Fixing

A disciplined approach to fixing bugs: reproduce first, fix second. Write a failing test that captures the bug, then fix the code to make it pass.

Core Principle

Never start by trying to fix the bug. Instead:

1. Understand the bug
2. Write a test that reproduces it (fails)
3. Fix the bug
4. Verify the test passes
5. Document what was wrong

When to Use This Skill

Use this skill when:

• Fixing reported bugs or defects
• Investigating unexpected behavior
• Resolving regression issues
• Debugging error reports

Workflow

Phase 1: Understand the Bug

Before writing any code:

1. Gather information

   • Read any linked issues or error reports
   • Ask clarifying questions if reproduction steps are unclear

2. Identify the scope

   • Which files/classes are likely involved?
   • Can this be reproduced with a test?

3. Confirm understanding

   • Summarize the bug in one sentence
   • State the expected vs actual behavior
   • Get user confirmation before proceeding

Phase 2: Write the Failing Test

This is the critical step. Write a test that:

1. Reproduces the exact scenario that triggers the bug
2. Fails with the current code (proving the bug exists)
3. Will pass when the bug is fixed

it('handles edge case with empty input', function () {
    // Arrange: Set up the scenario that triggers the bug
    $subject = new Subject(input: []);

    // Act: Perform the action that fails
    $result = $subject->process();

    // Assert: What SHOULD happen (currently fails)
    expect($result)->toBe('expected outcome');
});

Phase 3: Verify Test Fails

Run the test to confirm it fails:

vendor/bin/pest --filter=handles_edge_case_with_empty_array_input

If the test passes: The bug may not be what we thought. Revisit Phase 1.

If the test fails: Proceed to fixing.

Phase 4: Fix the Bug

Use a subagent to investigate and fix:

Task: Fix the bug causing [test name] to fail.

Context:
- Test file: tests/RelevantTest.php
- Test method: handles_edge_case_with_empty_array_input
- Current error: [paste error message]

The test reproduces the bug. Find and fix the root cause.
Do NOT modify the test - only fix the production code.
Run the test after each change to verify progress.

Phase 5: Verify the Fix

Run quality checks based on which files were changed:

• Use the backend-quality skill (Tier 1 only: Pint + related tests). PHPStan and the full test suite run at completion — see the backend-quality skill for details.

Phase 6: Document the Fix

When creating/updating a PR, include a technical description:

### What was the bug?

Description of the root cause with file and line references.

### How was it fixed?

Description of the fix and why it works.

### Test coverage

Added `test_name` to verify the fix and prevent regression.

Test Writing Guidelines

Test the Specific Scenario

Don't test general functionality - test the exact scenario that was broken:

// Good - tests the specific bug scenario
it('expands wildcards when data has missing nested keys')

// Bad - too generic
it('expands wildcards')

Name Tests Descriptively

Test names should describe the scenario and expected outcome:

// Good
it('compiles rules with nested each() calls')
it('handles nullable fields with required children')
it('validates arrays with mixed scalar and object children')

// Bad
it('works')
it('handles arrays')

When Tests Aren't Possible

For bugs that can't be reproduced with tests:

1. Document why — explain why automated testing isn't feasible
2. Provide manual steps — detailed reproduction steps
3. Add defensive code — consider adding validation or error handling