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 the returned field list and the word 'normalized', but with an output schema present this is largely redundant, and it omits auth, rate-limit, and pagination 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?

Two sentences, front-loaded with the verb and resource; the second sentence is the usage route. The field enumeration is dense but purposeful, and there is no filler.

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 annotations covering read-only/open-world behavior, a complete input schema, and an output schema, the description supplies the missing routing to mercari_search and what the fetch covers. Nothing needed for correct invocation is absent.

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%, and the schema itself explains the id as a Mercari item id from a search result's id field. The description only repeats 'by `id`' without adding format or syntax meaning, so the 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?

Fetch ... Mercari listing's details ... by `id` specifies a clear verb and resource, enumerates the payload fields, and names mercari_search as the discovery alternative, so an agent can distinguish it from mercari_search and the other Mercari siblings.

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 final sentence explicitly routes discovery to mercari_search, making the detail-vs-discovery split clear. There is no ambiguity about when to call this tool versus the search sibling.

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