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name: software-debugging description: Find and fix bugs systematically. Use when encountering errors, test failures, or unexpected behavior. Hypothesis-driven debugging to locate root causes and implement minimal fixes.

Software Debugging

You find and fix bugs systematically.

Method

Hypothesis-driven debugging. Test hypotheses. Find root cause. Fix minimally.

Process

1. Reproduce

• Isolate minimal steps
• Verify consistent reproduction
• Document exact conditions
• Note environment factors

2. Gather Evidence

CopyError: [Exact message]
Stack: [Key frames]
When: [Conditions]
Changed: [Recent changes]

Hypotheses:
1. [Most likely]
2. [Second possibility]
3. [Edge case]

3. Test Hypotheses

For each:

• Test: [How to verify]
• Expected: [What should happen]
• Actual: [What happened]
• Result: Confirmed/Rejected

4. Root Cause

CopyCause: [Actual problem]
Not: [What seemed wrong but wasn't]
Why missed: [Testing gap]

5. Fix

• Implement minimal fix
• Verify resolution
• Check side effects
• Add regression test

Investigation Pattern

Narrow down:

• Binary search through code paths
• Add strategic logging
• Isolate failing component
• Identify exact failure point

Fix:

• Minimal change only
• Root cause, not symptoms
• Preserve existing behavior
• Keep changes traceable

Verify:

Copyjust test        # Regression test passes
just check-all   # No side effects

Common Bug Types

Type bugs:

• None/null handling
• Type mismatches
• Undefined variables

State bugs:

• Race conditions
• Stale data
• Initialization order

Logic bugs:

• Off-by-one
• Boundary conditions
• Wrong assumptions

Integration bugs:

• API contract violations
• Version incompatibilities
• Configuration issues

Logging Strategy

Strategic points only:

Copylogger.debug(f"Entering {fn} with {args}")
logger.debug(f"State before: {state}")
logger.debug(f"Decision: {condition} = {value}")
logger.error(f"Expected {expected}, got {actual}")

Fix Principles

Minimal:

• Fix root cause only
• Don't refactor while fixing
• One change at a time

Defensive:

• Add appropriate guards
• Validate inputs
• Handle edge cases
• Fail gracefully

Tested:

• Add test for bug
• Test boundary conditions
• Verify error handling

Output Format

Copy## Bug: [Description]

### Reproduction
Steps: [Minimal]
Frequency: [Always/Sometimes/Rare]

### Root Cause
Problem: [Exact issue]
Location: file.py:123
Why: [Explanation]

### Fix
# Before
[problematic code]

# After
[fixed code]

### Verification
- [ ] Original issue resolved
- [ ] No side effects
- [ ] Regression test added

Prevention

After fixing, suggest:

1. Code improvements to prevent similar bugs
2. Testing gaps to fill
3. Documentation that would help
4. Monitoring to catch earlier

Red Flags

• Fixing symptoms, not root cause
• Refactoring during bug fix
• No regression test added
• Side effects not checked
• Changes too broad

Fix the root cause. Add a test. Move on.