/systematic-debugging

Workflow Diagram

4-phase root cause debugging methodology. Enforces the iron law: no fixes without root cause investigation first. Phases: root cause investigation, pattern analysis, hypothesis and testing (with isolated-testing and verifying-hunches sub-skills), and implementation with a 3-fix circuit breaker that escalates to architectural review.

flowchart TD
  Start([Start]) --> P1[Phase 1: Root Cause\nInvestigation]
  P1 --> ReadErrors[Read error messages\nand stack traces]
  ReadErrors --> Reproduce[Reproduce consistently]
  Reproduce --> Reproducible{Reproducible?}
  Reproducible -- No --> GatherData[Gather more data]
  GatherData --> Reproduce
  Reproducible -- Yes --> CheckChanges[Check recent changes\ngit diff]
  CheckChanges --> MultiComp{Multi-component\nsystem?}
  MultiComp -- Yes --> Instrument[Add diagnostic\ninstrumentation]
  Instrument --> RunDiag[Run once for evidence]
  RunDiag --> IdentifyLayer[Identify failing layer]
  IdentifyLayer --> TraceFlow
  MultiComp -- No --> TraceFlow[Trace data flow\nto source]
  TraceFlow --> P2[Phase 2: Pattern Analysis]
  P2 --> FindWorking[Find working examples]
  FindWorking --> CompareRef[Compare against\nreferences]
  CompareRef --> ListDiffs[Identify all\ndifferences]
  ListDiffs --> CheckDeps[Understand dependencies]
  CheckDeps --> P3[Phase 3: Hypothesis\nand Testing]
  P3 --> InvokeIsolated[/Invoke isolated-testing/]
  InvokeIsolated --> FormHypothesis[Form single hypothesis]
  FormHypothesis --> DesignTest[Design repro test\nwith predictions]
  DesignTest --> Execute[Execute test ONCE]
  Execute --> Verdict{Result?}
  Verdict -- Reproduced --> VerifyHunch[/Invoke verifying-hunches/]
  VerifyHunch --> P4[Phase 4: Implementation]
  Verdict -- Disproved --> FormHypothesis
  Verdict -- Inconclusive --> RefineTest[Refine test]
  RefineTest --> Execute
  P4 --> CreateTest[Create failing test\ncase]
  CreateTest --> TDD[/Invoke TDD skill/]
  TDD --> SingleFix[Implement single fix]
  SingleFix --> VerifyFix{Fix works?}
  VerifyFix -- Yes --> NoRegression{No regressions?}
  NoRegression -- Yes --> Done([Done])
  NoRegression -- No --> SingleFix
  VerifyFix -- No --> FixCount{Fixes\nattempted >= 3?}
  FixCount -- No --> P1
  FixCount -- Yes --> ArchReview[STOP: Question\narchitecture]
  ArchReview --> Discuss[Discuss with user\nbefore more fixes]
  Discuss --> ArchDecision{Refactor\narchitecture?}
  ArchDecision -- Yes --> Refactor([Architectural refactor])
  ArchDecision -- No --> P1

