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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds the formula (BMI = weight in kg / (height in metres squared)) and the NHS category output, which are useful behavioral details beyond the schema. It does not mention edge cases like invalid height/weight values, but the schema's exclusiveMinimum covers basic validation.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences with no filler. The core purpose is front-loaded, the unit instructions are compact, and the formula and source are included without bloating the description.

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

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

The description covers purpose, unit selection, formula, and output category. The output schema exists, so return values need not be explained. Minor gap: it doesn't state what happens with invalid inputs (e.g., zero or negative values), but the schema's exclusiveMinimum and the simple nature of the tool make this a small omission.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

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 five parameters. The description adds the unit-system mapping (metric vs imperial) and the formula, but it does not add meaning beyond what the schema's conditional requirements and parameter descriptions already provide. Baseline 3 is appropriate.

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

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

The description states a specific verb ('Work out'), a clear resource ('body mass index (BMI) from height and weight'), and the distinctive output ('with the NHS weight category'). It also names the two unit systems, which distinguishes it from sibling calculators like hopi_body_fat_calculator or hopi_weight_converter.

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

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

The description gives explicit usage instructions: use 'metric' with heightCm and weightKg, or 'imperial' with heightInches and weightPounds. It does not explicitly state when not to use it or name alternatives, but the unit-specific guidance is clear enough for an agent to select and invoke it correctly.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.