Hopi

Mortgage overpayment calculator

hopi_mortgage_overpayment_calculator

Read-onlyIdempotent

See how much time and interest a regular monthly overpayment saves on a repayment mortgage. Give the current balance, annual rate, remaining term in years and the extra amount paid each month. Returns the time saved, interest saved, new payoff time and the monthly payment. Amounts in GBP. Source: https://hopi.co.uk/mortgage-overpayment-calculator/

Input Schema

TableJSON Schema

Name	Required	Description
`rate`	Yes	Annual interest rate as a percentage (0 to 50)
`extra`	Yes	Extra monthly overpayment in GBP (0 or more)
`years`	Yes	Remaining term in years (greater than 0, up to 50)
`balance`	Yes	Current mortgage balance in GBP (greater than zero)

Output Schema

TableJSON Schema

Name	Required	Description
`rate`	No
`extra`	No
`years`	No
`balance`	No
`summary`	Yes
`timeSaved`	No
`source_url`	Yes	The tool's page on hopi.co.uk (cite or open to check the figure)
`monthsSaved`	No
`interestSaved`	No
`monthlyPayment`	No
`newPayoffMonths`	No
`overpaymentPayment`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 20, 2026
- addedOutput schema / required
```
Added value: +[
+  "source_url",
+  "summary"
+]
```
First observedSep 20, 2026

TDQS

A4.3/5.0

Behavior4/5

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

Annotations already establish readOnly, idempotent, and non-destructive behavior, so the description does not need to repeat safety details. It adds useful context by stating the input scenario, the outputs ('time saved, interest saved, new payoff time and the monthly payment'), the GBP currency, and a source link. No hidden or surprising behavior is left undisclosed.

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 three sentences with no filler. It front-loads the tool's purpose, then lists inputs, outputs, currency, and source. Every sentence earns its place.

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?

Given the 100% schema coverage, the output schema, and the simple calculator nature of the tool, the description is complete. It tells an agent what inputs to provide, what outputs to expect, the currency, and even a source for verification. Nothing needed for correct invocation is missing.

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 all four parameters. The description restates the inputs in natural language ('current balance, annual rate, remaining term in years and the extra amount paid each month') but adds little semantic detail beyond what the schema already provides. This meets the baseline but does not exceed it.

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 a specific verb ('see how much time and interest a regular monthly overpayment saves'), a specific resource ('a repayment mortgage'), and clearly distinguishes this tool from siblings like hopi_mortgage_calculator by restricting the scenario to overpayments. An agent can tell exactly what this tool does without opening the schema.

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?

The description provides clear context: it is for repayment mortgages with a regular monthly overpayment. It implicitly distinguishes itself from plain mortgage calculators, though it does not explicitly name alternatives or state when not to use it. This is clear enough for selection but lacks explicit exclusions.

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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