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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds that responses include a related-listings carousel, but discloses nothing about authentication, rate limits, or availability of the item (e.g., sold-out behavior).

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 that lists the payload without preamble or filler. The enumeration is slightly redundant given an output schema exists, but it is not wasteful.

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 an output schema and read-only annotations, the definition is adequate but thin: it spends its text restating return fields the output schema already covers, while omitting the one thing that matters — how to obtain or format the id.

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 0% for the single required id parameter, and the description says nothing about it — not its format (e.g., an 'm' prefixed id), not where to source it. With one undocumented parameter the description should compensate but does not.

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 the specific resource (a Mercari item) and enumerates the detail set it returns, which clearly differentiates it from mercari_search/mercari_home. It describes output content more than the retrieval action, but an agent can still tell what it does without ambiguity.

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 description never states when to use this tool instead of mercari_search or the sibling lookup tools, nor does it say that a valid Mercari item id (typically obtained from a search result) is a prerequisite. Usage is only inferable from the tool name.

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