  style Start fill:#4CAF50,color:#fff
  style Done fill:#4CAF50,color:#fff
  style Refactor fill:#4CAF50,color:#fff
  style InvokeIsolated fill:#4CAF50,color:#fff
  style VerifyHunch fill:#4CAF50,color:#fff
  style TDD fill:#4CAF50,color:#fff
  style Reproducible fill:#FF9800,color:#fff
  style MultiComp fill:#FF9800,color:#fff
  style Verdict fill:#FF9800,color:#fff
  style VerifyFix fill:#f44336,color:#fff
  style NoRegression fill:#f44336,color:#fff
  style FixCount fill:#f44336,color:#fff
  style ArchDecision fill:#FF9800,color:#fff
  style P1 fill:#2196F3,color:#fff
  style P2 fill:#2196F3,color:#fff
  style P3 fill:#2196F3,color:#fff
  style P4 fill:#2196F3,color:#fff
  style ReadErrors fill:#2196F3,color:#fff
  style Reproduce fill:#2196F3,color:#fff
  style GatherData fill:#2196F3,color:#fff
  style CheckChanges fill:#2196F3,color:#fff
  style Instrument fill:#2196F3,color:#fff
  style RunDiag fill:#2196F3,color:#fff
  style IdentifyLayer fill:#2196F3,color:#fff
  style TraceFlow fill:#2196F3,color:#fff
  style FindWorking fill:#2196F3,color:#fff
  style CompareRef fill:#2196F3,color:#fff
  style ListDiffs fill:#2196F3,color:#fff
  style CheckDeps fill:#2196F3,color:#fff
  style FormHypothesis fill:#2196F3,color:#fff
  style DesignTest fill:#2196F3,color:#fff
  style Execute fill:#2196F3,color:#fff
  style RefineTest fill:#2196F3,color:#fff
  style CreateTest fill:#2196F3,color:#fff
  style SingleFix fill:#2196F3,color:#fff
  style ArchReview fill:#f44336,color:#fff
  style Discuss fill:#2196F3,color:#fff

Legend

Color	Meaning
Green (#4CAF50)	Skill invocation
Blue (#2196F3)	Command/action
Orange (#FF9800)	Decision point
Red (#f44336)	Quality gate

Command Content

# Systematic Debugging

## Overview

Random fixes waste time and create new bugs. Quick patches mask underlying issues.

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

**Violating the letter of this process is violating the spirit of debugging.**

## The Iron Law

```
NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST
```

If you haven't completed Phase 1, you cannot propose fixes.

## When to Use

Use for ANY technical issue:
- Test failures
- Bugs in production
- Unexpected behavior
- Performance problems
- Build failures
- Integration issues

**Use this ESPECIALLY when:**
- Under time pressure (emergencies make guessing tempting)
- "Just one quick fix" seems obvious
- You've already tried multiple fixes
- Previous fix didn't work
- You don't fully understand the issue

**Don't skip when:**
- Issue seems simple (simple bugs have root causes too)
- You're in a hurry (rushing guarantees rework)
- Manager wants it fixed NOW (systematic is faster than thrashing)

## The Four Phases

You MUST complete each phase before proceeding to the next.

### Phase 1: Root Cause Investigation

<!-- SUBAGENT: CONDITIONAL - If searching codebase for patterns/similar code, use Explore subagent. If reading specific known files, use direct Read. Stay in main context for evidence accumulation. -->

**BEFORE attempting ANY fix:**

1. **Read Error Messages Carefully**
   - Don't skip past errors or warnings
   - They often contain the exact solution
   - Read stack traces completely
   - Note line numbers, file paths, error codes

2. **Reproduce Consistently**
   - Can you trigger it reliably?
   - What are the exact steps?
   - Does it happen every time?
   - If not reproducible → gather more data, don't guess

3. **Check Recent Changes**
   - What changed that could cause this?
   - 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):**

   **BEFORE proposing fixes, add diagnostic instrumentation:**
   ```
   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
     - Check state at each layer

   Run once to gather evidence showing WHERE it breaks
   THEN analyze evidence to identify failing component
   THEN investigate that specific component
   ```

   **Example (multi-layer system):**
   ```bash
   # Layer 1: Workflow
   echo "=== Secrets available in workflow: ==="
   echo "IDENTITY: ${IDENTITY:+SET}${IDENTITY:-UNSET}"

   # Layer 2: Build script
   echo "=== Env vars in build script: ==="
   env | grep IDENTITY || echo "IDENTITY not in environment"

   # Layer 3: Signing script
   echo "=== Keychain state: ==="
   security list-keychains
   security find-identity -v

   # Layer 4: Actual signing
   codesign --sign "$IDENTITY" --verbose=4 "$APP"
   ```

   **This reveals:** Which layer fails (secrets → workflow ✓, workflow → build ✗)

5. **Trace Data Flow**

   **WHEN error is deep in call stack:**

   See `root-cause-tracing.md` in this directory for the complete backward tracing technique.

