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by com.truecalci
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US mortgage PITI

mortgage_piti
Read-onlyIdempotent

Calculate monthly US mortgage payments broken down into PITI (Principal, Interest, Property Taxes, Homeowners Insurance, and Private Mortgage Insurance) along with full 30-year amortization schedule.

Behavior: Deterministic, idempotent calculation with zero external side effects. Computes standard monthly amortization using fixed-rate annuity formula, computes annual property taxes divided by 12, monthly hazard insurance, and conditional PMI (applied automatically if down payment is under 20% until 78% LTV threshold). Returns monthly total, principal/interest component, tax/escrow components, total lifetime interest, and payoff schedule.

Usage Guidelines: Use for US residential home purchase financing and refinancing scenarios. Do not use for international reducing-balance loans without escrow/PMI; use home_loan_emi instead.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
homePriceYesTotal purchase price or appraised property value in currency units (e.g. 450000). Must be positive.
as_of_dateYesCivil date YYYY-MM-DD. Selects the rule pack in force on that date. Required: the engine will not assume a date.
interestRateYesAnnual interest rate in percent, as a decimal string (e.g. 6.75).
loanTermYearsNoStandard US alias for tenureYears (loan term in years).
annualPmiPercentNoAnnual Private Mortgage Insurance premium percentage (e.g. 0.75 for 0.75% of original loan amount).
downPaymentPercentNoDown payment as a percentage of purchase price (e.g. 20 for 20%). Values below 20 automatically trigger PMI calculations.
annualHomeInsuranceNoAnnual hazard/homeowners insurance premium in currency units (e.g. 1400).
propertyTaxRatePercentNoAnnual local property tax rate as a percentage of home value (e.g. 1.2 for 1.2%).

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • addedInput schema / properties / as_of_date
      Added value: +{
+  "description": "Civil date YYYY-MM-DD. Selects the rule pack in force on that date. Required: the engine will not assume a date.",
+  "example": "2026-09-26",
+  "pattern": "^\\d{4}-\\d{2}-\\d{2}$",
+  "type": "string"
+}
    • removedInput schema / properties / tenureYears
      Removed value: -{
-  "default": 30,
-  "description": "Loan duration in years (typically 15, 20, or 30).",
-  "type": "integer"
-}
    • changedInput schema / required
      Previous value: -[
-  "homePrice",
-  "interestRate"
-]New value: +[
+  "homePrice",
+  "interestRate",
+  "as_of_date"
+]
  2. Changed7 schema fields changed
    • addedInput schema / properties / homePrice / example
      Added value: +"400000.00"
    • addedInput schema / properties / homePrice / pattern
      Added value: +"^\\d+\\.\\d{2}$"
    • changedInput schema / properties / homePrice / type
      Previous value: -"number"New value: +"string"
    • changedInput schema / properties / interestRate / description
      Previous value: -"Annual mortgage interest rate percentage (e.g. 6.8 for 6.8%). Must be positive."New value: +"Annual interest rate in percent, as a decimal string (e.g. 6.75)."
    • addedInput schema / properties / interestRate / example
      Added value: +"6.75"
    • addedInput schema / properties / interestRate / pattern
      Added value: +"^\\d+(\\.\\d+)?$"
    • changedInput schema / properties / interestRate / type
      Previous value: -"number"New value: +"string"
  3. Changed8 schema fields changed
    • changedInput schema / properties / annualHomeInsurance / description
      Previous value: -"Annual hazard insurance premium"New value: +"Annual hazard/homeowners insurance premium in currency units (e.g. 1400)."
    • changedInput schema / properties / annualPmiPercent / description
      Previous value: -"Annual PMI % if down payment < 20%"New value: +"Annual Private Mortgage Insurance premium percentage (e.g. 0.75 for 0.75% of original loan amount)."
    • changedInput schema / properties / downPaymentPercent / description
      Previous value: -"Down payment percentage (e.g. 20 for 20%)"New value: +"Down payment as a percentage of purchase price (e.g. 20 for 20%). Values below 20 automatically trigger PMI calculations."
    • changedInput schema / properties / homePrice / description
      Previous value: -"Purchase price of the home in currency units (e.g. 450000)"New value: +"Total purchase price or appraised property value in currency units (e.g. 450000). Must be positive."
    • changedInput schema / properties / interestRate / description
      Previous value: -"Annual interest rate in % (e.g. 6.8)"New value: +"Annual mortgage interest rate percentage (e.g. 6.8 for 6.8%). Must be positive."
    • changedInput schema / properties / loanTermYears / description
      Previous value: -"Loan duration in years (standard US alias for tenureYears)"New value: +"Standard US alias for tenureYears (loan term in years)."
    • changedInput schema / properties / propertyTaxRatePercent / description
      Previous value: -"Annual property tax rate %"New value: +"Annual local property tax rate as a percentage of home value (e.g. 1.2 for 1.2%)."
    • changedInput schema / properties / tenureYears / description
      Previous value: -"Loan duration in years (e.g. 15, 20, 30)"New value: +"Loan duration in years (typically 15, 20, or 30)."
  4. First observed

TDQS

A4.6/5.0
Behavior5/5

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint. The description adds substantial behavioral detail beyond that: 'Deterministic, idempotent calculation with zero external side effects', explains the calculation formula, conditional PMI logic (under 20% down until 78% LTV), and lists the return components. This goes far beyond the annotations and no contradiction exists.

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

Conciseness5/5

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

The description is efficient and well-structured: purpose, behavior, and usage guidelines are separated. It is front-loaded with the main purpose and then adds necessary detail. Every sentence earns its place without redundancy.

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

Completeness4/5

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

For a complex tool with 8 parameters, no output schema, and conditional PMI, the description covers the core functionality, returns, and usage context. It mentions the amortization schedule, lifetime interest, and PMI thresholds. It does not explicitly mention edge cases like negative values or currency handling, but these are likely covered by schema validation. Overall, it is quite complete.

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

Parameters3/5

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

Schema description coverage is 100%, so baseline is 3. The description does not repeat parameter meanings but does add context about how they interact, e.g., 'down payment is under 20%' and '78% LTV threshold', which aids understanding. However, it does not provide per-parameter details beyond the schema, so it stays at baseline.

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 clearly states the tool's purpose: 'Calculate monthly US mortgage payments broken down into PITI...' with a specific verb (calculate) and resource (US mortgage payments). It also distinguishes from sibling home_loan_emi by specifying 'US mortgage' and later naming the alternative for international loans.

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

Usage Guidelines5/5

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

The description contains an explicit 'Usage Guidelines' section: 'Use for US residential home purchase financing and refinancing scenarios. Do not use for international reducing-balance loans without escrow/PMI; use home_loan_emi instead.' This provides clear when-to-use, when-not-to-use, and names the alternative tool.

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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