AGENTS.md

Guidance for coding agents working on AlexandrAI Tools Loan Calculator in the alexandrai-tools project.

Project overview

This guide covers src/tools/loan-calculator/index.ts. The tool computes monthly payment with the standard amortization formula, handles zero-rate loans by spreading principal evenly across months, and derives total paid plus total interest through loanSummary.

Primary source files

• src/tools/loan-calculator/index.ts defines meta, monthlyPayment, and loanSummary.
• src/tools/loan-calculator/index.test.ts covers amortized payments, zero-rate loans, invalid principal or years, total-interest identity, and summary consistency.
• src/tools/loan-calculator/Body.astro owns browser inputs and display formatting.
• src/tools/loan-calculator/i18n.ts owns localized copy.
• src/lib/tools.ts registers metadata.

Setup commands

npm install npm test npm run typecheck npm run build npm run dev

Change rules

• Return zero payment when principal is non-positive or years is non-positive.
• Handle annualRatePct equal to zero without using the amortization denominator.
• Keep totalInterest equal to totalPaid minus principal.
• Do not include taxes, insurance, fees, or adjustable rates unless the formula and UI labels are expanded together.
• Do not present output as financial advice or loan approval guidance.
• Keep calculations pure and browser-local.
• When adding rounding, keep raw math and display formatting separate.

Testing

Test known amortization examples, zero-rate payment, zero principal, zero years, total paid, total interest, summary agreement with monthlyPayment, fractional rates, and display rounding if UI formatting changes.

PR checklist

• npm test passes for loan-calculator.
• Pure calculation stays independent from UI formatting.
• Browser-local behavior is preserved.
• Copy does not present calculator output as personalized financial advice.