/systematic-debugging

name: systematic-debugging
description: Root cause analysis for debugging. Use when bugs, test failures, or unexpected behavior have non-obvious causes, or after multiple fix attempts have failed.

Systematic Debugging

**Core principle:** Find root cause before attempting fixes. Symptom fixes are failure.

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

Phase 1: Root Cause Investigation

**BEFORE attempting ANY fix:**

1. **Read Error Messages Carefully**

• Read stack traces completely
• Note line numbers, file paths, error codes
• Don't skip warnings

2. **Reproduce Consistently**

• What are the exact steps?
• If not reproducible → gather more data, don't guess

3. **Check Recent Changes**

• Git diff, recent commits
• New dependencies, config changes
• Environmental differences

4. **Gather Evidence in Multi-Component Systems**

**WHEN system has multiple components (CI → build → signing, API → service → database):**

Add diagnostic instrumentation before proposing fixes:

   For EACH component boundary:
     - Log what data enters/exits component
     - Verify environment/config propagation
     - Check state at each layer

   Run once to gather evidence → analyze → identify failing component

Example:

   # Layer 1: Workflow
   echo "=== Secrets available: ==="
   echo "IDENTITY: ${IDENTITY:+SET}${IDENTITY:-UNSET}"

   # Layer 2: Build script
   env | grep IDENTITY || echo "IDENTITY not in environment"

   # Layer 3: Signing
   security find-identity -v

5. **Trace Data Flow**

See `references/root-cause-tracing.md` for backward tracing technique.

Quick version: Where does bad value originate? Trace up call chain until you find the source. Fix at source.

Phase 2: Pattern Analysis

1. **Find Working Examples** - Similar working code in codebase 2. **Compare Against References** - Read reference implementations COMPLETELY, don't skim 3. **Identify Differences** - List every difference, don't assume "that can't matter" 4. **Understand Dependencies** - Components, config, environment, assumptions

Phase 3: Hypothesis and 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** - Worked → Phase 4. Didn't work → form NEW hypothesis, don't stack fixes 4. **When You Don't Know** - Say so. Don't pretend.

Phase 4: Implementation

1. **Create Failing Test Case**

• Use the `test-driven-development` skill
• MUST have before fixing

2. **Implement Single Fix**

• ONE change at a time
• No "while I'm here" improvements

3. **Verify Fix**

• Test passes? Other tests still pass? Issue resolved?

4. **If Fix Doesn't Work**

• Count attempts
• If < 3: Return to Phase 1 with new information
• **If ≥ 3: Escalate (below)**

Escalation: 3+ Failed Fixes

**Pattern indicating architectural problem:**

• Each fix reveals new problems elsewhere
• Fixes require massive refactoring
• Shared state/coupling keeps surfacing

**Action:** STOP. Question fundamentals:

• Is this pattern fundamentally sound?
• Are we continuing through inertia?
• Refactor architecture vs. continue fixing symptoms?

**Discuss with human partner before more fix attempts.** This is wrong architecture, not failed hypothesis.

Red Flags → STOP and Return to Phase 1

If you catch yourself thinking:

• "Quick fix for now, investigate later"
• "Just try changing X"
• "I'll skip the test"
• "It's probably X"
• "Pattern says X but I'll adapt it differently"
• Proposing solutions before tracing data flow
• "One more fix" after 2+ failures

Human Signals You're Off Track

• "Is that not happening?" → You assumed without verifying
• "Will it show us...?" → You should have added evidence gathering
• "Stop guessing" → You're proposing fixes without understanding
• "Ultrathink this" → Question fundamentals
• Frustrated "We're stuck?" → Your approach isn't working

**Response:** Return to Phase 1.

Supporting Techniques

Reference files in `references/`:

• **`root-cause-tracing.md`** - Trace bugs backward through call stack
• **`defense-in-depth.md`** - Add validation at multiple layers after finding root cause
• **`condition-based-waiting.md`** - Replace arbitrary timeouts with condition polling

Related skills:

• **`test-driven-development`** - Creating failing test case (Phase 4)
• **`verification-before-completion`** - Verify fix before claiming success