Systematic Debugging

Core principle: NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST.

When to Use

Use for ANY technical issue: test failures, bugs, unexpected behavior, performance problems, build failures, integration issues.

Especially when:

• Under time pressure (emergencies make guessing tempting)
• "Just one quick fix" seems obvious
• Previous fix didn't work
• You don't fully understand the issue

The Four Phases

Complete each phase before proceeding to the next.

Phase 1: Root Cause Investigation

MUST complete ALL before Phase 2 (copy to TodoWrite): □ Error message copied verbatim | □ Reproduction confirmed | □ Recent changes reviewed (git diff) | □ Evidence from ALL components | □ Data flow traced (origin → error)

1. Read Error Messages - Stack traces completely, line numbers, file paths, error codes. Don't skip warnings.
2. Reproduce Consistently - Exact steps to trigger. Intermittent → gather more data.
3. Check Recent Changes - git diff, recent commits, new dependencies, config changes.
4. Multi-Component Systems - Log at each boundary: what enters, what exits, env/config state. Run once, analyze, identify failing layer.
5. Trace Data Flow - Error deep in stack? Use root-cause-tracing skill. Quick: Where does bad value originate? Trace up call stack, fix at source not symptom.

Phase 1 Summary: Error: [exact] | Reproduces: [steps] | Recent changes: [commits] | Component evidence: [each] | Data origin: [source]

Phase 2: Pattern Analysis

1. Find Working Examples - Similar working code in codebase. What works that's similar to what's broken?
2. Compare Against References - Read reference implementation COMPLETELY. Don't skim - understand fully.
3. Identify Differences - List EVERY difference (working vs broken). Don't assume "that can't matter."
4. Understand Dependencies - What components, config, environment needed? What assumptions does it make?

Phase 3: Hypothesis Testing

1. Form Single Hypothesis - "I think X is root cause because Y" - Be specific.
2. Test Minimally - SMALLEST possible change. One variable at a time.
3. Verify and Track - H#1: [what] → [result] | H#2: [what] → [result] | H#3: [what] → [STOP if fails] If 3 hypotheses fail: STOP immediately → "3 hypotheses failed, architecture review required" → Discuss with partner before more attempts.
4. When You Don't Know - Say "I don't understand X." Ask for help. Research more.

Phase 4: Implementation

Fix root cause, not symptom:

1. Create Failing Test - Simplest reproduction. Use ring:test-driven-development skill.
2. Implement Single Fix - Address root cause only. ONE change at a time. No "while I'm here" improvements.
3. Verify Fix - Test passes? No other tests broken? Issue resolved?
4. If Fix Doesn't Work - Count fixes. If < 3: Return to Phase 1. If ≥ 3: STOP → Architecture review required.
5. After Fix Verified - Test passes and issue resolved? Move to post-completion review.
6. If 3+ Fixes Failed - Pattern: each fix reveals new problem elsewhere, requires massive refactoring, creates new symptoms. STOP and discuss: Is architecture sound? Should we refactor vs. fix?

Time Limits

Debugging time boxes:

• 30 min without root cause → Escalate
• 3 failed fixes → Architecture review
• 1 hour total → Stop, document, ask for guidance

Red Flags

STOP and return to Phase 1 if thinking:

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• "Add multiple changes, run tests"
• "Skip the test, I'll manually verify"
• "It's probably X, let me fix that"
• "I don't fully understand but this might work"
• "One more fix attempt" (when already tried 2+)
• "Each fix reveals new problem" (architecture issue)

User signals you're wrong:

• "Is that not happening?" → You assumed without verifying
• "Stop guessing" → You're proposing fixes without understanding
• "We're stuck?" → Your approach isn't working

When you see these: STOP. Return to Phase 1.

