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

Annotations only declare readOnlyHint and openWorldHint; the description adds substantial context beyond that: result ordering, that prices are final payable amounts on vidi.one, how flexible-duration plans are priced relative to min_days, and that each plan carries a purchase URL with instant QR-code delivery. It does not address truncation at the limit or whether regional plans are interleaved when single-country options run out, which is the main remaining behavioral gap.

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 front-loaded sentences: scope and ordering first, then pricing semantics, then fulfillment details. Dense but every clause carries information the agent cannot get from the schema, with the minor exception of the instant-QR-code line, which is more marketing than selection guidance.

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 does so adequately — plan ordering, price basis, per-plan URL, and delivery mechanism. It is incomplete on result-set size behavior (default limit 20, max 100, whether results indicate more available), which matters for an openWorld search tool.

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 baseline is 3; the description earns above that by explaining semantic interactions the schema does not — that returned prices are denominated in the chosen currency and that flexible-duration plans are quoted against min_days. It stops short of clarifying ordering interplay between min_gb/max_price filters and the cheapest-first sort, or what limit does to the 'single-country then regional' ordering.

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 resource is stated precisely ('eSIM data plans that work in a country') and the ordering semantics (single-country first, then regional, cheapest first) make the result set unambiguous. The verb is implicit rather than stated, and no sibling tool is named, so differentiation from list_esim_countries or search_destinations is left to inference from the resource noun.

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 implied by 'plans that work in a country' — an agent can infer this is the retrieval step after resolving a country code. However, there are no explicit when/when-not statements, no mention of pairing with list_esim_countries for country resolution, and no exclusions relative to check_device_esim or the flight/destination siblings.

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