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

Annotations already declare readOnly, idempotent, closed-world and non-destructive, so the safety profile is covered. The description adds genuine value beyond that: results are per-site, offers are ordered by cheapest total, each carries a link, and the best overall price is surfaced, plus the market scoping rule that only sites serving the locale respond.

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?

Three tight sentences: identifier scope first, return shape second, no-match semantics last. The closing "never a guess" line earns its place by preventing pointless retries, though the phrasing is slightly promotional rather than purely informative.

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 only three parameters, full schema coverage and no output schema, the description carries the return-value burden well by describing per-site matches, offer ordering and the best price. Only the miss case's return shape (empty vs. per-site omissions) is left unstated.

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%, with the code formats, the sites default and the full locale enum already documented in the schema. The description reinforces the locale-market semantics and the "every site" default but adds no syntax or constraints the schema lacks, so it sits at the baseline.

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?

"Exact identity lookup" names a specific verb and scope, and enumerates the three accepted identifier types (GTIN/EAN, manufacturer item number, LEGO set number), which is enough to separate it from a fuzzy catalogue lookup. It never names the sibling it is not (search_products), so differentiation is inferred from the word "exact" rather than stated.

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 framing "exact identity lookup" plus "Never a guess: no match means no site lists that exact product" tells the agent this is the tool for a known code and that a miss is terminal, implicitly steering keyword queries elsewhere. It stops short of explicitly naming search_products as the alternative for partial or descriptive queries.

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