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

Annotations already cover the read-only, non-destructive, closed-world profile, so the description's job is to add semantics — which it does by pinning the supported range (1926–2080) and noting that a year falling on a 改元 returns both era names. It stops short of describing how the two result shapes differ or how errors surface, keeping it below 5.

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?

One compact sentence front-loads the primary year-conversion case, then the date case, then the argument rule. No filler and no repetition of the 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?

With no output schema, the description usefully enumerates the returned fields for each mode (和暦・干支・九星 vs 本命星・月命星), which is essential. Minor gap: it does not say whether both parameters may be passed at once or how the two result shapes are keyed.

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 only 50% (year has bounds but no description), and the description compensates: it restates the 1926–2080 range and adds the critical constraint that exactly one of year or date should be supplied, despite both being optional in the schema. It could add date-format expectations more explicitly, but the schema already carries YYYY-MM-DD.

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 (換算する / convert) plus the three target resources for each mode (和暦・十干十二支・九星 for a year; 和暦・本命星・月命星 for a date). No sibling tool in the list does era/eto/kyusei conversion, so an agent can select it unambiguously.

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?

The tool's niche is narrow enough that the intended use is implied, and the year/date either-or instruction steers invocation. However, it never states when to prefer this over siblings (e.g. get_koyomi_day) or what happens if neither/both arguments are supplied, so guidance is implicit rather than explicit.

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