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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), and the description adds genuinely non-obvious behavior: 15-minute result caching, per-search request cost that is charged even on cache hits, free failures, and the maximum page size of 200 rows. That is exactly the cost/limit context an agent needs before calling, and it goes well beyond the structured fields.

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?

Four sentences, front-loaded with the core purpose, then sizing, caching and cost, then the docs link. Every sentence carries distinct operational information and none restates the title or schema verbatim.

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 12-parameter read-only search with no output schema, the description covers the high-risk unknowns (cost, caching, page limits) and points to the docs URL. It stops short of describing the shape of a result row, which an agent with no output schema would benefit from knowing before committing requests.

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 already explains q, page, sort, count, fields, country, condition, prices, category_id, buying_format and item_location in detail. The description's only parameter-level additions are the 200-vs-40 row behavior and the country count, so it does not meaningfully exceed the schema. Baseline 3 applies when the schema carries the load.

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 and resource ('Search one page of eBay sold listings by keyword') plus the geographic scope (US or seven other countries). The word 'sold' cleanly separates it from the sibling ebay_listings, which covers active inventory, so an agent can route correctly without opening either 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?

Gives concrete operating context – page size semantics (200 rows with count=200, 40 without), 15-minute caching, and cost accounting (1 request per completed search including cache hits, failures free). It never explicitly names an alternative tool or when not to use this one, so the routing guidance is implied rather than stated.

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