   **Quick version:**
   - Where does bad value originate?
   - What called this with bad value?
   - Keep tracing up until you find the source
   - Fix at source, not at symptom

### Phase 2: Pattern Analysis

<!-- SUBAGENT: NO - Stay in main context. Sequential dependent work building on Phase 1 evidence. Accumulated state required. -->

**Find the pattern before fixing:**

1. **Find Working Examples**
   - Locate similar working code in same codebase
   - What works that's similar to what's broken?

2. **Compare Against References**
   - If implementing pattern, read reference implementation COMPLETELY
   - Don't skim - read every line
   - Understand the pattern fully before applying

3. **Identify Differences**
   - What's different between working and broken?
   - List every difference, however small
   - Don't assume "that can't matter"

4. **Understand Dependencies**
   - What other components does this need?
   - What settings, config, environment?
   - What assumptions does it make?

### Phase 3: Hypothesis and Testing

<CRITICAL>
**INVOKE `isolated-testing` SKILL BEFORE ANY EXPERIMENT.**

This phase requires patience and discipline. You are not "trying things." You are testing hypotheses.
</CRITICAL>

**Isolated Testing Protocol:**

1. **Form Single Hypothesis**
   - State clearly: "I think X is the root cause because Y"
   - Write it down
   - Be specific, not vague
   - **CRITICAL:** Before claiming you "found it," invoke `verifying-hunches` skill

2. **Design Repro Test BEFORE Execution**
   - Write the COMPLETE test procedure
   - Define what you will see if hypothesis is CORRECT
   - Define what you will see if hypothesis is WRONG
   - Get approval (unless autonomous mode)

3. **Execute ONCE**
   - Run the test EXACTLY as designed
   - Capture output
   - Compare to predictions

4. **Verdict**
   - **REPRODUCED:** Bug reproduces under this hypothesis -> FULL STOP, announce, wait (or proceed to fix if autonomous)
   - **DISPROVED:** Result matches "wrong" prediction -> Mark DISPROVED, form NEW hypothesis
   - **INCONCLUSIVE:** Neither matches -> Note what happened, refine test or continue
   - DON'T add more fixes on top
   - **Register disproven hypothesis** - prevents rediscovery after compaction

5. **When You Don't Know**
   - Say "I don't understand X"
   - Don't pretend to know
   - Ask for help
   - Research more

<HUNCH_CHECK>
When you feel like saying "I found it" or "this is the root cause":
1. STOP - that's a hypothesis, not a finding
2. Invoke `verifying-hunches` skill
3. Complete specificity check (exact location, mechanism, symptom link)
4. Define falsification criteria
5. Run test with prediction vs actual comparison
6. Only claim "confirmed" after evidence matches prediction
</HUNCH_CHECK>

<CHAOS_CHECK>
If you catch yourself doing ANY of these, STOP and return to step 1:
- "Let me try..." / "Maybe if I..." / "What about..."
- Running without a designed test
- Changing multiple things between tests
- Continuing after bug reproduces
- Testing theory A but making change related to theory B
</CHAOS_CHECK>

### Phase 4: Implementation

**Fix the root cause, not the symptom:**

1. **Create Failing Test Case**
   - Simplest possible reproduction
   - Automated test if possible
   - One-off test script if no framework
   - MUST have before fixing
   - Use the `test-driven-development` skill for writing proper failing tests

2. **Implement Single Fix**
   - Address the root cause identified
   - ONE change at a time
   - No "while I'm here" improvements
   - No bundled refactoring

3. **Verify Fix**
   - Test passes now?
   - No other tests broken?
   - Issue actually resolved?

