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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered. The description adds real context beyond that: the upstream source (eSIMCompare), the sample size (194 countries), and the exact plan thresholds used to compute the index. It does not disclose currency, data vintage, or behavior for an unmatched country name, which are meaningful for a price tool.

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 sentences, front-loaded with the metric definition before the mode-dependent return contents; nearly every clause carries information. The second sentence is a long comma-separated enumeration, but each item is a distinct output field rather than 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 no output schema, the description carries the burden of describing returns, and it does so for both branches (per-country plan, per-GB price, unlimited week, rank; global median, extremes, top providers). Gaps remain around currency/units, data recency, and unmatched-country behavior for a one-parameter tool that is otherwise simple.

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 already defines the single country param, so the baseline is 3. The description adds value by explaining that the presence or absence of the parameter changes the entire result shape, not just a filter, which the schema only hints at with 'Omit for the global summary.'

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 states a precise, operationalized metric (cheapest eSIM with >=5 GB valid >=7 days) and its scope (per country, 194-country index), which is clearly distinct from the plan-level siblings compare_esim_providers and search_esim_plans. It never names those siblings, so the differentiation is inferable 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?

The with-country vs without-country split is spelled out with the exact payload each mode returns, which effectively tells the agent when to use each mode. It stops short of naming alternatives or exclusion conditions against the sibling tools.

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