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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so safety is fully covered. The description adds that results are filtered by query and include lowest price, but doesn't disclose pagination, result size, or output ordering. With annotations carrying the safety profile, this is modest added value.

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?

Two tight sentences: returned fields first, filtering behavior second. Every clause carries information; nothing is padded.

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 zero-required-param read-only listing tool with fully documented schema, the description covers what it returns and how to filter. It stops short of output shape details (field types, whether results are paginated), which would be useful but are not strictly required. No output schema exists, so some return-shape explanation would have helped.

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%, and the schema already explains lang's default and that language never affects plan or price, and that query matches part of a name or code. The description's 'Filterable with query' merely restates what the schema says, adding no syntax or matching-behavior detail beyond it.

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 resource (countries with a plan) and enumerates returned fields (ISO code, localized name, plan count, lowest price), so the agent knows exactly what it yields. It is clearly a discovery/enumeration tool distinguishable from siblings like search_plans or get_plan, though the description doesn't explicitly name those alternatives.

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?

Says it is filterable with query, which implies a use case, but gives no explicit when-to-use vs alternatives guidance. An agent must infer that this is the entry point for discovering which countries have plans before calling get_plan or estimate_data.

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