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

Annotations already declare readOnly, idempotent, non-destructive and non-open-world, so the safety profile is covered. The description adds genuinely new behavior: result ordering (cheapest total first, out of stock last), the fields returned (price, shipping, stock, check time, shop link), and that each row is augmented with that version's best price in the site's other markets.

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?

It is a single front-loaded sentence that opens with the core action and pushes output detail into a parenthetical. The sentence is dense and slightly run-on with its trailing cross-market clause, but no wording is redundant.

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 no output schema, the description carries the burden of describing results and does so (ordering, field list, cross-market best price). Combined with the fully documented four-parameter schema, an agent has everything needed to call it correctly and interpret the response.

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 all four parameters are already documented, including the locale enum and the URL-or-id-plus-site pattern. The description reinforces the site/locale coupling ('the locale's market on the site that lists it') but adds no syntax or format detail beyond what the schema provides, so 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?

The description names a specific verb and resource ('Shop prices of one product version side by side') and scopes it to a locale market and the site that lists it, which separates it from search_products (finding products) and best_deals (aggregate deals). It stops short of naming those siblings explicitly, so the differentiation is inferable 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?

Usage is implied by the phrasing 'on the site that lists it' and by the cross-market extension, but there is no explicit when-to-use/when-not statement and no routing to search_products or list_sites for obtaining the site key or product id. The agent must infer the workflow from the schema descriptions rather than the description text.

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