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

Annotations declare readOnlyHint=true, so the description doesn't need to restate safety. It adds valuable behavioral context: the tool requires a key on the public server and returns key_required with user guidance if absent. It also implies the output is a side-by-side comparison of differences, which is useful. It doesn't detail output structure, but an output schema exists, so that's acceptable. Minor gap: no mention of how it handles invalid option_ids, but that's not a major omission.

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 sentences, each earning its place: the first defines scope, the second defines when to use it, the third discloses a critical prerequisite and failure mode. Information is front-loaded with the core purpose before the usage note. No fluff or repetition of schema details.

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 comparison tool with a full input schema and an output schema, the description covers the essential context: what is compared, when to use it, and the key requirement. The only notable omission is not naming a specific sibling alternative (e.g., price_plans) for when the user just wants prices, but the use-case framing is strong enough that an agent can infer the boundary. The key_required behavior is a nice completeness touch.

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 schema already documents all four parameters. The description adds context for option_ids ('from an earlier result') and age ('as the premium schedules state it') indirectly, but mostly it reinforces what the schema says. Baseline 3 is appropriate because the schema carries the heavy lifting and the description doesn't add significant new parameter-level meaning.

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 ('compare') and resource ('plans'), and immediately clarifies what the comparison covers: 'premium now and at 65, next-year range, deductible, room class, limits, area and each insurer's rate record.' It also distinguishes itself from siblings by focusing on 'what actually differs' and the trade-off question, which is a clear differentiator from tools like find_plans or price_plans.

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?

The description explicitly says when to use it: for 'which is better' and 'why is one cheaper' questions, and instructs the agent to 'state the trade-off and let the person choose.' It also names a prerequisite (a key on the public server) and the failure mode without it, which is strong usage guidance. It doesn't name a specific alternative tool, but the use-case framing is clear enough to route correctly among siblings.

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