Compound Mint Money Calculator

Mortgage payment calculator

calculate_mortgage_payment

Read-onlyIdempotent

Use when a user asks about a mortgage or house payment, e.g. "what is the monthly payment on a $400,000 house with 20% down at 6.5%?", "how much interest will I pay on a $300,000 mortgage?" or "15 vs 30 year mortgage payment". Monthly principal and interest for a fixed rate loan, total interest, first payment split, balance after 5 years and the same loan on a 15 or 30 year term. Optional property tax, home insurance, HOA and PMI give the full monthly payment. Example: $320,000 at 6.5% for 30 years = $2,022.62 a month.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "annual_rate": "<annual_rate>"
    },
    "name": "calculate_mortgage_payment"
  }
}

Input Schema

Name	Required	Description
`years`	No	Loan term in years (default 30).
`home_price`	No	Home price in USD (or send loan_amount).
`annual_rate`	Yes	Yearly mortgage rate in percent, e.g. 6.5.
`hoa_monthly`	No	HOA dues per month in USD (default 0).
`loan_amount`	No	Amount borrowed in USD (instead of home_price and down payment).
`down_payment`	No	Down payment in USD (default 0).
`pmi_annual_percent`	No	PMI as a yearly percent of the loan, e.g. 0.5 (default 0).
`property_tax_annual`	No	Property tax per year in USD (default 0).
`down_payment_percent`	No	Down payment as a percent of the price, e.g. 20 (instead of down_payment).
`home_insurance_annual`	No	Home insurance per year in USD (default 0).

Schema Changelog

Changes observed during successful MCP inspections.

AddedNov 3, 2026

TDQS

A4.4/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds real behavioral context beyond that: the loan is fixed-rate, the optional tax/insurance/HOA/PMI inputs roll up into a full monthly payment, and a worked example ($320,000 at 6.5%/30yr = $2,022.62) sets an expectation for output precision. It does not state rounding conventions or amortization assumptions in detail.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the triggering condition, then the computed outputs, then optional inputs, then a numeric example — a logical order with no filler. The three quoted user questions are slightly repetitive but serve as high-signal routing cues rather than waste.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With 10 parameters, no output schema, and read-only annotations, the description carries the burden of describing what comes back, and it does exactly that: monthly P&I, total interest, first payment split, 5-year balance, and term comparison. Nothing an agent needs in order to call this correctly appears to be missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3, but the description earns extra credit by explaining how optional cost parameters compose ('Optional property tax, home insurance, HOA and PMI give the full monthly payment') and by framing example calls that mix home_price with down_payment_percent. It stops short of clarifying the home_price vs loan_amount precedence beyond what the schema already notes.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description names a specific verb and resource — computing monthly principal and interest, total interest, first-payment split, 5-year balance, and 15/30-year comparison for a fixed-rate loan. It is unmistakably the mortgage calculator among siblings like calculate_compound_interest or calculate_debt_payoff, and the opening 'Use when a user asks about a mortgage or house payment' anchors it further.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Three quoted user phrasings give concrete trigger conditions, including the comparison scenario ('15 vs 30 year mortgage payment') and the interest-only question. It does not explicitly exclude neighboring tools (e.g., 'for general loan amortization use X' or 'not for refinance break-even analysis'), so routing among the many sibling calculators is left partly to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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