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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is fully covered outside the description. The description adds little beyond that — it restates the returned field names, which the output schema already provides — and says nothing about validation behavior (e.g. what happens for out-of-range circumference despite exclusiveMinimum) or rounding conventions for the size mapping.

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?

Two tight sentences, with the core capability front-loaded and the return shape trailing. The trailing 'See list_bundles' pointer is marginal filler for a single-purpose calculator but costs only a few words.

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?

For a one-parameter, non-destructive calculator with full annotations and an output schema, the description covers purpose, input unit, and output systems — everything needed to invoke it correctly. No behavioral gaps remain that the structured fields don't already close.

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 0%, so the description carries the burden for the single parameter; it does so by supplying the unit ('head circumference (cm)') that the schema omits. With only one clearly named required input, this is adequate, though it does not state the accepted range or precision expected.

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?

States a specific verb+resource ('Calculate hat size') and enumerates the exact output systems (FR/EU, US/UK, S/M/L/XL) plus the input basis (head circumference in cm). An agent can immediately tell this apart from sibling calculators such as calculate_bra_size or calculate_clothing_size_convert.

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?

Usage is only implied by the resource name; there is no explicit when-to-use, no prerequisites, and no named alternative among the many sizing/convert siblings. The one routing hint ('See list_bundles for related textile-mode calculators') points at a bundle-directory tool rather than a competing calculator, so it helps discovery but not tool selection.

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