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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description goes beyond these by disclosing that calculations are approximate, cashback expected values are site estimates, and construction costs/availability vary by building. It also explains the behavioral nuance that expected value can reverse face-value rankings, which is valuable context beyond the annotations.

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?

The description is front-loaded with the primary function in the first sentence, followed by a concise disclaimer paragraph. Every sentence earns its place, and the caveats are brief yet relevant. No redundant or filler content exists.

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 tool with 4 parameters and no output schema, the description covers the purpose, key inputs indirectly, and important limitations. It does not explicitly state the return format, but for a comparison tool the output is generally implied. Overall, it provides sufficient context for an agent to select and invoke the tool 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 all four parameters are fully documented in the schema. The description adds semantic value by listing the comparison dimensions such as set discounts, which clarifies why mobile_lines and mobile_carrier are needed. It reinforces the purpose of plan_slugs without redundant detail.

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 clearly states the tool 'compares 2-5 Hikari plans' including detailed factors such as real total cost, cashback receipt conditions, cancellation fees, and set discounts. This specific verb+resource scope distinguishes it from generic compare_plans and search/simulation tools.

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 provides clear context by specifying the exact number of plans (2-5) and the Hikari-specific scope, which signals when to use this tool. It does not explicitly name alternatives or exclusion conditions, but the disclaimer about checking official sites adds a limitation note.

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