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Cover image for Day 26: Building a Transparent Financial Scoring Engine with Python, AWS Lambda, and React
Eric Rodríguez
Eric Rodríguez

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Day 26: Building a Transparent Financial Scoring Engine with Python, AWS Lambda, and React

Data visualization is the first step of any application, but data interpretation is where we add real value. Until yesterday, my Cloud Financial Agent could tell me how much I spent on coffee. Today, it can tell me if that habit is ruining my future.

In this phase of the project, I faced an interesting challenge: How do I convert a raw JSON list of transactions into a quality-of-life metric (0-100)?

The Algorithm (Backend Strategy)

I avoided using LLMs for the core calculation because AI can hallucinate with strict math. Instead, I opted for a deterministic Python function injected into my AWS Lambda handler. The logic is strict:

Base: Every user starts with 50 points.

Savings Rate: If (Income - Expenses) / Income exceeds 50%, the algorithm injects +40 points.

Frugality Bonus: If total absolute expenses are under €500, it triggers a "Frugal Month" flag, adding +10 points.

Penalties: If cash flow is negative, the score plummets immediately.

The Transparency Layer (Frontend)

The biggest issue with fintech apps is ambiguity. To solve this, my API doesn't just return score: 100. It returns two key data structures:

short_reasons: A list of emojis for immediate visual impact (🔥 High Savings Rate).

audit_log: A technical breakdown that explains the math behind the number line-by-line.

Visual Integration

On the frontend (React + Tailwind + Recharts), I designed a custom Gauge component using dynamic SVGs that animate stroke-dashoffset based on the score received. Right below it, I implemented a console-style Audit Log to keep the developer aesthetic of the project.

The final result is a system that doesn't just show you your balance; it educates you on your own financial habits in real-time.

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