API Tool Calls

Loan Payment Calculator

loan_schedule

Read-onlyIdempotent

Computes fixed-rate monthly principal and interest payments, total interest, payoff month and a complete amortization table of at most 360 rows from entered amounts and a nominal annual rate. Optional down payment reduces principal; extra monthly principal payments shorten the modeled term. Uses exact rational arithmetic and nearest-cent half-up rounding. Taxes, insurance, PMI and fees are excluded.

Example tool call

This call uses example arguments supplied in the tool’s input schema.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "principal": 100,
      "termMonths": 2,
      "annualRatePercent": 12
    },
    "name": "loan_schedule"
  }
}

Input Schema

Name	Required	Description
`principal`	Yes	Starting amount in USD before the optional down payment; at most two decimal places.
`termMonths`	Yes	Original or remaining whole-month term. Every period is returned, at most 360 rows.
`downPayment`	No	USD amount subtracted from principal; must be less than principal; at most two decimal places; defaults to zero.
`extraMonthly`	No	Extra principal paid each month in USD; at most two decimal places; defaults to zero.
`annualRatePercent`	Yes	Fixed nominal annual interest rate in percent, excluding fees; at most six decimal places.

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

AddedOct 5, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations already cover safety (readOnly, idempotent, non-destructive), so the bar is lower. The description adds real behavioral context: exact rational arithmetic, nearest-cent half-up rounding, a 360-row cap, and explicit exclusion of taxes/insurance/PMI/fees. It does not describe error behavior for invalid inputs, but the disclosed computation semantics are substantial.

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?

Three dense sentences, each carrying distinct content (computation, optional inputs, arithmetic/rounding, exclusions), front-loaded with the core purpose. Slightly list-like, but no filler sentences.

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?

Output schema exists so return values need not be re-specified, yet the description still summarizes the amortization table shape. Combined with the 360-row cap, rounding rules, and exclusions, an agent has everything needed to call it correctly.

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 baseline is 3, but the description adds meaning beyond the schema: down payment 'reduces principal' and extra monthly principal 'shortens the modeled term'. That clarifies the interaction between optional parameters that the schema documents only individually.

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?

States a specific verb+resource ('Computes fixed-rate monthly principal and interest payments') plus the full output scope (total interest, payoff month, amortization table). It is clearly distinguishable from sibling calculators like compound_interest_calculator because it names mortgage-style P&I plus amortization.

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?

Gives clear context for when the tool applies (fixed-rate amortized loans) and explains optional behaviors like down payment and extra principal. It never explicitly routes the agent away from a sibling (e.g. compound_interest_calculator or debt_to_income), so it stops short of full when-not guidance.

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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