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SmartFin

by fyi.smartfin
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Loan payment and total interest

smartfin_calculate_loan_payment
Read-onlyIdempotent

Monthly payment, total interest and total paid for a fixed-rate loan with monthly payments: mortgage, car, student or personal loan. Give either loan_amount, or home_price with down_payment_pct for a mortgage. A mortgage takes the mortgage calculator's ranges: $1,000 to $100,000,000, a rate from 0% to 25%, 5 to 30 years. Use when the user asks what a loan costs per month or in total, or how the rate or term changes the cost. Do not use for variable-rate loans, interest-only loans, or credit cards. Excludes taxes, insurance and fees.

Input Schema

NameRequiredDescriptionDefault
currencyNoCurrency symbol for the amounts in the summary only; amounts are never converted. The link opens the page in US dollars, and how_to says to pick the currency there. Default USD
loan_typeNoType of loan; chooses the SmartFin calculator link. Default mortgage
home_priceNoMortgage only: home price, in the currency chosen, with down_payment_pct
term_yearsYesLoan term in years, e.g. 30 for a mortgage, 5 for a car loan
loan_amountNoAmount borrowed, in the currency chosen. Give this or home_price
annual_rate_pctYesAnnual interest rate in %, e.g. 6.5
down_payment_pctNoMortgage only: down payment as % of home_price. Default 20

Output Schema

NameRequiredDescriptionDefault
citeYesHow to credit this result
dataYes
nextYesSuggested follow-up calls
linksYes
how_toYesSteps for the user to see or redo this on smartfin.fyi
sourceYes
summaryYesPlain-language result the assistant can quote

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • addedInput schema / properties / currency
      Added value: +{
+  "description": "Currency symbol for the amounts in the summary only; amounts are never converted. The link opens the page in US dollars, and how_to says to pick the currency there. Default USD",
+  "enum": [
+    "USD",
+    "EUR",
+    "GBP",
+    "CHF",
+    "AUD",
+    "CAD",
+    "SGD",
+    "HKD",
+    "INR",
+    "JPY"
+  ],
+  "type": "string"
+}
    • changedInput schema / properties / home_price / description
      Previous value: -"Mortgage only: home price, in dollars, with down_payment_pct"New value: +"Mortgage only: home price, in the currency chosen, with down_payment_pct"
    • changedInput schema / properties / loan_amount / description
      Previous value: -"Amount borrowed, in dollars. Give this or home_price"New value: +"Amount borrowed, in the currency chosen. Give this or home_price"
  2. Changed1 schema field changed
    • addedOutput schema / properties / links / properties / page
      Added value: +{
+  "description": "SmartFin's page for this stock, when it has one",
+  "type": "string"
+}
  3. First observed

TDQS

A4.3/5.0
Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint and non-destructive, so safety is covered; the description adds genuinely useful domain behavior — what is excluded from the figures (taxes, insurance, fees) and the mortgage-specific input ranges. It does not mention the calculator link / page-opening side effect beyond what the schema's currency field already says.

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?

Outputs, input branching, ranges and exclusions are front-loaded in roughly five dense sentences with no filler. It is slightly overloaded with numeric ranges that could sit in the schema, but every sentence carries selection-relevant information.

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?

With an output schema present, the description needn't explain return values, and it covers the key gaps an agent would otherwise hit: input branching, mortgage range limits, unsupported loan structures, and cost exclusions. Only the absence of named alternative tools leaves a small hole.

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 adds real constraints beyond the schema: mortgage term is narrowed to 5–30 years (schema allows 1–50) and rate to 0–25% (schema allows 0–100), and it restates the loan_amount-vs-home_price+down_payment_pct branching. That extra narrowing meaningfully guides correct invocation.

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 opens with a concrete list of outputs (monthly payment, total interest, total paid) tied to a specific resource (fixed-rate amortizing loan) and enumerates the loan types covered. An agent can distinguish this from siblings like smartfin_calculate_mortgage_payoff or smartfin_calculate_simple_interest purely from the text.

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?

It gives an explicit when-to-use trigger ('what a loan costs per month or in total, or how the rate or term changes the cost') plus explicit exclusions (variable-rate, interest-only, credit cards). It stops short of naming the sibling tools to use instead, so it does not reach the top score for alternatives.

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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