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

Annotations already declare a read-only, idempotent, non-destructive operation, so the description does not need to repeat safety properties. It adds useful context by clarifying that the home-carrier rate is user-supplied and the travel-eSIM rate is illustrative, which helps the agent interpret the comparison.

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 two tightly written sentences, with the usage trigger front-loaded and the comparison behavior immediately following. There is no redundant or filler language.

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 simple two-parameter read-only calculator with a full input schema and an output schema, the description supplies the key behavioral caveat: the travel-eSIM rate is illustrative. It gives the agent enough context to select and invoke the tool appropriately without over-explaining return values.

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 both parameters are already documented in the schema. The description reinforces the meaning of the home-carrier daily rate and mentions trip days implicitly through the comparison, but it does not add substantial parameter 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 gives a specific comparison task: home carrier roaming fees versus a travel eSIM, using a user-supplied day-pass rate and an illustrative eSIM daily rate. It is more precise than the name alone, but it does not explicitly distinguish this tool from nearby siblings such as compare_plans or calculate_data_budget.

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 explicitly says when to use the tool: when the user wants to compare home-carrier roaming fees against a travel eSIM. It does not provide when-not-to-use conditions or name alternative tools, so it falls short of the top score.

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