<critical_constraints> ❌ NO fixing without reproducing first ❌ NO changing random things hoping it works ❌ NO fixing symptoms instead of root cause ✅ MUST read full error message including stack trace ✅ MUST form hypothesis before making changes ✅ MUST write test after fixing to prevent recurrence </critical_constraints>

<common_patterns>

• Off-by-one: range(len(x)-1) → should be range(len(x))

• Null reference: user.name without checking user is None

• Race condition: check-then-act pattern

• State mutation: items.sort() modifies original </common_patterns>

<when_stuck>

• Take a break

• Explain to someone (rubber duck)

• Check the obvious (is server running?)

• Search for similar issues

• Create minimal reproduction </when_stuck>