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

Annotations already provide readOnlyHint/openWorldHint/destructiveHint, and the description adds useful behavioral context: the result reflects current rank, the tool is lightweight and single-purpose, rank semantics are inverted (lower is better), and it costs one credit. It does not contradict annotations and provides more than the minimum without needing to repeat schema information.

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?

The description is compact, front-loaded with the core action, and every sentence is purposeful: current scope, output content, use cases, ranking direction, alternatives, and cost. It avoids redundancy and is easy to scan.

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 lightweight two-parameter tool with no output schema, the description is complete: it explains what is returned (rank and category node identifiers), how to interpret it, when to prefer alternatives, marketplace via domain, and cost. An agent has enough context to decide whether and how to invoke it.

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 input schema already covers both parameters with detailed descriptions (asin pattern/URL source and domain enum/marketplace guidance), so the baseline is 3. The description adds no extra meaning about parameter semantics beyond what the schema states; it mostly restates that the tool returns rank data.

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 opens with a specific verb and resource: 'Get a product's current Best Sellers Rank (BSR)', and immediately clarifies scope by mentioning numerical rank and sub-category node identifiers. It also differentiates from siblings by naming get_product_detail and get_sales_history as alternatives that cover broader detail or historical trends.

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?

It explicitly says when to use this tool ('when you only need the rank'), gives a concrete example ('Is this product selling well?'), clarifies the ranking direction ('Lower rank number means higher sales velocity'), and states exactly when to use get_product_detail or get_sales_history instead. It also adds cost awareness with 'Costs 1 credit'.

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