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

Annotations already establish read-only, non-destructive, closed-domain behavior, so the bar is lower; the description usefully adds that 明六つ/暮六つ follows the NAOJ definition and that prefecture defaults to Tokyo. It does not say anything about how location shifts the results, invalid-datetime behavior, or the shape of the returned values, and with no output schema that gap matters.

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?

A single dense sentence, front-loaded with what is returned, with parenthetical examples that earn their place by making the unfamiliar 刻 naming concrete. No filler; only the trailing location clause is slightly redundant with the schema.

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?

For a simple two-parameter read-only tool this is close to sufficient, and the description does enumerate the three output categories. However, with no output schema the description still leaves the return format (single string vs. structured fields, whether sun-event times are included) unspecified, so an agent cannot predict the response shape.

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; both schema fields already carry format descriptions. The description only restates that datetime is Japan time and that prefecture defaults to Tokyo — the latter duplicates the schema verbatim — without explaining how prefecture affects the computed 明六つ times.

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?

Names a specific verb (返す) and a concrete, unusual resource set: 明六つ・暮六つ, 不定時法の刻, 十二辰刻, with examples (暮六つ・暁九つ, 丑三つ時). This clearly separates it in spirit from siblings such as get_sun_moon_times or get_rokuyo_hours, but no sibling is named, so it stays at 4.

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?

Input expectations are stated (Japanese-time datetime, prefecture defaults to Tokyo), which implies the use case, but there is no explicit framing of when to pick this over get_rokuyo_hours or get_sun_moon_times, and no exclusions or prerequisites. Usage is inferable rather than guided.

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