Compound interest

Compute future value of a principal under compound interest. Send POST /api/compound-interest with the required fields principal, annualRate and years and pay $0.002 per call over x402 or MPP, or call it free by solving a proof-of-work challenge. It returns a JSON object with futureValue, totalInterest, effectiveAnnualRate and periods.

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.

Category: Live public data · Tags: finance interest compound future-value fv apy savings

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/compound-interest \
  -H "Content-Type: application/json" \
  -d '{"principal":1000,"annualRate":0.05,"years":10,"compoundingPerYear":12}'

Without payment this returns HTTP 402 Payment Required with the exact price for compound-interest; any x402 v2 or MPP client pays it and retries.

Example response

{
  "futureValue": 1647.01,
  "totalInterest": 647.01,
  "effectiveAnnualRate": 0.051162,
  "periods": 120
}

From an MCP client

catalog.call {
  "slug": "compound-interest",
  "params": {
    "principal": 1000,
    "annualRate": 0.05,
    "years": 10,
    "compoundingPerYear": 12
  }
}

On the hosted connector at https://agent402.tools/mcp, catalog.call runs compound-interest free (rate-limited, no wallet). Local install: npx -y agent402-mcp.

Errors and behavior

• principal, annualRate and years are required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• Free tier: no outbound network call leaves the server for this tool, so proof-of-work (16 leading zero bits of sha256) pays for it.
• A GET or HEAD to /api/compound-interest returns the same 402 quote, so the price can be read without a body.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (an answer larger than 1 MB is not replayed).

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/compound-interest", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "principal": 1000,
    "annualRate": 0.05,
    "years": 10,
    "compoundingPerYear": 12
  }),
});

No wallet? Pay with compute

Fetch a challenge, solve the sha256 puzzle (16 leading zero bits, a fraction of a second of CPU), and resend with the X-Pow-Solution header:

import { createHash } from "node:crypto";
const lz = (b) => { let t = 0; for (const x of b) { if (!x) { t += 8; continue; } t += Math.clz32(x) - 24; break; } return t; };
const c = await (await fetch("https://agent402.tools/api/pow/challenge?slug=compound-interest")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/compound-interest", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"principal":1000,"annualRate":0.05,"years":10,"compoundingPerYear":12}) });

Part of these workflows

Compound interest is one step in this skill pack, each sold as a single call:

• Loan comparison - Compare two or more loan offers - different rates, terms, fees, prepayment structures - on the metrics that actually matter (monthly payment, total interest, year-1 equity build, NPV at your discount rate, effective rate). Apples-to-apples math without opening a spreadsheet.

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