Systematic debugging

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
• Build failures
• Integration issues

Not here: when the output is right but late — a slow endpoint, query, page, job, build, or test suite — use razorback:diagnosing-performance. A defect has a wrong answer to trace back to a wrong line; slowness has no wrong line, so it needs measurement instead of tracing.

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

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):

   # Layer 1: Workflow
   echo "=== Secrets available in workflow: ==="
   echo "IDENTITY: $([ -n "${IDENTITY:-}" ] && echo SET || echo UNSET)"
   
   # Layer 2: Build script
   echo "=== Env vars in build script: ==="
   echo "IDENTITY: $([ -n "${IDENTITY:-}" ] && echo SET || echo UNSET)"
   
   # 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 ✗)

<!-- Canonical Redact rule: skills/security-review/SKILL.md — update all copies together. -->

Redact:

• Redact every secret in anything you show, quote, or send — write <REDACTED> in its place.
• Build loops against env vars so the credential stays in the environment rather than in displayed output.
• From captured artifacts, quote only the lines that carry the signal.

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

Use Miller for investigation:

• Inspect the buggy function — understand callers, callees, type flow with inspect(target, depth=full) (the symbol at issue earns full)
• Find references — find all call sites that might trigger the bug with trace
• Orient on the broader subsystem with context

Phase 2: Pattern Analysis

Find the pattern before fixing:

1. Find Working Examples

   • Search for similar working code in the codebase with Miller search
   • Orient for a token-budgeted view of the relevant area with Miller context
   • 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

   • Inspect the broken function — see callers, callees, types, children with Miller inspect(target, depth=full)
   • What other components does this need?
   • What settings, config, environment?
   • What assumptions does it make?

Phase 3: Hypothesis and Testing

Scientific method:

1. Form Single Hypothesis

   • State clearly: "I think X is the root cause because Y"
   • Write it down
   • Be specific, not vague

2. Test Minimally

   • Make the SMALLEST possible change to test hypothesis
   • One variable at a time
   • Don't fix multiple things at once

3. Verify Before Continuing

   • Did it work? Yes → Phase 4
   • Didn't work? Form NEW hypothesis
   • DON'T add more fixes on top

4. When You Don't Know

   • Say "I don't understand X"
   • Don't pretend to know
   • Research more with Miller, targeted docs, and the smallest relevant verification command
   • In an approved autonomous run, stop only if the uncertainty matches the blocker taxonomy
   • Outside an approved run, ask one specific question if the research path is exhausted

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 razorback:test-driven-development skill for writing proper failing tests

2. Implement Single Fix

   • Before writing the fix, assess the blast radius: Miller impact(target='<symbol being changed>') returns the impacted symbols plus the likely tests
   • 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?
   • Required affected scope still green?
   • 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?

   In an approved autonomous run, route this through the blocker taxonomy: if a plan-consistent architecture fix exists, take it and log the decision; if the plan is contradicted or tests are unresolvable, stop as a real blocker. Outside an approved run, discuss the architecture before attempting more fixes.

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

Red Flags - STOP and Return to Phase 1

Every row below — whether it is your own thought or a redirection from the user — means the same thing: STOP. Return to Phase 1.

Signal	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" / "Quick fix for now, investigate later"	Systematic debugging is FASTER than guess-and-check thrashing. The "later" investigation never happens.
"Just try changing X and see if it works" / "Just try this first, then investigate"	First fix sets the pattern. Do it right from the start.
"It's probably X, let me fix that" / "I see the problem, let me fix it"	Seeing symptoms ≠ understanding root cause.
"Here are the main problems: [lists fixes without investigation]" — proposing solutions before tracing data flow	Fixes proposed before investigation are guesses wearing a diagnosis.
"Add multiple changes, run tests" / "Multiple fixes at once saves time"	Can't isolate what worked. Causes new bugs.
"Skip the test, I'll manually verify" / "I'll write test after confirming fix works"	Untested fixes don't stick. Test first proves it.
"Pattern says X but I'll adapt it differently" / "Reference too long, I'll adapt the pattern"	Partial understanding guarantees bugs. Read the reference completely.
"I don't fully understand but this might work"	Not understanding IS the finding. Investigate it, don't route around it.
"One more fix attempt" (after 2+ failures)	3+ failures = architectural problem. Question the pattern, don't fix again.
Each fix reveals a new problem in a different place	The coupling is the bug, not the symptom you just patched.
User asks "Is that not happening?"	You assumed without verifying.
User asks "Will it show us...?"	You should have added evidence gathering.
User says "Stop guessing"	You're proposing fixes without understanding.
User says "Ultra-think this"	Question fundamentals, not just symptoms.
User asks "We're stuck?" (frustrated)	Your approach isn't working.

If 3+ fixes failed: the problem is structural, not local. Stop patching and question the architecture with razorback:architecture-quality.

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:

• razorback:diagnosing-performance - When the output is right but late; measurement replaces tracing
• razorback:test-driven-development - For creating failing test case (Phase 4, Step 1)
• razorback:verification-before-completion - Verify fix worked before claiming success
• razorback:fixing-small-issues - When the root-caused fix meets the quick-fix criteria, execute it there: in place, affected-scope verification, no worktree or baseline-suite ceremony