4. **If Fix Doesn't Work**
   - STOP
   - Count: How many fixes have you tried?
   - If < 3: Return to Phase 1, re-analyze with new information
   - **If ≥ 3: STOP and question the architecture (step 5 below)**
   - DON'T attempt Fix #4 without architectural discussion

5. **If 3+ Fixes Failed: Question Architecture**

   **Pattern indicating architectural problem:**
   - Each fix reveals new shared state/coupling/problem in different place
   - Fixes require "massive refactoring" to implement
   - Each fix creates new symptoms elsewhere

   **STOP and question fundamentals:**
   - Is this pattern fundamentally sound?
   - Are we "sticking with it through sheer inertia"?
   - Should we refactor architecture vs. continue fixing symptoms?

   **Discuss with your human partner before attempting more fixes**

   This is NOT a failed hypothesis - this is a wrong architecture.

## Red Flags - STOP and Follow Process

If you catch yourself 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"
- "Pattern says X but I'll adapt it differently"
- "Here are the main problems: [lists fixes without investigation]"
- Proposing solutions before tracing data flow
- **"One more fix attempt" (when already tried 2+)**
- **Each fix reveals new problem in different place**
- **"I found it!" or "This is the issue!"** (premature eureka - invoke verifying-hunches)
- **"I think I see what's happening"** (vague pattern-match - needs specificity)
- **Same theory you had before** (deja vu - check if previously disproven)
- **"Let me try..." / "Maybe if I..." / "What about..."** (chaos - invoke isolated-testing)
- **Making changes without a designed test** (action without design)
- **Testing multiple theories at once** (no isolation)
- **Continuing after bug reproduced** (stop on reproduction)

**ALL of these mean: STOP. Return to Phase 1.**

**If 3+ fixes failed:** Question the architecture (see Phase 4.5)

## your human partner's Signals You're Doing It Wrong

**Watch for these redirections:**
- "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, not just symptoms
- "We're stuck?" (frustrated) - Your approach isn't working

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

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Issue is simple, don't need process" | Simple issues have root causes too. Process is fast for simple bugs. |
| "Emergency, no time for process" | Systematic debugging is FASTER than guess-and-check thrashing. |
| "Just try this first, then investigate" | First fix sets the pattern. Do it right from the start. |
| "I'll write test after confirming fix works" | Untested fixes don't stick. Test first proves it. |
| "Multiple fixes at once saves time" | Can't isolate what worked. Causes new bugs. |
| "Reference too long, I'll adapt the pattern" | Partial understanding guarantees bugs. Read it completely. |
| "I see the problem, let me fix it" | Seeing symptoms ≠ understanding root cause. |
| "One more fix attempt" (after 2+ failures) | 3+ failures = architectural problem. Question pattern, don't fix again. |

## Quick Reference

| Phase | Key Activities | Success Criteria |
|-------|---------------|------------------|
| **1. Root Cause** | Read errors, reproduce, check changes, gather evidence | Understand WHAT and WHY |
| **2. Pattern** | Find working examples, compare | Identify differences |
| **3. Hypothesis** | Form theory, test minimally | Confirmed or new hypothesis |
| **4. Implementation** | Create test, fix, verify | Bug resolved, tests pass |

## When Process Reveals "No Root Cause"

If systematic investigation reveals issue is truly environmental, timing-dependent, or external:

1. You've completed the process
2. Document what you investigated
3. Implement appropriate handling (retry, timeout, error message)
4. Add monitoring/logging for future investigation

**But:** 95% of "no root cause" cases are incomplete investigation.

## Supporting Techniques

These techniques are part of systematic debugging and available in this directory:

- **`root-cause-tracing.md`** - Trace bugs backward through call stack to find original trigger
- **`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** - For creating failing test case (Phase 4, Step 1)
- **verification-before-completion** - Verify fix worked before claiming success

## Real-World Impact

From debugging sessions:
- Systematic approach: 15-30 minutes to fix
- Random fixes approach: 2-3 hours of thrashing
- First-time fix rate: 95% vs 40%
- New bugs introduced: Near zero vs common