Debug Mode

Systematic bug investigation and resolution.

Core Approach

"Don't guess. Form hypotheses. Test them."

The 4-Phase Process

Phase 1: Assessment 🔍

Goal: Understand and reproduce

• What is the expected behavior?

• What is the actual behavior?

• Can you reliably reproduce?

• What changed recently?

Key Questions:

• When did this start happening?

• Does it happen consistently or intermittently?

• What are the exact inputs that trigger it?

• What error messages or symptoms appear?

Phase 2: Investigation 🔬

Goal: Isolate and trace

• Trace execution from entry point

• Identify where expected diverges from actual

• Form hypotheses about root cause

• Test hypotheses systematically

Techniques:

• Add strategic logging/prints

• Use debugger breakpoints

• Simplify inputs to minimal reproduction

• Check boundary conditions

Phase 3: Resolution 🔧

Goal: Fix minimally and verify

• Implement the smallest fix that addresses root cause

• Don't fix symptoms, fix the disease

• Add regression test

• Verify fix doesn't break other things

If fix doesn't work:

• Count: How many fixes attempted?

• If < 3: Return to Phase 1, re-analyze with new information

• If ≥ 3: STOP. Question your understanding of the system.

Phase 4: Quality ✅

Goal: Prevent recurrence

• Add test covering the bug

• Document if the cause was non-obvious

• Consider if similar bugs exist elsewhere

• Clean up debug code

Debugging Checklist

• Reproduced: Can trigger bug consistently
• Isolated: Know which component is failing
• Root Cause: Understand WHY it fails
• Fixed: Minimal change addresses cause
• Tested: Regression test added
• Clean: Debug code removed

Hypothesis Template

For each hypothesis, record: Hypothesis (what's wrong) → Test (how to verify) → Result → Conclusion (confirmed/rejected/needs more info).

Common Root Causes

Symptom Often Caused By

Works locally, fails in CI Environment differences, missing deps

Intermittent failure Race condition, timing, external dependency

Wrong output Logic error, wrong variable, off-by-one

Crash/exception Null/None access, type mismatch, missing data

Performance issue N+1 queries, missing index, memory leak

Rationalization Prevention

Excuse Reality Required Action

"The fix is obvious" Obvious fixes mask root causes Form a hypothesis and verify before changing code

"It's probably X" "Probably" isn't evidence Test the hypothesis — name it, design a test, run it

"This is too simple to debug formally" Simple bugs waste the most time undiagnosed Follow Phase 1 — reproduce, isolate, then fix

"Logs look clean" You didn't add targeted logging Add debug logging at the suspected point

"I've tried 3 things, might as well try a 4th" Stacking guesses compounds confusion STOP. Return to Phase 1. Re-analyze with new info

"It works now" If you don't know why, it will break again Explain WHY it works and what changed

Red Flags - STOP and Re-Assess

If you catch yourself skipping reproduction ("I know what's wrong") or testing multiple hypotheses at once — STOP. Return to Phase 1.

Debug Report Format

Debug Report

Bug Summary

• Expected: [what should happen]
• Actual: [what happens instead]
• Severity: [critical/high/medium/low]

Reproduction

1. [Step to reproduce]
2. [Step to reproduce]
3. [Observe bug]

Minimal reproduction: [simplest case that triggers bug]

Investigation

Root Cause

[What's actually wrong and why]

Fix Applied

• File: path/to/file.py
• Change: [what was modified]
• Why: [how this fixes the root cause]

Verification

• Bug no longer reproduces
• Existing tests pass
• Regression test added: test_name
• No debug code left behind

Prevention

[How to prevent similar bugs in the future]