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

Annotations already declare readOnly, idempotent, non-destructive, non-open-world, so the safety profile is covered. The description adds genuinely new behavioral context beyond the schema: the Feb 29 -> Mar 1 rule and the Japan-timezone default for the reference date, which an agent could not infer otherwise.

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 verb and purpose, followed by a dense output list and two short edge-case sentences. Every clause carries information, though the long mid-sentence output enumeration is list-heavy rather than prose-optimized.

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?

With no output schema, the description correctly enumerates the returned fields and covers the key edge case (leap-day) and the reference-date default. The only thin spot is that lang's effect is left entirely to the schema, but overall it is complete enough for correct invocation.

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% and the schema documents all three parameters including the lang enum and date range. The description adds only one semantic not in the schema, the default reference date being today in Japan, so the baseline of 3 applies.

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 opens with a specific verb+resource ('Calculate exact age from a birth date') and then enumerates the concrete outputs, so an agent knows precisely what the tool computes. It is easily distinguished from siblings like convert_japanese_era or days_between because it bundles multiple derived values rather than performing one conversion.

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: the leap-day rule and the 'reference date defaults to today in Japan' note signal relevant contexts, but there is no explicit statement of when to pick this over days_between or convert_japanese_era. The guidance is adequate but leaves the routing decision to inference.

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