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

Annotations already declare readOnlyHint, idempotentHint, closed-world and non-destructive, so the safety profile is covered. The description adds real behavioral context beyond that: the not_found outcome for missing or withdrawn plans and the fact that stale cached prices should not be trusted. It stops short of describing auth needs or rate limits, so 4 rather than 5.

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, each load-bearing: verb+resource+returned fields first, usage/next-step second, error handling last. No filler, and the identification ('Retrieve one current eSIM plan by plan_id') is front-loaded.

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?

There is no output schema, and the description compensates by enumerating the returned fields (EUR price, coverage destination, data allowance, validity, availability, purchase terms). With 3 params, full schema coverage, and clear error semantics, nothing an agent needs to invoke it correctly is missing.

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 the baseline is 3. The description earns above that by clarifying that destination must be an exact slug rather than a free-text city and that plan_id comes from search_plans and is not a name, adding disambiguation the schema only partially conveys.

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 ('Retrieve one current eSIM plan by plan_id') and immediately scopes it against siblings by naming search_plans as the source of the ID and prepare_order as the downstream step. An agent can tell this apart from search_plans and compare_plans without opening any 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?

Explicitly prescribes when to call it ('confirm an offer before prepare_order') and what to do on failure ('A missing or withdrawn plan returns not_found; search_plans again instead of relying on cached prices'), which also warns against a wrong alternative behavior. Alternatives and their triggering conditions are named rather than implied.

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