SmartFin

Simple interest

smartfin_calculate_simple_interest

Read-onlyIdempotent

The interest earned and the final amount on a principal at a fixed yearly rate over a number of years, when interest is not compounded (interest = principal x rate x time). Use for questions about simple interest on a loan or deposit, such as a short-term note or a bond coupon at a flat rate. Do not use when interest is added back to the balance (use smartfin_calculate_future_value or smartfin_project_savings_growth), for loan payments (use smartfin_calculate_loan_payment), or for taxes and fees: it applies none.

Input Schema

TableJSON Schema

Name	Required	Description
`years`	Yes	Time in years; a part of a year is a decimal, e.g. 0.5
`currency`	No	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
`principal`	Yes	Starting amount in the currency chosen, above 0
`annual_rate_pct`	Yes	Yearly interest rate in %, from 0 to 100, e.g. 5

Output Schema

TableJSON Schema

Name	Required	Description
`cite`	Yes	How to credit this result
`data`	Yes
`next`	Yes	Suggested follow-up calls
`links`	Yes
`how_to`	Yes	Steps for the user to see or redo this on smartfin.fyi
`source`	Yes
`summary`	Yes	Plain-language result the assistant can quote

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedOct 18, 2026

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 / principal / description

Previous value: -"Starting amount in dollars, above 0"New value: +"Starting amount in the currency chosen, above 0"

AddedOct 13, 2026

TDQS

A4.5/5.0

Behavior4/5

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

Annotations already declare read-only, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds domain behavior beyond annotations: it is non-compounded simple interest and applies no taxes or fees, which helps set expectations for what the calculation includes.

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?

Three sentences, front-loaded with the definition, then use cases, then exclusions. Every sentence carries routing or calculation information, and there is no filler.

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?

For a simple, read-only calculator with annotations and an output schema, the description provides enough context: what it computes, when to use it, when not to use it, and which alternatives handle those cases. Return-value details are appropriately left to the output schema.

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 the schema already documents principal, annual_rate_pct, years, and currency. The description adds the relationship 'interest = principal x rate x time' and clarifies non-compounding, but it does not add per-parameter detail beyond the schema's own descriptions.

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 the exact computation: simple interest and final amount from principal, fixed yearly rate, and years. It also states the formula and distinguishes the tool from compounding, loan-payment, and tax/fee alternatives, so an agent can identify it precisely.

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?

It gives explicit positive use cases (simple interest on a loan or deposit, short-term note, flat-rate bond coupon) and explicit exclusions with named alternatives: compounding tools, loan-payment tool, and taxes/fees. This is exactly the when/when-not routing guidance needed among many sibling calculators.

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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