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

Annotations already cover readOnly/openWorld/idempotent/non-destructive, so the safety profile is handled. The description adds genuinely new behavioral context: the flat 5-point cost per successful run and a full enumeration of returned fields in lieu of an 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?

A single front-loaded sentence leading with the verb and resource, then the field list. The field enumeration is long but earns its place as the only return-value documentation; no filler sentences.

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 one-parameter read tool with no output schema, the description supplies both the scope (single item by id/URL) and the full return contract (price currency, status values, shipping, seller, features), plus cost. 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?

Only one parameter with 100% schema coverage, where the schema already says 'Item ID or URL' and supplies the example m47341865350. The description's 'by id or URL' adds nothing beyond the schema, so the baseline of 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 (Get) and resource (Mercari Japan item), plus the accepted lookup keys (id or URL), and enumerates the returned fields. It is clearly a single-item lookup rather than a search, though it never names the sibling run_mercari_items_search to make the distinction explicit.

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 phrase 'by id or URL' implies you need an already-known identifier, which suggests search-first workflows, but the description gives no explicit when-to-use/when-not guidance and never references the search sibling or any alternative.

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