Quick Reference

Phase	Key Activities	Success Criteria
1. Root Cause	Read errors, reproduce, check changes, gather evidence, trace data flow	Understand WHAT and WHY
2. Pattern	Find working examples, compare differences, understand dependencies	Identify what's different
3. Hypothesis	Form theory, test minimally, verify one at a time	Confirmed or new hypothesis
4. Implementation	Create test, fix root cause, verify	Bug resolved, tests pass

Circuit breakers:

• 3 hypotheses fail → STOP, architecture review
• 3 fixes fail → STOP, question fundamentals
• 30 min no root cause → Escalate

Integration with Other Skills

Required sub-skills:

• root-cause-tracing - When error is deep in call stack (Phase 1, Step 5)
• ring:test-driven-development - For failing test case (Phase 4, Step 1)

Complementary:

• defense-in-depth - Add validation after finding root cause
• verification-before-completion - Verify fix worked before claiming success

Blocker Criteria

STOP and report if:

Decision Type	Blocker Condition	Required Action
Cannot reproduce issue	Bug is intermittent and no consistent reproduction steps	STOP and gather more data before proceeding
3 hypotheses failed	Three consecutive hypotheses tested and all failed	STOP and escalate for architecture review
3 fixes failed	Three fix attempts made but issue persists	STOP and question fundamental assumptions
30 minutes without root cause	Extended investigation without identifying root cause	STOP and escalate for help
Fix reveals new problem	Each attempted fix creates different symptoms	STOP and request architecture review

Cannot Be Overridden

The following requirements CANNOT be waived:

• MUST complete Phase 1 (Root Cause Investigation) before any code changes
• MUST form evidence-based hypothesis before attempting fixes
• CANNOT skip creating a failing test in Phase 4
• MUST stop and escalate after 3 failed hypotheses or fixes
• CANNOT claim fix is complete without fresh verification

Severity Calibration

Severity	Condition	Required Action
CRITICAL	Production system affected	MUST prioritize and escalate immediately
CRITICAL	Data corruption or loss risk	MUST stop other work and focus exclusively
HIGH	User-facing functionality broken	MUST investigate before other tasks
HIGH	Tests failing on main branch	MUST resolve before merging new code
MEDIUM	Non-critical feature affected	Should investigate within current session
LOW	Minor behavior deviation	Fix in next iteration if time permits

Anti-Rationalization Table

Rationalization	Why It's WRONG	Required Action
"Quick fix for now, investigate later"	Later never comes. Quick fixes mask root causes.	MUST complete Phase 1 before any code change
"Just try changing X and see"	Guessing wastes time and introduces new bugs	MUST form hypothesis with evidence first
"I'm confident this is the issue"	Confidence without evidence is guessing	MUST verify with reproduction steps
"One more fix attempt" (after 2+)	Pattern indicates architecture issue, not bug	STOP and escalate for architecture review
"Each fix reveals new problem"	Symptom of deeper structural issue	STOP: architecture review REQUIRED
"I don't fully understand but this might work"	Partial understanding = partial fix = new bugs	MUST understand root cause before fixing
"Skip the test, I'll manually verify"	Manual verification is unreliable and non-repeatable	MUST create failing test (Phase 4, Step 1)
"Previous run showed it works"	Stale evidence is not evidence	MUST re-verify with fresh execution

Pressure Resistance

User Says	Your Response
"Just fix it quickly, we don't have time"	"MUST follow the phases - skipping investigation causes more delays through repeated fixes"
"Try this change, I think it'll work"	"I'll add that as a hypothesis and test it properly in Phase 3"
"We've tried 5 fixes, try one more"	"CANNOT proceed - 3+ failed fixes requires architecture review before more attempts"
"Skip the investigation, the cause is obvious"	"MUST verify the obvious before acting - obvious causes are often wrong"

Required Patterns

This skill uses these universal patterns:

• State Tracking: See skills/shared-patterns/state-tracking.md
• Failure Recovery: See skills/shared-patterns/failure-recovery.md
• Exit Criteria: See skills/shared-patterns/exit-criteria.md
• TodoWrite: See skills/shared-patterns/todowrite-integration.md

Apply ALL patterns when using this skill.