Savings goal

How much do I need to save each month to hit $X in N years? Pin down the required contribution with a clever PV-discount trick: discount the target back to today, then call loan-payment with the discounted target as principal — the 'payment' the tool returns IS your required monthly savings. Same PMT formula a mortgage uses; different decision.

When to use this pack

You have a concrete goal — save $1,000,000 for retirement in 30 years, $500k for a child's college in 18 years, $80k for a down payment in 5 years, $20k for a wedding in 2 years — and want the deterministic answer to: how much per month? What's the gap if I keep saving at my current rate? What return would I need on a fixed contribution? This pack walks the agent through projecting current savings, computing the gap, and back-solving the required PMT — all with the finance-math kit, all free over PoW.

Tools in this pack

• Compound interest $0.001 POST /api/compound-interest Compute future value of a principal under compound interest. Returns future value, total interest earned, and the effective annual rate (APY) given the compounding frequency. Matches Excel's FV(rate, nper, 0, -principal) and the classic (1+r/n)^(nt) textbook formula.
• Loan payment $0.001 POST /api/loan-payment Compute the monthly (or per-period) payment on a fully-amortizing loan: mortgage, auto, student loan, business loan. Returns the periodic payment, total paid over the term, and total interest. Matches Excel's PMT(rate, nper, -principal). Use this when you just need the payment number, not the full per-period schedule (see amortization).
• Net present value (NPV) $0.001 POST /api/npv Compute the net present value of a stream of cashflows at a given discount rate. Index 0 is treated as t=0 (today, not discounted); indices 1..n are discounted by (1+rate)^t. Matches Excel's NPV but with the conventional t=0 treatment most finance textbooks use (Excel itself starts discounting at t=1 — see notes). Use for capital-budgeting decisions: positive NPV = creates value at the discount rate; negative = destroys value.
• Internal rate of return (IRR) $0.001 POST /api/irr Compute the internal rate of return (IRR) of a cashflow stream — the discount rate at which NPV = 0. Index 0 is treated as t=0 (typically the negative initial investment); indices 1..n are inflows in subsequent periods. Solved via Newton-Raphson with bisection fallback. Requires at least one positive and one negative cashflow (otherwise IRR is undefined). Multiple sign changes in the cashflows can produce multiple IRR roots — we return the first one found.

Workflow

1. Project the current balance forward with compound-interest. Pass principal=current_savings, annualRate=expected_return, years=time_horizon, compoundingPerYear=12. This is the 'no further contributions' future value — what's already covered. Subtract from the target → the gap that new contributions need to fill.
2. Solve for the required monthly contribution with the PV-discount trick. The PMT formula loan-payment implements is PMT = PV · r / (1 - (1+r)^-n). The same formula in reverse computes: 'what regular contribution accumulates to a given FV?' To use loan-payment directly, first discount the gap back to present value: PV_of_gap = gap / (1 + r)^n (or use npv with cashflows=[0, ..., gap] to do this). Then call loan-payment(principal=PV_of_gap, annualRate=r, termYears=n, paymentsPerYear=12) — the 'payment' the tool returns IS your required monthly contribution. Same PMT formula, different decision: you're not borrowing, you're paying yourself.
3. Sanity-check the target in today's dollars with npv. A $1M target in 30 years isn't $1M in spending power — at 3% inflation it's worth ~$412k today. Build a single-cashflow stream [0, ..., target] over the horizon and call npv at discountRate = inflation rate (3% historical, 2.5% recent Fed target). The (positive) NPV is the target in today's-dollars terms. Surface both nominal and real targets to the user — many people set savings targets without realizing they're undershooting because they thought in nominal dollars.
4. If the user has a fixed contribution and wants to know 'what return do I need?', back-solve with irr. Build the cashflow stream [-current_savings, -annual_contribution, -annual_contribution, ..., +target] over the horizon and call irr. The returned rate is the required annual return to hit the target — compare to historical asset class returns (cash ~2%, bonds ~4%, balanced ~6%, stocks ~7-10%) to ground-truth whether the plan is plausible. If irr > 10% on a long horizon, the plan is aggressive; if irr > 15%, the user should expect to either save more, extend the horizon, or accept higher risk.

Run it in Claude

claude mcp add agent402 -s user -- npx -y agent402-mcp@latest

Then paste this prompt into Claude:

I want to save $1,000,000 for retirement in 30 years. I currently have $50,000 saved. Use Agent402 to compute how much I need to contribute monthly at a 7% expected return, and pressure-test the plan. (1) Project current $50k forward 30 years: compound-interest(principal=50000, annualRate=0.07, years=30, compoundingPerYear=12) — expect ~$406k. Gap = $1,000,000 - $406k = $594k FV still needed. (2) Discount the gap to PV: PV_of_gap = 594000 / (1+0.07/12)^360 ≈ $73,200. Now call loan-payment(principal=73200, annualRate=0.07, termYears=30, paymentsPerYear=12) → the 'payment' is your required monthly contribution. Expect ~$487/mo. (3) Sanity-check the target in today's dollars: build cashflows=[0, 0, ..., 1000000] (index 30 = $1M), call npv at discountRate=0.03 → ~$412k in today's-dollars terms. Surface both. (4) Optional back-solve: if the user can only afford $300/mo, build cashflow=[-50000, -3600, -3600, ..., +1000000] (30 years of $3600/yr contributions) and call irr → required return. If the irr > 10%, the plan is aggressive — recommend lowering the target, extending the horizon, or increasing the contribution. (5) Return: {requiredMonthlyContribution, targetInTodaysDollars, gapAfterCurrentSavings, plausibilityFlag: "realistic"|"aggressive"|"unrealistic", oneLineConclusion}. All four tools are free over PoW.

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