Tutorial: Mortgage Calculator

Finance

Data source: Standard amortization formulas

Overview

Mortgage Calculator performs computations on your inputs — no external data source required. This tutorial covers all 4 tools with parameter reference and code examples.

Prerequisites

Sign up at https://context.gnist.ai/signup for a free API key (100 calls/day).
Choose your integration method: MCP protocol or REST API.

Connect via MCP

Add to your MCP client config (Claude Desktop, Cursor, etc.):

MCP Config

{
  "mcpServers": {
    "gnist-mortgage-calculator": {
      "url": "https://context.gnist.ai/mcp/mortgage-calculator/",
      "headers": {
        "Gnist-API-Key": "YOUR_API_KEY"
      }
    }
  }
}

Tools (4)

`calculate_monthly_payment`

Calculate fixed monthly mortgage payment. Given a loan amount, interest rate, and term, computes the monthly payment, total amount paid over the life of the loan, and total interest.

Parameter	Type	Required	Description
`principal`	number	required	Loan amount.
`annual_rate`	number	required	Annual interest rate as a percentage (e.g. 5.5 for 5.5%).
`term_years`	integer	required	Loan term in years.

curl -X POST "https://context.gnist.ai/mcp/mortgage-calculator/" \
  -H "Content-Type: application/json" \
  -H "Gnist-API-Key: YOUR_API_KEY" \
  -d '{"jsonrpc": "2.0", "method": "tools/call", "id": 1, "params": {"name": "calculate_monthly_payment", "arguments": {"principal": 1.0, "annual_rate": 5.5, "term_years": 2025}}}'

import httpx

resp = httpx.post(
    "https://context.gnist.ai/mcp/mortgage-calculator/",
    headers={"Gnist-API-Key": "YOUR_API_KEY"},
    json={'id': 1,
 'jsonrpc': '2.0',
 'method': 'tools/call',
 'params': {'arguments': {'annual_rate': 5.5,
                          'principal': 1.0,
                          'term_years': 2025},
            'name': 'calculate_monthly_payment'}},
)
print(resp.json())

`calculate_amortization`

Generate a full amortization schedule for a mortgage. Returns monthly entries showing how each payment splits between principal and interest, plus the remaining balance after each payment.

Parameter	Type	Required	Description
`principal`	number	required	Loan amount.
`annual_rate`	number	required	Annual interest rate as a percentage (e.g. 5.5 for 5.5%).
`term_years`	integer	required	Loan term in years.

curl -X POST "https://context.gnist.ai/mcp/mortgage-calculator/" \
  -H "Content-Type: application/json" \
  -H "Gnist-API-Key: YOUR_API_KEY" \
  -d '{"jsonrpc": "2.0", "method": "tools/call", "id": 1, "params": {"name": "calculate_amortization", "arguments": {"principal": 1.0, "annual_rate": 5.5, "term_years": 2025}}}'

import httpx

resp = httpx.post(
    "https://context.gnist.ai/mcp/mortgage-calculator/",
    headers={"Gnist-API-Key": "YOUR_API_KEY"},
    json={'id': 1,
 'jsonrpc': '2.0',
 'method': 'tools/call',
 'params': {'arguments': {'annual_rate': 5.5,
                          'principal': 1.0,
                          'term_years': 2025},
            'name': 'calculate_amortization'}},
)
print(resp.json())

`calculate_affordability`

Estimate how much home a buyer can afford. Uses the 28% front-end DTI ratio: maximum housing payment is 28% of gross monthly income, minus existing debts.

Parameter	Type	Required	Description
`annual_income`	number	required	Gross annual income.
`annual_rate`	number	required	Annual interest rate as a percentage (e.g. 5.5 for 5.5%).
`monthly_debts`	number	optional	Existing monthly debt obligations. (default: `0.0`)
`down_payment`	number	optional	Cash available for down payment. (default: `0.0`)
`term_years`	integer	optional	Loan term in years. (default: `30`)

curl -X POST "https://context.gnist.ai/mcp/mortgage-calculator/" \
  -H "Content-Type: application/json" \
  -H "Gnist-API-Key: YOUR_API_KEY" \
  -d '{"jsonrpc": "2.0", "method": "tools/call", "id": 1, "params": {"name": "calculate_affordability", "arguments": {"annual_income": 1.0, "annual_rate": 5.5}}}'

import httpx

resp = httpx.post(
    "https://context.gnist.ai/mcp/mortgage-calculator/",
    headers={"Gnist-API-Key": "YOUR_API_KEY"},
    json={'id': 1,
 'jsonrpc': '2.0',
 'method': 'tools/call',
 'params': {'arguments': {'annual_income': 1.0, 'annual_rate': 5.5},
            'name': 'calculate_affordability'}},
)
print(resp.json())

`report_feedback`

Report a bug, feature request, or general feedback for this data source. Use this when something doesn't work as expected, when you'd like a new feature, or when you have suggestions for improvement. Args: feedback: Describe the issue or suggestion. feedback_type: One of 'bug', 'feature_request', or 'general'.

Parameter	Type	Required	Description
`feedback`	string	required
`feedback_type`	string	optional	(default: `general`)

curl -X POST "https://context.gnist.ai/mcp/mortgage-calculator/" \
  -H "Content-Type: application/json" \
  -H "Gnist-API-Key: YOUR_API_KEY" \
  -d '{"jsonrpc": "2.0", "method": "tools/call", "id": 1, "params": {"name": "report_feedback", "arguments": {"feedback": "example"}}}'

import httpx

resp = httpx.post(
    "https://context.gnist.ai/mcp/mortgage-calculator/",
    headers={"Gnist-API-Key": "YOUR_API_KEY"},
    json={'id': 1,
 'jsonrpc': '2.0',
 'method': 'tools/call',
 'params': {'arguments': {'feedback': 'example'}, 'name': 'report_feedback'}},
)
print(resp.json())

FAQ

What data does Mortgage Calculator provide?

Mortgage payment calculator — monthly payments, amortization schedules, and affordability estimates. It exposes 4 tools: calculate_monthly_payment, calculate_amortization, calculate_affordability, report_feedback.

Does this toolkit require external data?

No. It performs computations on your inputs directly. You only need a Gnist API key.

What format does the Mortgage Calculator API return?

JSON, via either MCP protocol (JSON-RPC 2.0) or REST API.