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by com.helpyself
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Body Fat Calculator

body_fat_calculator
Read-onlyIdempotent

A body fat calculator that works from a tape measure, using the US Navy method. A HelpySelf tool (helpyself.com).

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
hipNoHip circumference at the widest point, in the unit named by system. Required when sex is female; ignored when male.
sexYesWhich of the two US Navy equations to use; they are not interchangeable. female needs a hip measurement and uses waist plus hip; male uses the waist alone.
neckYesNeck circumference measured just below the larynx, in the unit named by system. Must be smaller than the waist (or waist plus hip for female) or the request is refused.
waistYesWaist circumference, relaxed, in the unit named by system: at the navel for men, at the narrowest point for women.
heightYesStanding height, in the unit named by system.
systemNoThe unit every measurement is in. metric (the default) reads height, neck, waist and hip in centimetres and weight in kilograms; imperial reads lengths in inches and weight in pounds.metric
weightNoBody weight in kg (metric) or lb (imperial). Not part of the percentage; it is only used to also return fat mass and lean mass in the same unit. Omit it and those come back null.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed7 schema fields changed
    • addedInput schema / properties / height / description
      Added value: +"Standing height, in the unit named by system."
    • addedInput schema / properties / hip / description
      Added value: +"Hip circumference at the widest point, in the unit named by system. Required when sex is female; ignored when male."
    • addedInput schema / properties / neck / description
      Added value: +"Neck circumference measured just below the larynx, in the unit named by system. Must be smaller than the waist (or waist plus hip for female) or the request is refused."
    • addedInput schema / properties / sex / description
      Added value: +"Which of the two US Navy equations to use; they are not interchangeable. female needs a hip measurement and uses waist plus hip; male uses the waist alone."
    • addedInput schema / properties / system / description
      Added value: +"The unit every measurement is in. metric (the default) reads height, neck, waist and hip in centimetres and weight in kilograms; imperial reads lengths in inches and weight in pounds."
    • addedInput schema / properties / waist / description
      Added value: +"Waist circumference, relaxed, in the unit named by system: at the navel for men, at the narrowest point for women."
    • addedInput schema / properties / weight / description
      Added value: +"Body weight in kg (metric) or lb (imperial). Not part of the percentage; it is only used to also return fat mass and lean mass in the same unit. Omit it and those come back null."
  2. First observed

TDQS

B3.3/5.0
Behavior3/5

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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, covering the safety profile. The description adds the method and tape-measure basis but no additional behavioral details such as error conditions, refusal cases, or output format. Since annotations carry the safety burden, a 3 is appropriate.

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 a single sentence, concise and front-loaded with the core purpose and method. No wasted words; it's appropriately sized for a simple calculator tool.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness3/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The description is minimal and doesn't mention the output (e.g., body fat percentage, fat mass/lean mass when weight is provided) or the conditional hip requirement for female subjects. While the schema covers parameters, the lack of output description leaves a gap for agents expecting a result format. However, the tool is simple and the schema is rich, so a 3 is fair.

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%, with each parameter explained in detail (e.g., hip required for female, neck must be smaller than waist). The description adds no parameter-specific information beyond what the schema provides, so the baseline 3 applies.

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 a specific purpose: a body fat calculator using the US Navy method from tape measurements. It identifies the resource and method, distinguishing it from generic calculators, though it doesn't explicitly name sibling alternatives like BMI or calorie calculators.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

No guidance on when to use this tool versus alternatives. It doesn't mention prerequisites, exclusions, or context that would help an agent decide between this and other calculators. The agent must infer usage from the name and schema.

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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