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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered. The description adds genuine behavioral value beyond them: it defines the three recommendation tiers (best_fit = cheapest covering the estimate, budget = below estimate, roomier = next size up) and states that each option carries a direct link to checkout confirmation. It does not mention auth, rate limits, or what happens when no destination is supplied.

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 sentences, front-loaded with what the tool does and what it returns, with the input conditions appended. The final sentence about destinations partially repeats schema content already covered, but nothing is padded or circular.

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 takes on the burden of explaining returns and does so concretely (three tiers, their definitions, and the checkout link). It also explains the multi-country input pattern and the optional data-usage input. Only the behavior when neither destination nor destinations is supplied is left unstated, a minor gap against otherwise complete coverage.

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 description coverage is 100%, so every parameter is already documented in the schema, including the destination/destinations alternative and the usage/activities/min_gb override chain. The description restates the multi-country destination pattern and the total-days convention but adds no format or defaulting detail beyond what the schema provides, which is the baseline-3 case.

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 and resource (recommend a data plan from destination + trip length) and goes further by describing the exact shape of the result: up to 3 named options (best_fit, budget, roomier) with a reason and a checkout link each. That output profile is distinctive enough to separate it from search_plans and estimate_data, which do not return curated, purchase-ready recommendations.

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?

It gives clear context for use: supply destination (or every country of a multi-country trip in destinations), trip length, and data usage 'if known', implying the tool works with partial input. It never names an alternative or an exclusion (e.g. when to use estimate_data instead), so it stops short of full routing guidance.

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