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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behavioral context: the return is a bare number with no range or reference comparison ('does not place it against a range'), which tells the agent not to expect a percentile or interpretation.

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 compact clauses: definition/formula first, then scope, then the return-value caveat. Every sentence earns its place, though the formula clause is dense enough that it could be split for faster scanning.

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?

There is no output schema, so the description carries the burden of describing the return value, and it does so explicitly. With full annotation coverage and a simple deterministic calculation, little else is needed; only edge behavior at the weight/length bounds is left to the schema.

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% and the units, ranges and locale enum are all documented in the schema itself. The description's formula adds a small amount of unit context (grams vs kg per cubic metre) but nothing about how to pass locale or handle the min/max bounds, so this is the baseline 3.

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 names the exact measure (Rohrer's ponderal index), the inputs it derives from (weight and length), and the formula, so there is no ambiguity about what is computed. It also scopes the tool to newborn proportions, which separates it from the percentile siblings such as bmi_percentile and growth_percentile.

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?

It states the population this is for ('a newborn proportion measure') and names the alternative measure and the condition that selects it ('BMI-for-age is the usual measure for older children'), which maps directly onto the sibling tools. An agent can route between this and bmi_percentile without opening either schema.

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