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

Annotations already declare readOnly/idempotent/non-destructive, so the added value here is the access-tier behavior: without Library Pro only the first 3 US screenshots come back, other countries and remaining indexes require the $19/month plan. That is exactly the kind of gate an agent needs before calling, though output format beyond 'markdown images' and any rate limits remain unstated.

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 sentences, front-loaded with purpose, then parameter guidance, then the pricing caveat. Every sentence carries information; only the '$19 a month' phrasing leans slightly promotional but still conveys a real constraint.

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 does the right work by naming the return format ('ordered', 'markdown images a vision model can inspect') and disclosing the truncation/gating behavior. For a simple 3-parameter read tool this is close to complete, though the exact response envelope is 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%, so all three parameters are already documented. The description adds two genuinely useful bits — that index is 1-based and mirrors the /apps/:slug/screens/:index route, and that country selects both screenshots and in-app events — but otherwise restates what the schema carries.

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?

States a specific verb and resource ('Get the ordered App Store listing screenshots for a library app') plus the return shape (markdown images inspectable by a vision model). It does not explicitly differentiate from the sibling get_screenshots or get_app_videos, so an agent must infer the split from the 'directory'/library-app framing.

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 implied rather than stated: pass country for localized storefronts, omit index for the ordered list. There is no explicit when-to-use/when-not statement and the close sibling get_screenshots is never named as the alternative for non-directory apps.

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