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

No annotations are provided, so the description carries the full burden, and it does well: it discloses data provenance (values checked daily on Rakuten Market and Yahoo! Shopping), the computation basis, sorted output, and the 'no estimates' (推定なし) qualifier, which is meaningful for a price tool. It omits output-shape details, but the read-only, computed-report nature is clear.

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?

Four compact sentences with the core purpose front-loaded. Every clause earns its place: scope, sort order, data source, and the estimation policy. No filler, no redundancy.

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?

For a 0-parameter tool with no annotations and no output schema, the description covers purpose, scope, ordering, data source, and a transparency guarantee—nearly everything an agent needs to invoke it correctly. The only gap is that it does not describe the returned fields, which matters a bit more because no output schema exists.

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?

The tool has zero parameters, so the baseline is 4. The description adds the semantic context an agent needs to interpret this fixed report: scope (6 mounts, two marketplaces), sort order, and the price-drop threshold it tracks.

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 description names a specific verb and resource: a list (一覧) of interchangeable lenses whose price fell below the previous best price, computed across six mounts. It adds the sort criterion (drop amount, descending) and the data basis, which together clearly distinguish it from the lens siblings (lens_filter, lens_index, lens_item).

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 use context is implied rather than explicit: an agent can tell this is the tool for 'which lenses got cheaper' queries, but the description never names alternatives or states when not to use it. With four lens-related siblings (lens_filter, lens_index, lens_item) and pricing tools like hikaku and prices_now, explicit routing would materially help.

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