Execute

→ Bug? Test failure? Unexpected behavior? → Find root cause first. No fixes without evidence.

Isolate: read error → reproduce → check git diff → drill upward through diagnostic layers: L1 symptom → L2 logic → L3 system → L4 architecture → L5 cross-system contract → L6 platform constraint → L7 spec gap. Stop when no deeper "why" remains OR terminal unactionable (T1-T4).
Identify owner: compare with working code → locate canonical owner → flag duplicate owners as a finding
Before fixing, run Patch-Shape Triage and Ripple Signal Triage if the candidate fix touches shared/core/cross-module behavior, contract, source-of-truth, fallback, adapter, duplicate owner, producer+consumer, or consumer-side patching. Also run Pre-Edit Complexity Check when the candidate fix touches an overloaded owner or may worsen source complexity.
Prove: one hypothesis → minimal test → iterate. 3+ failed fixes = question architecture, do not attempt another code fix. After fix, if any symptom persists → differential diagnosis (Phase 4 Step 4bis).
Fix: failing test → minimal code at canonical owner → verify → Reflection + architecture review → repair + retirement track → Done when: confidence ≥ B, both tracks explicit, DeeperCause answered "no" with evidence, no H-class hard signal still active.

Systematic Debugging

Random fixes waste time and create new bugs. Symptom fixes are failure.