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

Annotations already declare readOnlyHint and openWorldHint, so the bar is lower, but the description adds genuinely useful non-schema facts: prices are in USD, results are data-only with no phone number, and one plan is flagged as recommended. It does not say how many plans are returned or whether unavailable destinations yield an empty result.

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?

Four short sentences, front-loaded with purpose and scope, then unit and content caveats. The only near-redundancy is 'Read only', which merely echoes the annotation, and it costs almost nothing.

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 three-parameter read-only lookup with no output schema, the description covers what the tool returns (plans plus a recommendation), currency, and the data-only limitation. It stops short of describing result shape or count, which would help an agent decide whether pagination or narrowing is needed.

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 the schema already documents destination examples and the use/days semantics (including the 14-day default), so the description adds essentially no per-parameter meaning. It only restates the destination forms in prose.

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 specific verb (find) and resource (NUM data eSIM plans) plus the accepted destination granularity, and the 'one recommended plan for the trip' clause distinguishes it from the checkout and order-status siblings, which cannot return a plan catalog.

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 mention of a recommended plan suggests this is the discovery step preceding esim_start_checkout, but the description never names that sibling or states prerequisites/exclusions. An agent must infer the browse-then-checkout workflow from the sibling names alone.

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