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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description's added value is the output shape: code, name, ISO codes covered, plan count, and cheapest USD price. Since no output schema exists, this return-value disclosure is genuinely useful and goes beyond structured fields, though pagination or result-size behavior remains 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?

One front-loaded sentence with zero filler, and the field enumeration is information-dense rather than redundant. It reads slightly as a run-on, but every clause carries meaning.

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 one-optional-parameter read tool with no output schema, listing the returned fields is exactly the compensating context an agent needs. Only minor gaps remain: no guidance on how the filter behaves or whether the full catalog is always returned.

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 the single `query` parameter is fully documented in the schema with examples ('japan', 'JP'). The description adds no matching semantics (substring vs exact ISO, case sensitivity), so this is the baseline case where the schema does all the work.

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?

Uses a specific verb ("List") on a precisely scoped resource ("destinations (countries and multi-country regions) with eSIM plans on shibaSIM") and enumerates the returned fields, which implicitly separates it from the plan-level siblings get_plan and search_plans. It stops short of naming those siblings, so the differentiation is inferred rather than stated.

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 is only implied: the agent can infer this is the catalog/discovery call that precedes search_plans or get_plan, but the description never says so, nor does it state any when-not-to-use condition. No mention of the optional query parameter as a narrowing mechanism either.

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