When building full-stack financial apps, your standard math logic needs serious guardrails.

Today (Day 80 of my build!), I was fine-tuning the Month-End Projection widget for my AI Financial Agent.

The Problem: My React dashboard looked great, showing €86 in expenses. But my Serverless Python backend was returning a projection of €168. To make matters worse, if I clicked on "April" (a past month), the system would project a completely broken number instead of just showing the final closed amount.

The Fix: I had to implement three architectural changes to force my backend to respect the passage of time and match the frontend's business logic.

Aligning Business Rules (The Mirror Filter) My React code was hiding internal currency conversions (like Wise "To EUR" transfers) from the expense chart. But my Python Lambda was just summing up everything that wasn't an income.…