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

Annotations already cover the safe read-only, idempotent, non-destructive profile, so the bar is lower. The description adds genuine behavioral context beyond them: which chart applies at which age, that from 24 months a CDC percentile-range bucket is returned, and explicitly that no interpretation is 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?

Front-loaded with the core action and the age-based standards; every sentence carries information. Slightly long, and the website attribution sentence is marginal, but nothing is padding.

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?

No output schema exists, so the description must cover returns — and it does, describing the percentile-range buckets and stating that no interpretation is included. Combined with the full schema coverage, an agent has everything needed to call it correctly.

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 coverage is 100%, so the baseline is 3. The description goes beyond the schema by explaining why sex and age matter (charts published separately, WHO/CDC split at 24 months), giving the ageMonths parameter real interpretive meaning.

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 (computes BMI and places it on the BMI-for-age chart) and specifies the reference standards by age. This clearly separates it from siblings like growth_percentile and ponderal_index.

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?

Gives explicit when-to-use context: WHO below 24 months, CDC 24–240 months, and warns that childhood BMI is never read against adult 18.5–25 bands. It does not, however, explicitly route the agent among sibling tools such as growth_percentile or zscore_percentile.

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