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

Annotations already mark this as read-only and open-world, but the description adds critical behavioral context beyond them: the upstream data source (iTunes Lookup API), how ratings are counted per country, and the payment model ($0.002 USDC per successful call via x402, with failed calls uncharged). The payment/auth detail is exactly the kind of operational fact an agent needs before invoking.

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 front-loaded sentences: purpose and data source first, scope ('iPhone and universal apps') next, then the ratings caveat and pricing. Every sentence contributes distinct, non-redundant information with no filler.

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 an output schema present, the description does not need to explain return values, and it covers everything else an agent needs: input identification, platform scope, the country-dependent ratings nuance, and the x402 payment requirement. Nothing material for correct invocation is missing.

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 the baseline is 3, but the description adds meaning beyond the schema: 'by App Store id' reinforces the required key, and 'Ratings are Apple's counts for the chosen country' explains how the optional country parameter affects results. That country-to-ratings linkage is functional semantics the schema's short parameter descriptions do not spell out.

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 the specific resource (App Store iPhone app metadata), the source (Apple's public iTunes Lookup API), the lookup key (App Store id), and enumerates the returned fields. The clause 'iPhone and universal apps' explicitly distinguishes it from the Mac App Store and Google Play siblings, letting an agent identify the correct tool without inspecting schemas.

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 context is clear: it is an id-based lookup for iPhone/universal apps, and the note that ratings are scoped to the chosen country implies the country parameter's role. It stops short of explicitly naming alternatives like dev_mac_app_store or dev_google_play_store, so the when-not-to-use routing is left implicit.

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