Systematic Debugging

Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST.

Fixing symptoms creates whack-a-mole debugging. Every fix that doesn't address root cause makes the next bug harder to find. Find the root cause, then fix it.

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Phase 1: Root Cause Investigation

Gather context before forming any hypothesis.

1. Collect symptoms: Read the error messages, stack traces, and reproduction steps. If the user hasn't provided enough context, ask one question at a time.

2. Read the code: Trace the code path from the symptom back to potential causes. Use grep to find all references, read to understand the logic.

3. Check recent changes:

   git log --oneline -20 -- <affected-files>
   

   Was this working before? What changed? A regression means the root cause is in the diff.

4. Reproduce: Can you trigger the bug deterministically? If not, gather more evidence before proceeding.

Output: "Root cause hypothesis: ..." — a specific, testable claim about what is wrong and why.

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Phase 2: Pattern Analysis

Check if this bug matches a known pattern:

Pattern	Signature	Where to look
Race condition	Intermittent, timing-dependent	Concurrent access to shared state
Nil/null propagation	NoMethodError, TypeError	Missing guards on optional values
State corruption	Inconsistent data, partial updates	Transactions, callbacks, hooks
Integration failure	Timeout, unexpected response	External API calls, service boundaries
Configuration drift	Works locally, fails in staging/prod	Env vars, feature flags, DB state
Stale cache	Shows old data, fixes on cache clear	Redis, CDN, browser cache

Also check:

• Project TODOs for related known issues

• git log

  for prior fixes in the same area — recurring bugs in the same files are an architectural smell, not a coincidence

External pattern search: If the bug doesn't match a known pattern, search for:

• "{framework} {generic error type}" — sanitize first: strip hostnames, IPs, file paths, SQL, customer data. Search the error category, not the raw message.
• "{library} {component} known issues"

If a documented solution or known dependency bug surfaces, present it as a candidate hypothesis in Phase 3.

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Phase 3: Hypothesis Testing

Before writing ANY fix, verify your hypothesis.

1. Confirm the hypothesis: Add a temporary log statement, assertion, or debug output at the suspected root cause. Run the reproduction. Does the evidence match?

2. If the hypothesis is wrong: Before forming the next hypothesis, search for the error. Sanitize first — strip hostnames, IPs, file paths, SQL fragments, customer identifiers. Search only the generic error type and framework context. Then return to Phase 1. Gather more evidence. Do not guess.

3. 3-strike rule: If 3 hypotheses fail, STOP. Tell the user:

   3 hypotheses tested, none match. This may be an architectural issue
   rather than a simple bug.
   
   Options:
   A) Continue investigating — I have a new hypothesis: [describe]
   B) Escalate for human review — this needs someone who knows the system
   C) Add logging and wait — instrument the area and catch it next time
   

Red flags — if you see any of these, slow down:

• "Quick fix for now" — there is no "for now." Fix it right or escalate.
• Proposing a fix before tracing data flow — you're guessing.
• Each fix reveals a new problem elsewhere — wrong layer, not wrong code.

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Phase 4: Implementation

Once root cause is confirmed:

1. Fix the root cause, not the symptom. The smallest change that eliminates the actual problem.

2. Minimal diff: Fewest files touched, fewest lines changed. Resist the urge to refactor adjacent code.

3. Write a regression test that:

   • Fails without the fix (proves the test is meaningful)
   • Passes with the fix (proves the fix works)

4. Run the full test suite. Paste the output. No regressions allowed.

5. If the fix touches >5 files: Flag the blast radius to the user:

   This fix touches N files. That's a large blast radius for a bug fix.
   A) Proceed — the root cause genuinely spans these files
   B) Split — fix the critical path now, defer the rest
   C) Rethink — maybe there's a more targeted approach
   

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Phase 5: Verification & Report

Fresh verification: Reproduce the original bug scenario and confirm it's fixed. This is not optional.

Run the test suite and paste the output.

Output a structured debug report:

DEBUG REPORT
════════════════════════════════════════
Symptom:         [what the user observed]
Root cause:      [what was actually wrong]
Fix:             [what was changed, with file:line references]
Evidence:        [test output, reproduction attempt showing fix works]
Regression test: [file:line of the new test]
Related:         [prior bugs in same area, architectural notes]
Status:          DONE | DONE_WITH_CONCERNS | BLOCKED
════════════════════════════════════════

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Important Rules

• 3+ failed fix attempts → STOP and question the architecture. Wrong architecture, not failed hypothesis.
• Never apply a fix you cannot verify. If you can't reproduce and confirm, don't ship it.
• Never say "this should fix it." Verify and prove it. Run the tests.
• If fix touches >5 files → ask the user about blast radius before proceeding.
• Completion status:
  • DONE — root cause found, fix applied, regression test written, all tests pass
  • DONE_WITH_CONCERNS — fix works but architectural concern remains
  • BLOCKED — cannot determine root cause, needs human expertise
  • NEEDS_CONTEXT — missing reproduction steps or access