Hopi

Mortgage repayment calculator

hopi_mortgage_calculator

Read-onlyIdempotent

Work out the monthly payment on a repayment mortgage at a constant interest rate, using the standard amortising formula. Returns the monthly payment, total repaid and total interest. Fees and rate changes are not included. Amounts in GBP. Source: https://hopi.co.uk/mortgage-calculator/

Input Schema

TableJSON Schema

Name	Required	Description
`rate`	Yes	Annual interest rate as a percentage (0 to 50)
`years`	Yes	Mortgage term in years (greater than 0, up to 50)
`amount`	Yes	Amount borrowed in GBP (greater than zero)

Output Schema

TableJSON Schema

Name	Required	Description
`rate`	No
`years`	No
`amount`	No
`months`	No
`summary`	Yes
`source_url`	Yes	The tool's page on hopi.co.uk (cite or open to check the figure)
`totalRepaid`	No
`totalInterest`	No
`monthlyPayment`	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

A4/5.0

Behavior4/5

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

Beyond the annotations (readOnlyHint, idempotentHint, destructiveHint), the description discloses what the tool returns and explicitly excludes fees and rate changes. This adds valuable functional context without contradicting the annotations.

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 compact and well-structured: it front-loads the core purpose, then lists outputs, exclusions, and currency. Every sentence contributes information, with no unnecessary verbiage.

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?

Given the output schema exists and the tool is a simple read-only calculator, the description sufficiently explains inputs, outputs, and limitations. It does not discuss edge cases like zero interest rates, but these are minor and easily inferred from the 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?

The input schema covers all three parameters with descriptions, units, and constraints (100% coverage). The description adds no parameter-specific details beyond the schema, so it meets the baseline of 3 without exceeding it.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's purpose: it works out the monthly payment on a repayment mortgage at a constant interest rate, and it lists the returned values. It does not explicitly name a sibling tool, but the exclusions (fees, rate changes) provide implicit differentiation from broader calculators.

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 a clear when-not condition by stating 'Fees and rate changes are not included,' which helps an agent avoid using this tool for variable-rate or fee-inclusive mortgages. However, it does not name alternative tools, so it falls short of a 5.

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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