Hopi

Age calculator

hopi_age_calculator

Read-onlyIdempotent

Work out an exact age from a date of birth, as of today or an optional 'as of' date. Dates use the YYYY-MM-DD format. Returns the age in years, months and days, the total days lived, and the next birthday. Source: https://hopi.co.uk/age-calculator/

Input Schema

TableJSON Schema

Name	Required	Description	Default
`dob`	Yes	Date of birth in YYYY-MM-DD format
`asOf`	No	Optional date to measure the age at, in YYYY-MM-DD format (defaults to today)

Output Schema

TableJSON Schema

Name	Required	Description
`dob`	No
`asOf`	No
`days`	No
`years`	No
`months`	No
`summary`	Yes
`totalDays`	No
`source_url`	Yes	The tool's page on hopi.co.uk (cite or open to check the figure)
`nextBirthdayInDays`	No
`nextBirthdayWeekday`	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 declare readOnlyHint, idempotentHint, and non-destructive behavior. The description adds meaningful behavioral detail by listing the computed outputs (years, months, days, total days lived, next birthday) and clarifying the optional as-of date behavior beyond the structured 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?

Three short sentences deliver purpose, input format, optional parameter behavior, and output enumeration with no filler. The most important usage detail is front-loaded, and the source URL adds provenance without bloating the description.

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 rich annotations and a complete schema, the description covers what the tool does, what inputs it accepts, what it returns, and how dates should be formatted. Nothing essential is missing for correct invocation.

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%, and both parameters already have clear descriptions and date formats. The description adds only minor value by restating the optional as-of date and YYYY-MM-DD format, so the baseline of 3 is appropriate.

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 uses a specific verb ('work out an exact age') and names the precise resource (a date of birth) with optional as-of-date semantics. It clearly differentiates this from date-related siblings like days_between_dates or working_days_calculator by focusing on exact age breakdown and next birthday.

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 gives clear context for when to use the tool: computing an exact age from a DOB, either today or at a chosen date. It does not explicitly name alternative tools or exclusions, but the use case is unambiguous enough for an agent to select it correctly among the 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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