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by uk.co.hopi
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Compound interest calculator

hopi_compound_interest_calculator
Read-onlyIdempotent

Project savings growth from a starting deposit plus a regular monthly contribution, compounded monthly with contributions added at the end of each month. Returns the final balance, total paid in and interest earned. Amounts in GBP, rate is the annual rate as a percentage. Source: https://hopi.co.uk/compound-interest-calculator/

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
rateYesAnnual interest rate as a percentage
yearsYesTerm in years (1 to 100)
monthlyNoMonthly contribution in GBP (default 0)
principalNoStarting deposit in GBP (default 0)

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
rateYes
yearsYes
monthsYes
monthlyYes
summaryYes
principalYes
source_urlYesThe tool's page on hopi.co.uk (cite or open to check the figure)
totalPaidInYes
finalBalanceYes
interestEarnedYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • addedOutput schema / required
      Added value: +[
+  "finalBalance",
+  "interestEarned",
+  "monthly",
+  "months",
+  "principal",
+  "rate",
+  "source_url",
+  "summary",
+  "totalPaidIn",
+  "years"
+]
  2. First observed

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, and destructiveHint, so the safety profile is covered. The description adds valuable behavioral detail beyond annotations: compounding frequency (monthly) and contribution timing (end of month), plus return values and currency. No contradictions.

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 with no wasted words. Purpose is front-loaded, return values are stated, units and rate basis are given, and a source link is included. Exceptionally concise and well-structured.

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 calculator with an output schema (not shown but indicated), the description adequately covers inputs, calculation method, and outputs. It lacks details on edge cases or default behavior nuances, but the combination of annotations, schema, and output schema leave little uncovered. The description is complete for an agent to call 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?

With 100% schema coverage, the schema already documents all parameters. The description adds meaning beyond the schema by clarifying the compounding frequency (monthly) and that contributions are added at month-end, which is not in the schema. It also reinforces that amounts are in GBP and rate is annual percentage.

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 with a specific verb ('Project savings growth') and resource ('starting deposit plus a regular monthly contribution'), and details the compounding method. It also lists what it returns (final balance, total paid in, interest earned), making it distinct from sibling savings tools like hopi_savings_goal_calculator.

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 conveys clear usage context: projecting savings with monthly contributions compounded monthly. However, it does not explicitly mention when to use this over alternatives or any exclusions, though the unique combination of features implicitly differentiates it from other financial 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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