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

Beyond the annotations (readOnlyHint, destructiveHint, non-idempotent), the description adds real context: credit cost per successful call, the fact that repeated calls may be charged again (consistent with idempotentHint=false), and a warning to treat returned data as untrusted. That last point is valuable and not derivable from structured fields. No return-format or rate-limit detail, hence not a 5.

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 with the core purpose front-loaded and no filler. The cost sentence is slightly boilerplate but earns its place given non-idempotent billing behavior.

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 read-only lookup with no output schema and 100% schema coverage, the description covers purpose, billing, and data-trust posture. It does not mention pagination despite 'size' being a per-page parameter, which is the only notable gap.

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 all three parameters (asin, size, marketplace) are fully documented with patterns, enums, defaults, and ranges in the schema. The description adds no parameter-level meaning, so the baseline of 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?

Clear verb+resource: 'Retrieve comparable product records for an ASIN'. An agent immediately knows this returns competitor products keyed by ASIN. It does not, however, distinguish itself from the near-identically named sibling amazon_product_competitor_lookup, leaving the agent to guess which of the two to pick.

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?

No when-to-use guidance, no prerequisites, and no mention of the competing amazon_product_competitor_lookup tool. The only usage-adjacent content is a cost note, which is billing information rather than selection guidance.

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