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

With no annotations, the description carries the full burden and does so well: it discloses the default excluded-boundary rule, weekend/Japanese-holiday exclusion, the added banking closure range (12/31〜1/3), and — critically — that from > to returns a negative count instead of erroring, including the antisymmetry identity and its from≠to precondition. That is exactly the non-obvious behavior an agent could not infer.

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?

Core purpose and default boundary are front-loaded, and the input example at the end is immediately usable. The antisymmetry parenthetical is verbose and largely answers a question few agents will ask, costing it the top score.

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 6-parameter, annotation-free tool the description covers boundary semantics, exclusion rules, and edge-case return behavior; the remaining parameters are fully documented in a 100%-covered schema, and no output schema means the numeric return is implicit in "営業日数を数えます". It stops just short of stating the return type explicitly or mentioning extra_closed_dates.

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, but the description adds meaning beyond the schema: it explains why include_from defaults false and include_to true via a worked example, and specifies the concrete dates added by calendar="banking" where the schema only says "銀行休業日も除外". Only extra_closed_dates is left entirely to the schema.

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 and resource (count business days between from and to) and immediately scopes it with the default boundary semantics. It is clearly distinguishable from siblings add_business_days (shifting a date), is_holiday (single-date test), and list_holidays (enumeration), without needing to open any schema.

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?

Provides a concrete use scenario ("今日から支払期日まで何営業日か") and tells the agent how to alter behavior via include_from/include_to and calendar="banking". However, it never explicitly routes the agent away from or toward named alternatives, so it stays at clear-context rather than explicit when/when-not.

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