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A null pointer exception surfacing in a generic service layer shows where the crash happens, while the null value originates elsewhere. Tracing it backwards through multiple service and data layers manually leaves the true origin uncertain. Debug traces the full execution path through the knowledge graph from the point of failure back to the source, identifies exactly where the null value enters without a guard, and returns a targeted fix. Issue: 
Users are getting null pointer errors during checkout with empty carts.
Potpie traces:
  1. The checkout controller that receives the request
  2. The cart service and how it handles empty state
  3. The downstream order service that fails on null cart data
Root cause citation:
  • Exact file path and line where the null value passes through without a null check
  • The dependent call site that throws the exception
Fix includes:
  • Null guard at the correct layer with the minimal change required
  • Verification across all call sites for the same missing check
Build generates the corrected code as a reviewable diff.