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

Annotations only declare readOnlyHint=true; the description goes well beyond by disclosing the result shape (one row per segment × metric), the unit convention (values in table unit, multiply by unitMultiplier for JPY), the EDINET source link, and the empty-result behavior. These are exactly the traits an agent cannot infer from the annotation or schema.

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?

The core content is front-loaded and useful, but the entire paragraph is then repeated verbatim in Japanese, and a generic ISO 8601 date-format note is appended that is irrelevant to a tool whose only parameters are a ticker and a year-month. Roughly half the text is duplication or boilerplate.

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 no output schema, the description correctly compensates by explaining row structure, units, relatedItems, and the empty-items case. Minor gaps remain: no mention of error behavior for an unknown ticker or how many rows/segments to expect, but nothing an agent needs to call it correctly is missing.

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% and both parameters are documented in the schema (ticker format and YYYY-MM fiscal period). The description only restates that fiscalPeriod omitted yields the latest report, which the schema already says, and adds nothing about symbol semantics. 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?

Names a specific resource and scope: "Reportable-segment breakdown from the latest annual securities report (有価証券報告書)", and enumerates the metrics returned (external sales, segment profit, assets, depreciation, capex). It also distinguishes itself from adjacent data by stating that product/service and geographic tables live in relatedItems, separating it from siblings like get_sga_breakdown or get_subsidiaries.

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 clear usage context: omit fiscalPeriod for the latest period, expect an explanatory note when items is empty (single segment / unsupported layout), and look in relatedItems for related-information tables. It does not explicitly name an alternative tool to use instead for other segment-like data, so it falls short of full when/when-not routing.

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