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 results come from a 'live' resale marketplace and lists the returned fields, but says nothing about pagination, result limits, or sorting, which matters for a search tool.

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?

One tightly packed sentence, front-loaded with the tool's identity and scope. It is slightly redundant in enumerating return fields that the output schema already defines, but nothing is wasted.

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?

An output schema exists, so return-field detail in the description is a bonus rather than a necessity. For a one-parameter search tool the definition is adequate but leaves query construction and pagination/result-count behavior unexplained.

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 single 'query' parameter has 0% schema description coverage and the description adds no guidance on what to put in it (keywords, brand names, item titles, filters). With a required param that is completely undocumented on both sides, the description fails to compensate.

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 states a specific verb and resource ('Mercari listing search') and even enumerates what the results contain (title, price, thumbnail, condition, seller, total count). It is clearly distinguishable from mercari_item (detail) and mercari_autocomplete (suggestions), though it never explicitly names those alternatives.

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?

There is no when-to-use guidance, no conditions, and no mention of alternatives such as mercari_item for detail lookups or mercari_autocomplete for query completion. Usage is only implied by the word 'search'.

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