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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behavior: the pinned 2026-2027 coverage window, the computation rule (weekday baseline and no Cabinet Office statutory holiday), and that outputs carry dataset_version and sources. No auth or rate-limit notes, hence not a 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?

Labeled sections (when, when-not, computes, example requests, inputs, outputs, formula, fallback, docs) make it scannable and the highest-value content is front-loaded. It is longer than strictly necessary — the formula line and the REST fallback are edge-case detail — but nothing is actively wasteful.

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?

An output schema exists, yet the description still names the coverage window, the exclusions, an example request and a fallback path, so an agent has everything needed to call it correctly and interpret results. Nothing material is missing for a single-parameter read-only calculator.

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?

One parameter with 100% schema coverage, and the schema already documents it as a Gregorian date in the pinned Cabinet Office 2026-2027 window. The description repeats 'date (date)' and the same range constraint, adding essentially no syntax or format detail beyond the schema, so the baseline 3 applies.

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 (checks a date against Japan's statutory national holidays) plus the exact scope window (2026-2027) and the explicit boundary against other calendar types (company, school, bank, exchange, prefectural). An agent can distinguish this from the other check_* country tools and from add_/count_japan_national_working_days without opening a 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?

Gives an explicit 'use this when' condition, an explicit 'do not use this when' exclusion (non-statutory closure calendars), and instructs the agent to call the tool directly rather than answer from memory or stop at discovery. That is the full when/when-not/alternative triad.

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