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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds genuinely useful context the annotations do not carry: results are relevance-ordered and an empty list is a normal outcome for obscure input. It does not cover auth or rate-limit behavior, but the annotation baseline makes that tolerable.

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, zero waste, with the core purpose front-loaded ahead of the sibling routing. Every clause earns its place.

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 single-parameter autocomplete tool with a full schema, an output schema, and annotations covering the safety profile, the description supplies exactly what the structured fields do not: purpose, sibling routing, and empty-result semantics. Nothing needed 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?

Schema coverage is 100%, so the single `query` parameter is fully documented in the schema itself. The description's mention of 'partial `query`' restates the schema wording rather than adding new syntax or format detail. Baseline 3 is correct when the schema does the heavy lifting.

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 ('Fetch') and resource ('Mercari's relevance-ordered search suggestions') plus the input scope ('partial `query`'). It is immediately distinguishable from the search/item/master siblings without opening any schema.

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?

Explicitly routes the agent to alternatives by intent: mercari_search for listings, mercari_item for a single listing, mercari_master for catalog details. It also sets expectations for the empty-result case, so the agent knows not to retry or treat it as an error.

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