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

Annotations only cover readOnly/openWorld, so the description carries real weight: it discloses the archive's temporal limit ('before this archive started'), the full status vocabulary (found, not_archived, unknown, no_issuance_at_that_time), and the report_datetime vs fetched_at distinction. That is materially more than the annotations provide, though return-field detail partly overlaps the output 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?

Front-loaded with the core purpose, then parameter usage, return contents, statuses, and the routing caveat, with almost no filler. Dense but every clause carries information; the run-on return sentence could be split for readability.

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?

A complete definition for a read-only archival lookup: purpose, parameter selection rules, return payload, issue-time vs fetch-time semantics, status outcomes, and the explicit pointer away from the live forecast. With annotations and an output schema present, 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 description coverage is 100%, so the schema already defines area codes and the ISO-8601 format of 'at' and 'issued_at'. The description restates those semantics in terser form and distinguishes the two time params, but adds little that the schema does not already say; 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: an archived JMA prefectural weather forecast as it was originally issued. It clearly contrasts itself with the live/current forecast, though it never names its actual siblings (forecast_issuances, catalog), so sibling differentiation is only partial.

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 concrete selection guidance between the two time parameters: use 'at' for the issuance in effect at a time, 'issued_at' for an exact JMA issue time, plus area requirements. It does not spell out forbidden cases beyond 'not the current forecast', so no explicit when-not clause against siblings.

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