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

Annotations already declare this a safe, idempotent, closed-world read, so the bar is lower. The description adds meaningful behavior beyond that: it estimates GB before selecting, optimizes on cheapest, and sources prices from the 'real published catalog' – useful disclosure of how the pick is derived.

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?

Two sentences with the usage trigger front-loaded and the mechanism second; nothing is redundant. The first sentence is slightly packed with three conditions but still scannable.

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 read-only recommender with an output schema, the description covers what an agent needs: the trigger, the inputs it will use, and the nature of the result (a concrete cheapest plan with real prices). Return-value detail is correctly delegated to the output schema.

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 all three parameters are already documented, and the description largely restates them ('where they're going, for how long, how much they'll use data'). It hints that usage_profile is converted into a GB estimate, but adds no formats, ranges, or multi-country handling detail beyond the schema.

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 ('picks the cheapest matching plan') and adds the internal step of estimating GB need, which separates it from lookup tools like list_available_plans or get_plan_details. It does not name a sibling directly, so the differentiation is implicit rather than explicit.

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 a concrete trigger ('Use when the user says where they're going, for how long, and roughly how much they'll use data, and wants a concrete plan pick'). That is clear context, but it offers no explicit when-not clauses or pointers to alternatives such as compare_plans or estimate_data_usage.

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