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Systematic Debugger

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you haven't completed Phase 1, you cannot propose fixes. Quick patches mask underlying issues.

The Four Phases

Phase 1: Root Cause Investigation (MANDATORY FIRST)

1. Read Error Messages Carefully — stack traces completely, note line numbers and error codes
2. Reproduce Consistently — can you trigger it reliably? if not, gather more data
3. Check Recent Changes — git diff, recent commits, new dependencies
4. Gather Evidence in Multi-Component Systems
   • Add diagnostic logging at each component boundary
   • Log what enters and exits each layer
   • Run once to gather evidence showing WHERE it breaks
5. Trace Data Flow — where does the bad value originate? trace backward up the call stack

Phase 2: Pattern Analysis

1. Find Working Examples — similar working code in the same codebase
2. Compare Against References — read reference implementations completely
3. Identify Differences — list every difference, however small
4. Understand Dependencies — what config, environment, or state does this assume?

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis — "I think X is the root cause because Y"
2. Test Minimally — smallest possible change to test hypothesis
3. Verify Before Continuing — if it worked → Phase 4; if not → form NEW hypothesis
4. One variable at a time — never add multiple changes simultaneously

Phase 4: Implementation

1. Create Failing Test Case — before fixing, write a test that reproduces the bug
2. Implement Single Fix — address the root cause, not the symptom
3. Verify Fix — test passes? no other tests broken?
4. If Fix Doesn't Work (3+ attempts) — STOP and question the architecture

When 3+ Fixes Fail

Three failures indicate an architectural problem, not a surface bug:

• Each fix reveals new shared state/coupling elsewhere
• Fixes require "massive refactoring" to implement
• Each fix creates new symptoms elsewhere

Stop and discuss with the user before attempting more fixes.

Red Flags — Stop and Return to Phase 1

If you catch yourself thinking:

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• "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+)

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

Common Rationalizations vs. Reality

Supporting Techniques

• Add diagnostic logging at component boundaries first
• Binary search through the call stack to find failure point
• Isolate the minimal reproduction case
• Read the error message — it often contains the exact answer

Real-World Impact

• Systematic approach: 15-30 minutes to fix
• Random fixes approach: 2-3 hours of thrashing
• First-time fix rate: 95% vs 40%