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

Annotations already establish a safe, read-only, idempotent, non-destructive operation, so the bar is lower. The description still adds real behavioral value by disclosing the return shape ('returns the number and does not place it against a range'), which is important with no output schema, but it does not mention the disclaimer/clinician note that the locale parameter implies is also produced.

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 sentences, front-loaded with the formula and scope, then the return contract. Every clause carries information; nothing is padding or repetition of the name/title.

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 pure calculation with no output schema and full annotation coverage, the description is nearly complete: formula, scope, alternative, and return semantics are all present. The only gap is that it describes the return as just the number while the locale parameter signals an accompanying disclaimer and clinician note, leaving that part of the response undisclosed.

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 baseline is 3. The description goes beyond the schema by spelling out the unit algebra (grams and cm cubed versus kg per cubic metre), clarifying how weightKg and lengthCm are consumed and reported. It adds nothing about the locale parameter, which the schema already handles well.

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 computation with its exact formula (100 x weight in grams / length in cm cubed), names the resource, and explicitly scopes it as a newborn proportion measure rather than a general growth metric. It is easily separable from bmi_percentile and growth_percentile siblings because it declares what it does not do (place the value against a range).

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?

Provides clear context for when it applies ('newborn proportion measure') and names the alternative for the excluded case ('BMI-for-age is the usual measure for older children'). It lacks explicit prerequisites or input-format guidance beyond what the schema supplies, so it falls just short of full routing instructions.

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