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

Annotations already declare readOnly, idempotent, non-destructive and closed-world behavior, so the safety profile is covered. The description adds real context beyond that: which fields come back, and the non-obvious constraint that the destination must be the one the plan was found under, not merely where the traveller is going.

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, no padding, and the trigger condition is front-loaded before the returned-field list and prerequisite note. Every clause carries information the agent needs.

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 needn't explain return values, and annotations cover the safety profile. Purpose, triggering context, required inputs and the destination-matching constraint are all covered, leaving no gap that would cause a misinvocation.

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% with rich per-parameter descriptions (including the language default and the accepted destination formats), so the baseline is 3. The description adds the pairing rule linking planId to the destination under which it was found, which the schema states only implicitly.

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+resource (return the detail of one plan) and enumerates exactly what detail: coverage (country count, region), features, validity, price. It also names the sibling tools that produce the planId, so an agent can distinguish it from search_esim_plans and get_destination_overview without opening a schema.

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?

"Use this when someone wants the detail of one plan returned by search_esim_plans or get_destination_overview" gives clear triggering context and a prerequisite chain. It stops short of an explicit exclusion (e.g. use search_esim_plans first to discover plans), so it is strong context rather than full when/when-not routing.

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