💵 Financial Gateway MCP Server

Give your AI assistant a built-in financial calculator. This Model Context Protocol (MCP) server hands Claude, Cursor, or any MCP client three precise money tools — client estimates, loan amortization, and currency conversion — so it can crunch real numbers instead of guessing them.

Every result is pure deterministic math: the same input always gives the same answer, computed entirely from the values you pass in. No API keys. No outbound calls. No made-up data.

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✨ Why use it

LLMs are notoriously unreliable at arithmetic — they'll happily "estimate" a loan payment and be off by hundreds of dollars. This Actor gives your AI a tool that returns exact, auditable, repeatable figures every time, with the formula and inputs visible in the output.

Perfect for:

• 🧾 AI agents that draft quotes & estimates for clients (line items → subtotal → discount → tax → total)
• 🏦 Assistants that answer "what would the monthly payment be?" with the real amortization formula
• 💱 Workflows that need deterministic currency math from a rate you control (no surprise live-rate drift)
• 🤖 Any MCP client (Claude Desktop, Cursor, MCP Inspector) that should calculate, not hallucinate

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🛠️ The three tools

Tool	What it does
calculate_estimate	Builds a line-item estimate — per-line totals, subtotal, optional discount, optional tax, and grand total.
amortize_loan	Fixed-rate loan summary — monthly payment, total paid, and total interest (standard amortization formula).
currency_convert	Converts an amount using an FX rate you supply (no live-rate lookups, so results stay reproducible).

Speaks MCP over JSON-RPC 2.0 — supports initialize, tools/list, and tools/call.

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🚀 Quick start

On Apify — paste a single JSON-RPC request into the query input and run. The Actor executes it, saves the response as OUTPUT, logs it to the dataset, and exits. (Leave query empty to get the tool catalog back.)

Example — amortize a $25,000 loan at 6.5% over 60 months:

{
  "query": "{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"tools/call\",\"params\":{\"name\":\"amortize_loan\",\"arguments\":{\"principal\":25000,\"annualRatePct\":6.5,\"termMonths\":60}}}"
}

Locally as an MCP server — run node src/main.js with no input. It reads newline-delimited JSON-RPC on stdin and replies on stdout — the standard transport for Claude Desktop and the MCP Inspector.

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📥 Tool arguments

calculate_estimate

Argument	Type	Required	Notes
items	array of { description?, quantity, unitPrice }	Yes	Non-empty; quantity and unitPrice must be numbers.
discountPct	number	No	0–100, applied to the subtotal. Default 0.
taxPct	number	No	≥ 0, applied after the discount. Default 0.
client	string	No	Echoed back on the result.
currency	string	No	Label only; defaults to "USD".

amortize_loan

Argument	Type	Required	Notes
principal	number	Yes	> 0.
annualRatePct	number	Yes	≥ 0 (a 0% rate becomes straight-line repayment).
termMonths	integer	Yes	Integer > 0.

currency_convert

Argument	Type	Required	Notes
amount	number	Yes
rate	number	Yes	FX multiplier > 0 — you supply it, it is not fetched.
from / to	string	No	Labels only.

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📤 What you get back

The JSON-RPC response is saved as OUTPUT, and every tool call is appended to the dataset with its arguments, result, and timestamp.

Example — amortize_loan for $350,000 @ 5.5% over 180 months:

{
  "principal": 350000,
  "annualRatePct": 5.5,
  "termMonths": 180,
  "monthlyPayment": 2859.79,
  "totalPaid": 514762.58,
  "totalInterest": 164762.58
}

Example — calculate_estimate for 8 units @ $175 with 7% tax:

{
  "client": "Acme LLC", "currency": "USD",
  "lineItems": [{ "description": "Consulting", "quantity": 8, "unitPrice": 175, "lineTotal": 1400 }],
  "subtotal": 1400, "discountPct": 0, "discountAmount": 0,
  "taxPct": 7, "taxAmount": 98, "total": 1498
}

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🔒 No setup, no surprises

• Zero configuration. No API keys, OAuth, or credentials. The Actor makes no outbound network calls — your numbers never leave the run.
• Deterministic & auditable. Same input → same output, every time. The result echoes the inputs so the math is checkable.
• Honest by design. It does not connect to QuickBooks, Stripe, Xero, banks, or any FX feed, and it does not store or retrieve real invoices or accounts. It's a stateless calculator — nothing is faked.

⚠️ Good to know

• amortize_loan uses the standard fixed-rate formula with monthly compounding — it doesn't model fees, escrow, variable rates, or extra payments.
• calculate_estimate applies one flat discount and one flat tax rate.
• Results are arithmetic only and rounded to 2 decimals — verify before using in any financial, accounting, or regulatory context. This is not financial or tax advice.
• Bad arguments return a clean JSON-RPC error (-32602), unknown tools return -32601, and a malformed query fails with an explanatory message — never a crash.