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

Beyond the read-only/idempotent annotations, the description discloses important behavior: end defaults to today UTC, leap-day handling is specified with examples, parts are negative when end precedes start, and total_days is signed. This is rich, call-relevant behavioral context.

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?

The description is front-loaded with purpose and usage, and the examples earn their place by resolving leap-day ambiguity. However, the final sentences about the result and non-repeated dates are somewhat redundant given the output schema, keeping it from a 5.

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?

The tool is simple, both parameters are fully documented in the schema, annotations cover the safety profile, and an output schema exists. The description adds the edge-case behavior an agent needs to call and interpret the tool 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 already 100%, so the baseline is 3, but the description adds useful semantics: end defaults to today UTC, start can represent a birthdate, and the output parts are signed when end is before start. The leap-day example further clarifies how the date parameters are interpreted.

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 states a specific calculation: age/duration from start to end expressed as calendar years, months, and days. This clearly distinguishes it from a total-days sibling like days_between.

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 gives a clear usage condition: when the user asks for an age or the length of time between two dates in years, months, and days. It does not explicitly name days_between as an alternative or state when not to use it, so it falls short of a 5.

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