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

readOnlyHint=true already establishes this is a safe read, so the description's job is to add return context — which it does by disclosing that results carry Japanese/English labels and a category, and by naming the actual key vocabulary. It does not state ordering or whether unknown categories error, which is the only remaining gap.

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?

Front-loaded with the payload (enumerated keys) and the routing instruction in two tight sentences. The Japanese half is a verbatim restatement, which is redundant for most agents but serves the stated ja/en bilingual audience.

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 and one optional filter parameter, the description has to sketch the return shape itself, and it does: keys plus ja/en labels plus category. A note about whether the list is stable/cacheable would make it complete, but an agent can call this correctly as written.

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% with a single enum parameter whose six values and per-value meaning are already in the schema. The description does not expand on what each category contains, so it adds nothing beyond the structured field — 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 ('List the reason keys that classify EDINET extraordinary reports'), enumerates actual example values, and distinguishes itself from the sibling get_extraordinary_report_events by positioning itself as the master lookup feeding that tool's `reason` field.

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?

Explicitly tells the agent the workflow: use a returned key as `reason` in get_extraordinary_report_events, and omit the category filter for all values. There is no equivalent alternative master-list tool among the siblings, so the absence of when-not guidance costs nothing.

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