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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered. The description adds genuinely new context: a public-server key is required and a missing key yields a key_required response with guidance on obtaining one, which is auth behavior the annotations do not convey.

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?

Front-loaded with what differs and why to use it, then the auth caveat. The telegraphic comma-list style is dense but every clause carries information; nothing is wasted.

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 an output schema present, return-value explanation is unnecessary, and the description covers the essential extras: the comparison dimensions, usage intent, and the key requirement. What remains missing is explicit routing against sibling tools, but overall it is sufficient to call correctly.

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 the baseline is 3. The description restates that comparisons typically need rate-record context and that option_ids come from an earlier result, but the schema already documents age, gender, smoker, and option_ids semantics, adding little beyond it.

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?

Names a specific verb (compare) and enumerates exactly what is compared: premium now and at 65, next-year range, deductible, room class, limits, area, and each insurer's rate record. This is far more specific than siblings like find_plans or price_plans, so an agent can tell them apart without reading schemas.

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?

States clear usage contexts ("which is better", "why is one cheaper") and even gives a behavioral instruction ("state the trade-off and let the person choose"). It does not, however, explicitly name or exclude sibling tools like find_plans or price_plans, so the when-not path is left to inference.

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