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

Annotations already declare the safe-read profile (readOnly, idempotent, non-destructive), so the description's job is to add beyond that. It does: cities are not distinct destinations but covered by their country's plans, and the returned slug is the key other tools expect — both are non-obvious behavioral facts. It omits any note on result volume, fallback when nothing matches, or ranking behavior.

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?

A single dense sentence that front-loads the purpose, then the accepted value forms, then the return value. No filler and nothing that could be dropped without losing information.

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?

No output schema exists, so the description must cover the return value — and it does, stating that matching destinations are returned along with the slug consumed by get_plans and create_checkout_link. For a one-parameter lookup tool, an agent has everything needed to call it and use its result.

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?

With a single parameter at 100% schema coverage, the baseline is 3. The description goes beyond the schema by enumerating the accepted query shapes (country, region, city) and showing that regions like 'Europe', 'Asia' and 'Global' are valid, which the schema does not state.

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 ('Find the eSIMpass travel eSIM for a place') and enumerates the accepted place categories (country, region, city) with concrete examples. It also names the downstream consumers (get_plans, create_checkout_link), which separates it from those siblings without the agent 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?

Gives clear context for when to reach for this tool: any place lookup, including cities that resolve to their country's plans, and it explains the workflow role of the result (feeding get_plans and create_checkout_link). It stops short of explicit exclusions (e.g., when to use the generic 'search' sibling instead), so it is not a full when/when-not statement.

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