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

Annotations already declare readOnly/idempotent/non-destructive/openWorld=false, and the description goes well beyond them by disclosing the exact data-coverage boundary (which holiday classes are and are not inferred), the reporting formula, the full output field list, and a documented REST fallback path. This is unusually rich behavioral context for a read-only calculator.

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?

Usage guidance is correctly front-loaded, but the body repeats the input schema as a prose list, restates output fields that an output schema already defines, and adds formula plus REST/docs boilerplate. Several of these blocks duplicate structured data rather than earning their space.

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 an output schema present and annotations covering the safety profile, the description supplies the remaining essentials: domain scope, exclusions, argument example, and a fallback endpoint. The one real gap is routing guidance among the count/check/add working-day siblings and any statement of supported date-range limits beyond the schema's 2026-2027 hints.

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 description coverage is 100% and every parameter already carries a description and example, so the schema carries the semantics. The description only restates the four inputs with types/optionality and one combined JSON example, adding no new interpretation (e.g., what happens when both include flags are false or how inclusive ranges behave).

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?

"Counts authority-scope working and non-working dates in a range" is a specific verb + resource, and the Belgium/authority-scope qualifier is clear. However, in a set of 200+ siblings it never distinguishes itself from check_be_working_day (single date) or add_be_working_days (date arithmetic), so the agent must infer the boundary from the name alone.

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 both a "use this when" trigger (auditable Belgium working-day count) and a "do not use this when" exclusion covering substitute dates, sector agreements, and employer/school/bank schedules, plus an instruction to call the tool rather than answer from memory. The exclusion is awkwardly worded ("private-sector statutory core only") and no alternative sibling tool is named for the excluded cases.

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