Root Cause Tracing

Core Principle

TRACE BACKWARD TO THE BUG'S ORIGIN
Don't patch symptoms - fix the disease

The Backward Trace Method

SYMPTOM
   ↓ Where does this value come from?
IMMEDIATE CAUSE
   ↓ What set that to wrong value?
DEEPER CAUSE
   ↓ Why did that happen?
ROOT CAUSE ← Fix HERE

Practical Steps

1. Start at Symptom

Q: What exactly went wrong?
→ Document the exact symptom
→ Note file, line, error message

2. First Backward Step

Q: Where does this value/state come from?
→ Find the direct source
→ Check: is THIS the bug or just a carrier?

3. Keep Tracing

Q: What set THAT to the wrong value?
→ Follow the data flow backward
→ Each step: is this the origin or just passing it along?

4. Identify Root

SIGNS YOU'VE FOUND ROOT:
- Logic error in requirements interpretation
- Initial state incorrectly set
- Wrong assumption in algorithm
- Missing validation at entry point
- Broken invariant at creation

Stop Conditions

You've found root cause when:

• There's no earlier source to trace
• The error was introduced HERE (not inherited)
• Fixing HERE prevents the bug entirely
• The "why" answer is: "wrong logic/design"

You're still at symptoms if:

• The wrong value came from somewhere else
• Another function passed bad data
• It's a consequence, not origin
• Fixing here = adding a workaround

Common Patterns

Bug Pattern	Root Usually At
Wrong value displayed	Data calculation, not display code
Null pointer	Where object should've been created
Off-by-one	Loop bounds definition, not body
Race condition	Shared state design, not specific access
Invalid state	State transition logic, not current handler

Anti-Patterns (Don't Do This)

❌ Symptom Patching

// Bad: Fix where error happens
if (value === undefined) value = defaultValue
// Good: Find why value is undefined

❌ Guard Stacking

// Bad: Add guards everywhere
if (x) if (y) if (z) doThing()
// Good: Ensure x, y, z correct at source

❌ Quick Fix Pressure

"Just make it work" → Technical debt
Better: "5 more minutes tracing saves 5 hours debugging later"

Verification

After identifying root cause:

1. Explain the chain - Symptom ← caused by ← caused by ← ROOT
2. Confirm origin - "This is where wrong value first appears"
3. Fix at root - Change the origin, not the symptoms
4. Verify fix - Original symptom gone + no new bugs

Integration with Checkers

Use with:

• assumption-checker → Was root cause an assumption?
• dependency-validator → Was root cause a broken dependency?
• pre-action-verifier → Verify root cause fix before applying