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

Beyond the annotations, the description discloses pricing ($0.003/call), determinism, primary-source citation, and a billing policy that errors and unverified rules are never charged. That is meaningful behavioral context for a paid endpoint. It does not explain the readOnlyHint=false flag or payment handshake flow, which the schema's x_payment field covers instead.

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?

Capabilities are front-loaded, followed by price, guarantees, and the mode contract — a sensible priority order. The mode mapping duplicates text already present in the schema's mode description, which is minor redundancy rather than noise.

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 full schema coverage, the description need not explain return values, and it covers all five modes and their required inputs. Only the optional x_payment flow and defaults like saturday_off are left entirely to the schema, which is acceptable.

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% across all 10 parameters, so the schema already documents each field including the mode enum. The description's mode-to-param mapping adds orientation but is largely a repeat of the mode parameter's own schema description, so baseline 3 is appropriate.

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?

The opening sentence names specific capabilities: Japanese holidays, business-day arithmetic, statutory filing deadlines with holiday shift, and wareki conversion. No sibling (jp_payroll_net, jp_consumption_tax, jp_social_insurance, etc.) covers holiday calendars or day arithmetic, so an agent can route here 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 mode-to-parameter contract ('add -> from, days; between -> from, to; holidays -> year; deadline -> kind, base_date; wareki -> date') tells the agent exactly how to configure a call for each use case. It gives clear context but no explicit when-not or alternative-tool guidance, since no sibling offers overlapping functionality.

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